U.S. patent application number 14/548326 was filed with the patent office on 2015-03-19 for advanced temperature compensation and control circuit for single photon counters.
The applicant listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Thomas FRACH.
Application Number | 20150076357 14/548326 |
Document ID | / |
Family ID | 42710061 |
Filed Date | 2015-03-19 |
United States Patent
Application |
20150076357 |
Kind Code |
A1 |
FRACH; Thomas |
March 19, 2015 |
ADVANCED TEMPERATURE COMPENSATION AND CONTROL CIRCUIT FOR SINGLE
PHOTON COUNTERS
Abstract
A PET scanner includes a ring of detector modules encircling an
imaging region. Each of the detector modules includes one or more
sensor avalanche photodiodes (APDs) that are biased in a breakdown
region in a Geiger mode. The sensor APDs output pulses in response
to light from a scintillator corresponding to incident photons. A
reference APD also biased in a breakdown region in a Geiger mode is
optically shielded from light and outputs a voltage that is
measured by an analog to digital converter. Based on the
measurement, a bias control feedback loop directs a variable
voltage generator to adjust a bias voltage applied to the APDs such
that a difference between a voltage of a breakdown pulse and a
preselected logic voltage level is minimized.
Inventors: |
FRACH; Thomas; (Aachen,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
EINDHOVEN |
|
NL |
|
|
Family ID: |
42710061 |
Appl. No.: |
14/548326 |
Filed: |
November 20, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13148055 |
Aug 5, 2011 |
8921754 |
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PCT/IB2010/050539 |
Feb 5, 2010 |
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14548326 |
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61157923 |
Mar 6, 2009 |
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Current U.S.
Class: |
250/370.08 |
Current CPC
Class: |
G01T 1/2018 20130101;
G01T 7/005 20130101; G01T 1/244 20130101; G01T 1/248 20130101; A61B
6/037 20130101; G01T 1/247 20130101; G01T 1/40 20130101 |
Class at
Publication: |
250/370.08 |
International
Class: |
G01T 1/24 20060101
G01T001/24 |
Claims
1. A method of maintaining a signal to noise ratio substantially
constant in an imaging detector, the method comprising: measuring a
breakdown voltage of one or more photodiodes in the detector;
comparing the breakdown voltage to a preselected voltage level;
determining a difference between the preselected voltage level and
the measured breakdown voltage; adjusting a temperature in the
detector based on the difference.
2. A radiation detector for use in imaging comprising: a plurality
of avalanche photodiodes; a biasing circuit configured to bias the
photodiodes to operate in a Geiger mode in which the photodiodes
breakdown in response to receiving radiation generating an output
pulse and the biasing circuit being configured to bias each
photodiode back to the Geiger mode after each breakdown; a first
cooling element thermally coupled to the photodiodes and configured
to remove heat from the photodiodes; a control circuit configured
to measure the breakdowns and control the first cooling element in
accordance with the measured breakdowns.
3. The detector module as set forth in claim 2, wherein at least
one of the diodes is a reference photodiode which is shielded from
light, the control circuit measuring the breakdowns of the at least
one reference photodiode.
4. The detector module as set forth in claim 3, wherein the control
circuit includes a counter configured to count the output pulses
generated by the at least one reference photodiode, the control
circuit controlling the first cooling element in accordance with
the count.
5. The detector module as set forth in claim 4, wherein the control
circuit is configured to control the first cooling element in
accordance with a rate at which the output pulses are counted by
the counter.
6. The detector module as set forth in claim 4, wherein the biasing
circuit applies a recharge pulse to bias each photodiode back to
the Geiger mode after each breakdown; and wherein the counter is
configured to count the recharge pulses.
7. The detector module as set forth in claim 3, wherein the control
circuit is further configured to measure a breakdown voltage across
the at least one reference photodiode and adjust the bias voltage
of the photodiodes to a predetermined characteristic logic voltage
level.
8. The detector module as set forth in claim 2, wherein the first
cooling element includes a Peltier cooling element which is
electrically controlled by the controller and further including: a
second cooling element which transfers heat from the Peltier
cooling element to ambient surroundings.
