U.S. patent application number 13/198120 was filed with the patent office on 2013-02-07 for circuit and method for dynamically controlling op-amp offset for photodetector applications.
This patent application is currently assigned to TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS, INC.. The applicant listed for this patent is GONGGUI XU. Invention is credited to GONGGUI XU.
Application Number | 20130033320 13/198120 |
Document ID | / |
Family ID | 47562294 |
Filed Date | 2013-02-07 |
United States Patent
Application |
20130033320 |
Kind Code |
A1 |
XU; GONGGUI |
February 7, 2013 |
CIRCUIT AND METHOD FOR DYNAMICALLY CONTROLLING OP-AMP OFFSET FOR
PHOTODETECTOR APPLICATIONS
Abstract
Provided herein are a circuit and method for dynamically
controlling operational amplifier (op-amp) offset for photodetector
applications using a variable trimming circuit coupled to a test
node and an op-amp.
Inventors: |
XU; GONGGUI; (PLANO,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
XU; GONGGUI |
PLANO |
TX |
US |
|
|
Assignee: |
TEXAS ADVANCED OPTOELECTRONIC
SOLUTIONS, INC.
PLANO
TX
|
Family ID: |
47562294 |
Appl. No.: |
13/198120 |
Filed: |
August 4, 2011 |
Current U.S.
Class: |
330/252 |
Current CPC
Class: |
H03F 2203/45101
20130101; H03F 3/087 20130101; H03F 3/45475 20130101 |
Class at
Publication: |
330/252 |
International
Class: |
H03F 3/45 20060101
H03F003/45 |
Claims
1. A circuit for controlling op-amp offset in a photodetector
application comprising: a photodetector configured to produce a
current in response to detecting light; an op-amp having an output
terminal, a positive trim terminal, a negative trim terminal, and
first and second input terminals, wherein at least one of the first
and second input terminals is coupled to the photodetector and
wherein an output produced at the output terminal of the op-amp
when operational is based on the current produced by the
photodetector; a variable trimming circuit configured to receive a
current I, wherein the variable trimming circuit includes a
plurality of current branches each configured to provide a portion
of the current I to an output node of the variable trimming
circuit; a test node; and a selector having an input portion
coupled to the output node and an output portion that can be
coupled to both the test node and a transmission node, wherein the
transmission node is configured to couple to one of the positive
and negative trim terminals, and wherein the selector is
configurable to direct current received from the output node to one
of the test node and the transmission node.
2. The circuit of claim 1 further comprising a current amplifier
positioned between the output portion of the selector and the test
node.
3. The circuit of claim 1 wherein at least one of the plurality of
current branches is configured to provide a variable amount of
current.
4. The circuit of claim 1 wherein the plurality of current branches
are configured in a binary arrangement.
5. The circuit of claim 1 wherein a current branch that is
configured to provide the smallest current of the plurality of
current branches defines a resolution of a trimming current that
can be provided to the transmission node.
6. The circuit of claim 1 further comprising a packaging material
covering the op-amp, variable trimming circuit, test node,
transmission node, and selector, wherein the test node is
accessible only before the application of the packaging
material.
7. The circuit of claim 1 further comprising a packaging material
covering the op-amp, variable trimming circuit, test node,
transmission node, and selector, wherein the test node is
accessible after the application of the packaging material.
8. The circuit of claim 1 further comprising: a register coupled to
the variable trimming circuit; and at least one input terminal
coupled to the variable trimming circuit, wherein the input
terminal is configured to carry configuration parameters to the
variable trimming circuit that can be stored in the register for
the plurality of current branches.
9. The circuit of claim 8 wherein a feedback loop couples the
output terminal of the op-amp to the variable trimming circuit.