9. The detector module as set forth in claim 2, wherein the control
circuit is configured to control the first cooling element in
accordance with a rate of the measured breakdowns.
10. The detector module as set forth in claim 2, wherein the
control circuit is configured to measure a voltage of the
breakdowns and control the first cooling element in accordance with
the measured breakdown voltages.
11. An imaging apparatus comprising: a gantry defining an imaging
region; a subject support configured to support a subject in the
imaging region; a detector array that includes a plurality of
detector modules as set forth in claim 2; an event verification
processor that analyzes detected radiation to determine whether the
detected radiation originated from valid events; a reconstruction
processor configured to reconstruct the valid events into an image
representation.
12. A method of controlling a detector array for use imaging, the
method comprising: biasing a plurality of avalanche photodiodes to
operate in a Geiger mode in which the photodiodes breakdown in
response to receiving radiation generating an output pulse; after
each breakdown, biasing each photodiode back to the Geiger mode;
cooling the photodiodes; measuring the breakdowns and controlling
the cooling in accordance with the measured breakdowns.
13. The method as set forth in claim 12, further including
shielding at least one of the photodiodes from light; and wherein
measuring the breakdowns measures the breakdowns of the at least
one shielded photodiode.
14. The method as set forth in claim 13, wherein measuring the
breakdowns includes counting the output pulses generated by the at
least one shielded photodiode and controlling the cooling in
accordance with the count.
15. The method as set forth in claim 14, wherein the cooling is
controlled in accordance with a rate at which the output pulses
from the at least one shielded photodiode are counted.
16. The method as set forth in claim 14, wherein biasing the
photodiodes back to the Geiger mode after each breakdown includes
applying a recharge pulse and wherein the counting includes
counting the recharge pulses.
17. The method as set forth in claim 12, wherein the temperature is
controlled in accordance with a rate of the measured
breakdowns.
18. The method as set forth in claim 12, further including:
measuring a voltage of the breakdowns and wherein the cooling is
adjusted in accordance with the measured breakdown voltage.
19. The method as set forth in claim 13, further including:
measuring a breakdown voltage across the shielded photodiode and
adjusting the biasing of the photodiodes to a predetermined
characteristic logic voltage level.
20. The method as set forth in claim 12, wherein the cooling
includes electrically controlling a Peltier cooling element to
adjust the temperature, and further including: transferring heat
from the Peltier cooling element to ambient surroundings.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/148,055 filed Aug. 5, 2011 which is a US National Stage
Entry of PCT application Serial No. PCT/IB2010/050539, filed Feb.
5, 2010, published as WO 2010/100574 A2 on Sep. 10, 2010, which
claims the benefit of U.S. provisional application Ser. No.
61/157,923 filed Mar. 6, 2009, which is incorporated herein by
reference.
DESCRIPTION
[0002] The following relates to the diagnostic imaging arts. It
finds particular application in conjunction with radiation
detectors for nuclear medical imagers employing radiation
transmission or radiopharmaceuticals, such as single photon
emission computed tomography (SPECT) imagers, positron emission
tomography (PET) imagers, planar x-ray imagers, and the like, and
will be described with particular reference thereto. It will be
appreciated that the invention may also be applicable to other
radiation imaging modalities, and in systems and methods employing
radiation detectors such as astronomy and airport luggage
screening.
[0003] In SPECT, a radiopharmaceutical is administered to an
imaging subject, and one or more radiation detectors, commonly
called gamma cameras, are used to detect the radiopharmaceutical
via radiation emission caused by radioactive decay events.
Typically, each gamma camera includes a radiation detector array
and a honeycomb collimator disposed in front of the radiation
detector array. The honeycomb collimator defines a linear or
small-angle conical line of sight so that the detected radiation
comprises projection data. If the gamma cameras are moved over a
range of angular views, for example over a 180.degree. or
360.degree. angular range, then the resulting projection data can
be reconstructed using filtered back-projection,
expectation-maximization, or another imaging technique into an
image of the radiopharmaceutical distribution in the imaging
subject. Advantageously, the radiopharmaceutical can be designed to
concentrate in selected tissues to provide preferential imaging of
those selected tissues.
[0004] In PET, a radiopharmaceutical is administered to the imaging
subject, in which the radioactive decay events of the
radiopharmaceutical produce positrons. Each positron interacts with
an electron to produce a matter/anti-matter annihilation event that
emits two oppositely directed gamma rays. Using coincidence
detection circuitry, an array of radiation detectors surrounding
the imaging subject detects the coincident oppositely directed
gamma ray events corresponding to the positron-electron
annihilation. A line of response (LOR) connecting the two
coincident detections contains the position of the
positron-electron annihilation event. Such lines of response are
analogous to projection data and can be reconstructed to produce a
two- or three-dimensional image. In time-of-flight PET (TOF-PET),
the small time difference between the detection of the two
coincident y ray events is used to localize the annihilation event
along the LOR.
[0005] In planar x-ray imaging, a radiation source irradiates a
subject, and a radiation detector array disposed on the opposite
side of the subject detects the transmitted radiation. Due to
attenuation of radiation by tissues in the imaging subject, the
detected radiation provides a two-dimensional planar representation
of bones or other hard, radiation-absorbing structures in the
imaging subject. Such transmission-based imaging is improved upon
in computed tomography (CT) imaging, in which the radiation source
is revolved around the imaging subject to provide transmission
views or projection data over an extended angular range, for
example over a 180.degree. or 360.degree. span of angular views.
Using filtered back-projection or another image reconstruction
technique, this radiation projection data is reconstructed into a
two- or three-dimensional image representation.
[0006] SPECT, PET, and other radiation-based medical imaging share
a common need for compact and robust radiation detector modules. In
the past, SPECT and PET radiation detector modules have typically
consisted of an array of photomultiplier tubes (PMT'S) optically
coupled with scintillator crystals. The scintillator crystal
absorbs the radiation particle and converts it into a light burst
which is measured by the photomultiplier tubes. Photomultiplier
tubes provide high detection and gain (.about.10.sup.6)
characteristics but they are bulky, fragile, require high voltages,
and are very sensitive to magnetic fields. In some radiation
detection systems, the photomultiplier tubes have been replaced by
photodiodes that produce an analog signal proportional to the
intensity of the light bursts. Even though photodiodes offer a
cost-effective, low voltage alternative to photomultiplier tubes in
high light situations, they do not provide the adequate gain in low
light (low gamma ray flux) sensing applications, thus leading to
poor signal-to-noise ratios.
[0007] To address these difficulties, silicon photomultiplier
(SiPM) detectors have been developed that incorporate the high gain
and stability of photomultiplier tubes along with the
cost-effective, low voltage nature of the photodiodes. SiPM
detectors use an array of small avalanche photodiodes (APDs) that
are each optically coupled to a corresponding scintillation
crystal. The APDs are biased in a breakdown region. In this region,
the APDs become sensitive to single carriers, such as may be caused
by an incident photon. These carriers, electrons and/or holes, can
also be thermally generated, thus leading to dark counts that cause
noise. Both electrons and holes can initiate the breakdown of the
diode, thereby producing a strong output signal. In analog SiPMs,
the output signal consists of the cumulative charge of a large
number of passively quenched diodes. In contrast, digital SiPMs
detect breakdown events individually based on voltage pulses that
are digitized by logic gates and counted by digital counters that
are located approximate to the APDs.
[0008] In digital Geiger-mode, APDs break down in response to a
photon of light from a radiation event in the corresponding
scintillation crystal and produce an output pulse. The output pulse
functioning as binary 1's are counted to determine the number of
photons generated by the radiation event striking the corresponding
scintillator. This photon count corresponds to the energy of the
detected radiation event.
[0009] While sensitive to individual photon events, breakdown
voltage of each APD is affected by various ambient factors, such as
magnetic fields and temperature. Drift of the breakdown voltage
leads to a corresponding change of an excess voltage. Photon
detection is affected by changes in excess voltage because: (1) the
excess voltage determines the field strength inside the device,
thus leading to a drift of the photon detection probability, and
(2) the charge pulse produced during breakdown is proportional to
the product of the diode capacitance and the excess voltage. Analog
SiPMs, which count detected photons as a measured charge signal,
are affected by both factors and become very sensitive to ambient
conditions. The dark current rate (DCR) is doubled approximately
every 8.degree. C. To reduce the DCR of the sensor and avoid errors
due to variations in the APDs, cooling can help, but even with
cooling, temperature fluctuations can occur.