10. An integrated circuit package for an optical sensing
application comprising: a photodetector configured to produce a
current in response to detecting light; an op-amp having an output
terminal, a positive trim terminal, a negative trim terminal, and
first and second input terminals, wherein at least one of the first
and second input terminals is coupled to the photodetector and
wherein an output produced at the output terminal of the op-amp is
based on the current produced by the photodetector; a variable
trimming circuit configured to receive a current I, wherein the
variable trimming circuit includes a plurality of current branches
each configured to provide a portion of the current I to an output
node of the variable trimming circuit; a selector having an input
portion coupled to the output node and an output portion that can
be coupled to both a test node and a transmission node, wherein the
transmission node is configured to couple to one of the positive
and negative trim terminals, and wherein the selector is
configurable to direct current received from the output node to one
of the test node and the transmission node; and a packaging
material covering the op-amp, variable trimming circuit,
transmission node, and selector.
11. The integrated circuit package of claim 10 further comprising:
a register coupled to the variable trimming circuit; and at least
one input terminal coupled to the variable trimming circuit,
wherein the input terminal is configured to carry configuration
parameters to the variable trimming circuit that can be stored in
the register for the plurality of current branches.
12. The integrated circuit package of claim 11 wherein the
packaging material further covers the test node.
13. The integrated circuit package of claim 12 wherein the test
node is accessible via the at least one input terminal.
14. A method comprising: testing a plurality of current branches in
a variable trimming circuit to determine whether each of the
current branches is capable of transmitting a defined amount of
current, wherein each current branch provides a fraction of a total
current I received by the variable trimming circuit to an output
node of the variable trimming circuit; identifying an offset value
for an operational amplifier (op-amp) that receives an input signal
from a photodetector, wherein the offset value causes an output
level of the op-amp when no signal is being received from the
photodetector; identifying at least one of the current branches
needed to provide a trimming current to the output node, wherein
the trimming current is selected to minimize the offset value; and
selecting the at least one identified current branch to provide the
trimming current to the op-amp.
15. The method of claim 14 further comprising selecting the
trimming current as the closest match to the offset value that can
be produced by the variable trimming circuit.
16. The method of claim 14 wherein testing the plurality of current
branches includes: selecting a single one of the plurality of
current branches; providing current to the selected single current
branch; and identifying whether an output current provided by the
variable trimming circuit matches the fraction of the total current
I to be provided by that current branch.
17. The method of claim 14 wherein testing the plurality of current
branches includes: selecting a plurality of current branches;
providing current to the selected plurality of current branches;
and identifying whether an output current provided by the variable
trimming circuit matches the fraction of the total current I to be
provided by the selected plurality of current branches.
18. The method of claim 14 further comprising: identifying a second
offset value for the op-amp; identifying at least one of the
current branches needed to provide a second trimming current to the
output node, wherein the second trimming current is the closest
match to the second offset value that can be produced by the
variable trimming circuit; and selecting the at least one
identified current branch to provide the second trimming current to
the op-amp.
19. The method of claim 18 wherein the steps of identifying,
identifying, and selecting are repeated each time a new offset
value is needed.
20. The method of claim 18 further comprising retesting the
plurality of current branches prior to identifying at least one of
the current branches needed to provide the second trimming current
to the output node.
Description
BACKGROUND
[0001] Integrated circuits (ICs) may include various components for
use in applications such as optical sensing applications. One such
component is an operational amplifier (op-amp) that has positive
and negative input terminals. An ideal op-amp has no input offset
voltage. In other words, the positive and negative input terminals
are balanced so that connecting the two terminals to one another
results in a zero output. Practically however, a finite offset
exists due to imperfections in the op-amp itself and/or
environmental factors such as heat that may affect op-amp
performance. This offset may affect the performance of an optical
sensing application. Accordingly, improvements are needed to
address op-amp offset in optical sensing applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] For a more complete understanding, reference is now made to
the following description taken in conjunction with the
accompanying Drawings in which:
[0003] FIG. 1 illustrates one embodiment of a circuit having an
op-amp coupled to an optoelectronic sensor.
[0004] FIG. 2 illustrates one embodiment of a circuit having a
variable trimming circuit coupled to a test node and the op-amp of
FIG. 1.
[0005] FIG. 3 illustrates one embodiment of the variable trimming
circuit of FIG. 2.
[0006] FIG. 4 is a flow chart illustrating one embodiment of a
method that may be used with the circuit of FIG. 2.
[0007] FIG. 5 is a diagram of one embodiment of a system with which
the circuit of FIG. 2 may be used.