[0010] The present application contemplates a new and improved
nuclear imaging detector apparatus and method that overcomes the
above-referenced problems and others.
[0011] In accordance with one aspect, a radiation detector module
is provided. A plurality of detector pixels each have a
scintillator optically coupled to at least one sensor photodiode
operated in a Geiger mode. At least one reference photodiode is
shielded from light and is operated under the same conditions as
the at least one sensor photodiode. The module includes a control
circuit that measures a breakdown voltage across the reference
photodiode, and adjusts a bias voltage across the at least one
reference photodiode and the at least one sensor photodiode. This
brings the dark current pulses generated by the at least one
reference photodiode into substantial equality with the
characteristic logic voltage level.
[0012] In accordance with another aspect, a method of compensating
for drift in a sensitivity of a portion of a radiation detector
array is provided. A bias voltage is applied to a plurality of
sensor photodiodes and a parallel connected reference photodiode.
The reference photodiode is covered with an opaque covering,
preventing it from receiving light from an associated scintillator.
The bias voltage biases the photodiodes to a Geiger mode, sensitive
to single photons. Following breakdown of the reference photodiode,
a breakdown voltage of the reference photodiode is measured. A
difference between a value of a digitized pulse from the reference
photodiode and a logic voltage level is determined. The bias
voltage is adjusted to minimize the difference.
[0013] One advantage resides in improved breakdown voltage control
for avalanche photodiodes operated in the Geiger mode.
[0014] Another advantage lies in compensation for several ambient
factors that affect the sensitivity of the photodiodes.
[0015] Another advantage lies in the flexibility to be used in
either analog or digital systems.
[0016] Another advantage lies in the freedom of the system builder
to relax requirements on temperature stabilization without
compromising system performance.
[0017] Still further advantages and benefits will become apparent
to those of ordinary skill in the art upon reading and
understanding the following detailed description.
[0018] The present application may take form in various components
and arrangements of components, and in various steps and
arrangements of steps. The drawings are only for purposes of
illustrating the preferred embodiments and are not to be construed
as limiting the present application.
[0019] FIG. 1 is a diagrammatic illustration of a nuclear imaging
scanner in accordance with the present application;
[0020] FIG. 2 depicts a cutaway view of a detector module, in
accordance with the present application;
[0021] FIG. 3 is a flow diagram depicting bias control and
temperature control feedback loops;
[0022] FIG. 4 shows certain circuit components used to realize the
feedback loops of FIG. 3;
[0023] FIG. 5 depicts waveforms detailing one cycle of the circuits
of FIGS. 3 and 4, with an accurate bias voltage;
[0024] FIG. 6 depicts waveforms detailing one cycle of the circuits
of FIGS. 3 and 4, with a bias voltage that is too high;
[0025] FIG. 7 depicts waveforms detailing one cycle of the circuits
of FIGS. 3 and 4, with a bias voltage that is too low.
[0026] With reference to FIG. 1, a diagnostic imaging device 10
includes a housing 12 and a subject support 14. Enclosed within the
housing 12 is a detector array. The detector array includes a
plurality of individual detector modules 16. The array may include
hundreds or thousands of radiation detector modules 16. While one
particular embodiment is described with reference to a positron
emission tomography (PET) scanner, it is to be understood that the
present application is also useful in other medical applications,
such as single photon emission computed tomography (SPECT) as well
as x-ray astrophysics, gamma ray telescopes, radiography, security,
and industrial applications. Generally, the present application
finds use in imaging x-rays, gamma rays, or charged particles with
high energy and spatial resolution. The array is arranged so that
detector elements 16 are disposed adjacent to an imaging region 18
and oriented to receive radiation from the imaging region 18. The
subject support 14 is movable in to and out of the imaging region
18. This allows the detector array to be sensitive to multiple
views of the subject, without having to reposition the subject on
the support 14. The detector array can be a ring of detectors 16,
multiple rings, one or more discrete flat or arced panels, or the
like.