DETAILED DESCRIPTION
[0008] The present disclosure is directed to circuits and methods
for op-amp offset control. It is understood that the following
disclosure provides many different embodiments or examples.
Specific examples of components and arrangements are described
below to simplify the present disclosure. These are, of course,
merely examples and are not intended to be limiting. In addition,
the present disclosure may repeat reference numerals and/or letters
in the various examples. This repetition is for the purpose of
simplicity and clarity and does not in itself dictate a
relationship between the various embodiments and/or configurations
discussed.
[0009] Referring to FIG. 1, one embodiment of a circuit 100 is
illustrated with an optoelectronic sensor 102 (e.g., a photodiode)
that produces a current in response to detected light. The
photodiode 102 is coupled to an op-amp 104. The circuit 100 is a
simplified circuit and it is understood that the photodiode 102 may
be coupled to the op-amp 104 in many different configurations.
Furthermore, while the circuit 100 includes an element 106 (e.g., a
capacitor or a resistor), it is understood that the circuit 100 may
include many different components, including resistive elements,
capacitive elements, inductive elements, other op-amps, and/or
other components needed to provide desired functionality.
[0010] The op-amp 104 includes a negative input terminal NEG, a
positive input terminal POS, and an output terminal OUT. In the
present example, the anode of the photodiode 102 is coupled to
ground via a node 108 and the cathode of the photodiode 102 is
coupled to the negative input terminal NEG of the op-amp 104 via a
node 110. The positive input terminal 110 is coupled to ground via
a node 112. The output terminal OUT is coupled to a node 114, which
is coupled to the node 110 via the element 106.
[0011] The photodiode 102 of circuit 100 is a light sensor: a photo
current will be produced from the photodiode 102 when light hits
the photodiode. At low light, the generated photo current might be
very small. In order to detect low light, the photodiode 102 is
preferred to have an extremely low leakage current. One way to
reduce photodiode leakage current is to make sure the voltage
across the photodiode is close to zero. This can be achieved by a
high gain op-amp 104. For the ideal op-amp with no input offset,
the feedback configuration of the circuit 100 will drive the op-amp
NEG input terminal very close to the POS input terminal. When the
POS input terminal is at ground voltage level, the NEG input
terminal will also be close to ground voltage level. Since the
photodiode 102 is between the NEG input terminal and ground, the
voltage across the photodiode 102 is close to zero voltage, and
therefore an extremely low leakage current (also called dark
current) can be achieved.
[0012] Although an ideal op-amp has no offset voltage, in the real
world the op-amp 104 of the circuit 100 has a finite offset. In
photodetector applications (e.g., ambient light sensing, motion
detection, or proximity detection), this offset is undesirable
because this offset voltage is the voltage across the photodiode
102. When the photodiode 102 is biased up by this offset voltage, a
finite amount of leakage current (also called dark current) will be
generated from the photodiode 102 and this will limit the
photodetector's performance. For example, the dark current may
result in a detectable signal even when the photodiode 102 is not
actually detecting light. For photodetector applications, this is
problematic as it may interfere with sensing applications that rely
on extremely low levels of detected light. Accordingly, the offset
voltage needs to be minimized or eliminated in order to more
accurately detect light using the photodiode 102.
[0013] Due to the offset voltage that may exist between the
positive and negative input terminals POS and NEG of the op-amp
104, the op-amp 104 also includes a negative trim terminal TRNEG
and a positive trim terminal TRPOS that may be used to bias the
op-amp 104 either negatively or positively, respectively, to
account for offset. Adjusting for this offset is referred to as
"trimming" and typically involves the use of a trimming current
that compensates for the mismatches between differential branches
within the op-amp 104 and serves to minimize or cancel the offset
when applied. However, offset trimming may be difficult to
accomplish if the offset is so small as to be difficult to detect.
Furthermore, trimming may be performed for a certain set of
parameters (e.g., a particular environmental temperature or range
of temperatures) and may change if the circuit 100 is used under
other environmental conditions.
[0014] Trimming introduces an additional problem that involves
reliability. More specifically, there is a question as to whether
the trimming circuit itself is reliable. For example, if a desired
current is to be supplied to the TRNEG terminal of the op-amp 104,
the trimming circuit may be set to provide that current. However,
there may be no testing of the trimming circuit to ensure that it
is actually providing the desired current.
[0015] Referring to FIG. 2, one embodiment of a circuit 200 is
illustrated with the op-amp 104 of FIG. 1. Although not shown, the
op-amp 104 may be part of the circuit of FIG. 1 or may be
configured differently. The circuit 200 also includes a variable
trimming circuit 202 that controls current for two different
purposes. One purpose is to provide trimming circuitry for
providing a trimming current to the op-amp 104 and another purpose
is to provide a test current to ensure that the trimming current is
correct (e.g., that the variable trimming circuit 202 is
functioning correctly). For purposes of illustration, the variable
trimming circuit 202 is shown with a variable current source 204.
The variable current source 204 is coupled to a VDD node 206 and
provides current to a selector 208, which represents any means for
selectively diverting current. The selector 208 is coupled to an
output of the variable current source 204 and the position of the
selector 208 results in the output current being either the test
current or the trimming current.
[0016] In other embodiments, the selector 208 may be configured to
provide current from selected current branches (as described below)
as the trimming current and the remaining current as the test
current. This configuration enables the difference between the
total current and the trimming current to be calculated to
determine whether the trimming current is correct without affecting
the supply of the trimming current to the op-amp 104. In still
other embodiments, each current branch or group of current branches
may be selectable to couple to either a test node or a trim node,
and the selector 208 may be omitted.
[0017] To provide the test current, the selector 208 may be
manipulated to provide current to a current amplifier. In the
present example, the current amplifier is a current mirror formed
by transistors M2 and M1 that provides a gain of K:1, but it is
understood that other current amplifiers may be used. For purposes
of illustration, the transistors M1 and M2 are both N-channel metal
oxide semiconductor field effect transistors (MOSFET), but it is
understood that other transistors and transistor configurations may
be used. The gates of M1 and M2 are coupled together to form a node
210 and the sources of both M1 and M2 are coupled to a ground node
212. The selector 208 is coupled to the drain of M1 and to the node
210. The drain of M2 is coupled to a test pin 216 by a node
214.
[0018] To provide the trimming current polarity, the selector 208
may be manipulated to provide current to a switch 220 via a node
218. The switch 220 represents any means for selectively diverting
current received from the variable current source 206 to one of
TRNEG and TRPOS.
[0019] With additional reference to FIG. 3, one embodiment of the
variable trimming circuit 202 is illustrated as a current source
having multiple current branches identified as I.sub.1 through
I.sub.n+m. Each current branch I.sub.1 through I.sub.n+m may be
individually selectable and the variable trimming circuit 202 may
be configured to provide varying amounts of current based on the
selected branches with a total available current of I.sub.fullscale
(I.sub.fs) that is the sum of all the branches.
[0020] As described previously, some embodiments may enable the
current branches I.sub.1 through I.sub.n+m to be individually
selectable, which enables testing of branches not used for trimming
to occur simultaneously with trimming. For example, current
branches I.sub.1 through I.sub.n may be selected for the trimming
current (I.sub.trim) and the remainder (I.sub.n+1 through
I.sub.n+m) may be available for a testing current
(I.sub.fs-I.sup.trim).
[0021] It is understood that many different configurations of
current branches I.sub.1 through I.sub.n+m may be provided. For
example, the current branches may be configured to supply an
identical amount of current or different amounts of current. One
possible configuration is a binary configuration where I.sub.1 is
the smallest current branch (where "smallest" refers to the amount
of current provided and provides the minimum resolution of the
variable trimming circuit 202) and is configured to provide an
amperage of X, I.sub.2 is configured to provide twice as much
current as I.sub.1 (2*X), I.sub.3 is configured to provide twice as
much current as I.sub.2 (2.sup.2*X), and so on until I.sub.n+m
(2.sup.n+m-1*X). In another possible configuration, each current
branch is identical and a sufficient number of current branches are
summed until the desired current is obtained. In yet another
possible configuration, each current branch other than I.sub.1 and
I.sub.2 may be the summation of previous current branches. For
example, I.sub.3 may be the summation of I.sub.1 and I.sub.2,
I.sub.4 may be the summation of I.sub.3, I.sub.2, and I.sub.1, and
so on. Accordingly, the actual configuration of the circuit
branches I.sub.1 through I.sub.n+m may vary considerably as long as
the value of each is known so that the proper current branches can
be selected to provide the desired trimming current.