[0027] In PET, pairs of gamma rays are produced by a positron
annihilation event in the imaging region and travel in
approximately opposite directions. Such an event may be produced
from the nuclear decay of .sup.82Rb. These gamma rays are detected
as pairs, with a slight time difference (on the order of
nanoseconds or fractions thereof) between detections if one gamma
ray travels farther to reach a detector than the other.
Accordingly, in PET scanners, the detector arrays typically
encircle the imaging region.
[0028] Before the PET scan commences, a subject is injected with a
radiopharmaceutical. In one common exam, the radiopharmaceutical
contains a radioactive element, such as .sup.82Rb, coupled to a tag
molecule. The tag molecule is associated with the region to be
imaged, and tends to gather there through body processes. For
example, rapidly multiplying cancer cells tend to expend abnormally
high amounts of energy duplicating themselves. The
radiopharmaceutical can be linked to a molecule, such as glucose,
or an analog thereof, that a cell typically metabolizes to create
energy, which gathers in such regions and appear as "hot spots" in
the image. Such a tag is also useful in cardiac perfusion imaging,
since the heart expends relatively large amounts of energy. Other
techniques monitor tagged molecules flowing in the circulatory
system. In such a technique, it is beneficial to tag a molecule
that is not quickly absorbed by tissues of the body.
[0029] When a gamma ray strikes the detector array, a time signal
is generated. A triggering processor 20 monitors each detector 16
for an energy spike, e.g., integrated area under the pulse,
characteristic of the energy of the gamma rays generated by the
radiopharmaceutical. The triggering processor 20 checks a clock 22
and stamps each detected gamma ray with a time of leading edge
receipt stamp. The time stamp, energy estimate and position
estimation is first used by an event verification processor 24 to
determine if the event data is valid, e.g., if the pair of events
are coincident, have the proper energy, and the like. Accepted
pairs define lines of response (LORs). Because gamma rays travel at
the speed of light, if detected gamma rays arrive more than several
nanoseconds apart, they probably were not generated by the same
annihilation event and are usually discarded. Timing is especially
important in time of flight PET (TOF-PET), as the minute difference
in substantially simultaneous coincident events is used to further
localize the annihilation event along the LOR. As the temporal
resolution of events becomes more precise, so too does the accuracy
with which an event can be localized along its LOR.
[0030] LORs are stored in an event storage buffer 26. In one
embodiment, the LORs are stored in a list-mode format. That is, the
events are stored in temporal order with time indicators
periodically inserted. Alternatively, the events can be
individually time stamped. A reconstruction processor 28
reconstructs all or a portion of the LORs into an image
representation of the subject using filtered backprojection or
other appropriate reconstruction algorithms. The reconstruction can
then be displayed for a user on a display device 30, printed, saved
for later use, and the like.
[0031] Each detector module 16 includes a plurality of photodiodes
in one embodiment. While operating the photodiodes in Gieger mode,
a reverse bias voltage is applied to allow the photodiodes to be
sensitive to single photons of light generated by associated
scintillation crystals optically coupled to the photodiodes. The
scintillators are selected to provide high stopping power for
incumbent radiation with rapid temporal decay of the scintillation
burst. Some suitable scintillator materials include LSO, LYSO, MLS,
LGSO, LaBr, CsI(Ti), and mixtures thereof. The bias voltage is
applied such that the photodiodes produce an avalanche current when
struck by the scintillated photons, earning them the moniker
avalanche photodiodes (APDs). The optimum bias voltage is sensitive
to multiple factors, such as temperature, pressure, ambient light,
and the like. Bias voltage control circuitry 32 monitors the
detector modules 16 and adjusts the applied bias voltage as
conditions dictate.
[0032] With reference to FIG. 2, a pixelated detector module 16
includes at least one sensor APD 34, more particularly one or more
SiPMs each including an array of the APDs 34, optically coupled to
a scintillation crystal 35. Additionally, each module 16 also
includes at least one reference detector 36, such as a reference
APD. The reference APDs 36 are covered with an opaque enclosure,
such as a metal cap, to prevent light (ambient light or
scintillation bursts) from reaching the reference APDs 36. The
reference APDs 36 are placed among the sensor APDs 34, as it is
desirable to have the sensor APDs 34 and the reference APDs 36
operating in the same environment, aside from the reception of
light. In the illustrated embodiment, the sensor APDs 34 and
reference APDs 36 are formed on a common substrate 38.
[0033] A digital circuitry layer 40 is electronically connected to
the sensor photodiodes 34 and the reference photodiodes 36. The
digital circuitry layer 40 includes circuitry that collects and
outputs photon detection specific information such as radiation
detector module identification, pixel identification, timestamps,
and photon counts. The digital circuitry may also include digital
biasing circuitry, digital triggering circuitry, and readout
circuitry. The bias control circuitry 32 can be located in the
digital circuit layer 40. Alternately, the bias control circuitry
32 can be located on a separate chip or die. With reference now to
FIG. 3, the bias control circuitry 32 includes a first, bias
control feedback loop 42. Instead of detecting photo-generated
electron-hole pairs, the reference APD 36 detects thermally
generated electron-hole pairs or dark current. Thermally generated
electron-hole pairs are created by generation-recombination
processes within the semiconductor and can trigger an avalanche
current in the absence of scintillated photons, producing noise in
the system. The bias voltage across the APDs 34, 36 can be adjusted
to make the APDs 34, 36 more or less sensitive, commensurate with
the ambient surroundings.
[0034] When the reference APD 36 breaks down, an analog-to-digital
converter (ADC) 44 converts the resulting anode voltage into a
digital value, equivalent to the breakdown voltage. The AD
converter converts the anode voltage after the avalanche current
has decayed through the diode (there is no current flowing outside
the diode during the breakdown). The current inside the diode
discharges the diode capacitance and thus leads to a voltage drop
at the anode (the cathode is pinned to a fixed voltage level, while
the anode is left floating by leaving the reset transistor open).
The internal current stops flowing when the voltage over the diode
has reached the breakdown voltage, below that voltage, there is no
multiplication possible and therefore most of the current stops and
only a tiny leakage current continues to discharge the diode. The
signal is processed and changed back into an analog signal by a
digital-to-analog converter (DAC) 46, and is used to adjust a
variable voltage source 48 that reverse biases the sensor APDs 34
and the reference APDs 36. The avalanche current, which is in the
order of 10.sup.6 electrons per photon, will continue to flow until
the voltage over the diode has reached the breakdown voltage. The
time for this to happen is typically 200-300 ps depending on the
excess voltage, diode capacitance and internal resistance. After
that, there is no current flowing and the anode voltage reflects
the breakdown voltage. This steady-state anode voltage is measured
by the AD converter and the bias voltage is adjusted so that the
anode voltage equals the logic level. A recharge transistor 50 is
used to charge the diode back above the breakdown voltage for the
next measurement cycle. That recharge pulse is about 10-15 ns long
while the time to the next discharge can be in the millisecond
range. A more detailed discussion of the bias control loop 42 is
undertaken hereinbelow, in reference to FIG. 4.
[0035] With continuing reference to FIG. 3, a second, temperature
control loop 52 is illustrated. The digital pulses from the ADC 44
are counted by a dark pulse counter 54 within a predetermined time
period. Alternately, the dark pulse counter 54 could detect and
count the activity of the recharging circuit 50. The dark pulse
counter 54 outputs a digital value representative of the dark count
rate. As temperature is proportional to the dark count rate, a
driver 56 uses the dark count rate to drive a primary temperature
control element 58, such as a Peltier cooling element, to quickly
fine-tune the operating temperature of the APDs 34, 36. A secondary
cooling element 60, which can use water, air, or other coolants,
can be used to remove heat from the system. Limiting temperature
variance is desirable to to limit the variance of the temperature
in order to always get the same number of counts per photon.
[0036] With reference now to FIG. 4, the reference APD 36 is
reverse biased with the variable voltage source 48. The anode is
connected to a transistor 62 that is used to recharge the reference
APD 36 to a selected voltage over the reference APD's 36 breakdown
voltage. This is done by applying a short pulse to the gate of the
transistor 62 with sampling and recharge circuitry 64, allowing the
transistor 62 to turn it conductive. In one embodiment, the
transistor 62 is an NMOS transistor. After this recharge, the
reference APD 36 is left sensitive to carriers and will eventually
break down. With reference to FIG. 5 and continuing reference to
FIG. 4, during breakdown, the voltage at node 66 increases rapidly
from zero, forming a voltage pulse 68, to a voltage dictated by the
current operating conditions of the module 16. It is desirable for
this voltage to be as close to a logic voltage level 70 as
possible. The voltage pulse 68 is sensed by an inverter 72, which
digitizes and passes the signal to the sampling and recharge
circuitry 64, and the dark rate counter 54. The sampling and
recharge circuitry 64 starts the ADC 44 to measure the actual
voltage after the pulse 68 over the broken-down reference APD 36.
Once the measurement is complete, the measurement is filtered 73
and passed to the bias voltage control feedback loop 42. More
specifically, a bias voltage controller 75 controls the voltage
output of the variable voltage source 48, described in more detail
below. Additionally, the sampling and recharge circuitry 64 applies
a pulse 74 that recharges the reference APD 36 resetting it so that
it is once again sensitive to carriers. While the reference diode
36 is broken down, the voltage at node 76 drops to zero, as
indicated by the waveform 78.
[0037] If the voltage pulse 68 is equal to the logic voltage level
70, then the bias voltage 80 is on target. Thus, a bias voltage
control signal 82 produced by the bias control feedback loop 42 is
correct, that is, half of the logic voltage level 70. If the bias
voltage 80 is on target, no corrections are needed.
[0038] FIG. 6 depicts a situation in which the bias voltage 80 is
too high, and is corrected. Such a situation may be caused by a
shift in the breakdown voltage 84 of the APDs 34, 36 brought about,
for example, by a lower ambient temperature. In this case, the
voltage measured by the ADC 44 (i.e. the voltage of pulse 68)
exceeds the logic voltage level 70 by a difference 86. In this
situation, the bias control feedback loop 42 directs the variable
voltage source 48 to lower the bias voltage 80, thus minimizing the
difference 86 between the voltage pulse 68 and the logic voltage
level 70. As in the previous example of FIG. 5, the sampling and
recharge circuitry 64 applies the pulse 74 resetting the reference
APD 36.
[0039] Similarly, FIG. 7 depicts a situation in which the bias
voltage 80 is too low. Such a situation may be caused by a higher
ambient temperature. In this case, the voltage measured by the ADC
44 (i.e. the voltage of pulse 68) is less than the logic voltage
level 70 by the difference 86, which is now a negative value. In
this situation, the bias control feedback loop 42 directs the
variable voltage source 48 to raise the bias voltage 80, again
minimizing the difference 86 between the voltage pulse 68 and the
logic voltage level 70. Again, the sampling and recharge circuitry
64 applies the pulse 74 resetting the reference APD 36. In the
illustrated embodiments, the bias voltage correction is done while
the reference APD 36 is in its broken-down state. This allows the
ADC 44 to monitor the difference 86 in real time.
[0040] In one embodiment, the circuitry depicted in FIGS. 3 and 4
can be integrated on the same die next to the APDs 34, 36 if the
bias voltage is generated by a charge pump on the chip and enough
chip area is available. Parts of the circuitry can be located on
separate chips, thus allowing application in conjunction with
analog silicon photomultipliers.
[0041] In an alternate embodiment, the bias control loop 42 can be
implemented in a purely analog way, eliminating the ADC 44 and the
DAC 46. In this embodiment, the reference photodiode 36 is operated
at the breakdown voltage by impressing a well defined current
(about 1 .mu.A) and using the resulting voltage as a control signal
for the variable voltage source 48. This embodiment would have the
advantage of making the overall circuit more compact. In the
digital embodiments, the ADC 44 can also be re-used to monitor
other voltages. This can be useful for functional and parametric
testing at the wafer level, and during the power-on sequence of the
sensor module.
[0042] The present application has been described with reference to
the preferred embodiments. Modifications and alterations may occur
to others upon reading and understanding the preceding detailed
description. It is intended that the present application be
construed as including all such modifications and alterations
insofar as they come within the scope of the appended claims or the
equivalents thereof.
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