[0022] Referring to FIG. 4, a method 400 illustrates one embodiment
of a process that may be executed using the circuit 200 of FIG. 2
with the variable trimming circuit 202 as illustrated in FIG. 3.
The method 400 is a multi-step process that involves both testing
the variable trimming circuit 202 and trimming the offset of the
op-amp 104.
[0023] In step 402, each circuit branch is tested individually to
ensure that each branch is functioning properly. This step involves
directing the output of the variable trimming circuit 202 to the
test pin 216 and measuring the current off of the test pin 216.
Various methods may be used to test the current branches. For
example, each current branch may be stepped through on an
individual basis, with I.sub.1 being tested, then I.sub.2, then
I.sub.3, etc, to determine whether each branch provides the proper
current. Alternatively, calculations may be performed based on
multiple branches. For example, I.sub.1 may be tested and then
I.sub.1+I.sub.2 may be tested. The difference may be calculated as
the value of I.sub.2, or the summed result may be compared to a
desired value. Accordingly, while the actual testing process may
vary, a determination is made as to whether each current branch is
functioning properly.
[0024] Although not shown in FIG. 4, if a current branch is not
functioning properly, the variable trimming circuit 202 may be
discarded (which may entail discarding some or all of the remainder
of the circuit 200). Alternatively, adjustments may be made. For
example, if the branch I.sub.3 is not functioning and the current
branches are identical, I.sub.3 may be marked as malfunctioning and
the number of available current branches may be reduced to
I.sub.n+m-1. It is understood that this may be performed in
software and so may entail the circuit 200 being coupled to a
processor with which it will be used as the processor would need to
know which current branch is not functioning. Due to the more
complicated nature of such adjustments, discarding of the variable
trimming circuit 202 may be the typical method of handling a
malfunctioning current branch, particularly when the testing of
step 402 is performed during manufacture.
[0025] In step 404, the offset of the op-amp 104 is identified.
This step may involve coupling the input pins of the op-amp
together (e.g., via ground), directing the output of the variable
trimming circuit 202 to the op-amp 104, and measuring the output of
the op-amp 104 while stepping through the current branches. As the
op-amp 104 theoretically has infinite gain, the testing process may
look for the lowest value current branch or branch combination that
causes the output of the op-amp 104 to transition from positive to
negative or vice versa. It is understood that any method may be
used to determine the offset of the op-amp 104 using the variable
trimming circuit 202. For example, the current branches may be
stepped through in order from smallest to largest, from largest to
smallest, or in other methodologies, such as using larger step
increments to more rapidly narrow the offset into a smaller range
of possibilities and then using smaller current branches to more
carefully identify the offset.
[0026] In step 408, the current branches needed to produce the
desired trimming current are selected. This may be accomplished by
configuring the variable trimming circuit 202 to close the desired
current branches and open the remaining current branches. For
example, if I.sub.1, I.sub.2, and I.sub.5 are needed, those three
current branches may be closed and the remaining branches opened.
In the embodiments providing for both testing and trimming
currents, this would result in an I.sub.frim equal to I.sub.1+2+5
and an I.sub.fs-I.sub.trim of I.sub.n+m-(I.sub.1+2+5).
[0027] In step 410, a determination may be made as to whether
parameters and/or the offset have changed. For example, the offset
may drift over time or parameters changes (e.g., moving from one
environmental temperature to another environmental temperature) may
cause offset changes. If the parameters/offset have changed, the
method 400 returns to step 404. If the parameters/offset have not
changed, the method 400 may return to step 410.
[0028] It is understood that step 410 may occur some time after the
preceding steps. For example, steps 402-408 may be performed
initially during manufacture, while a user may execute step 410 at
a later time to determine whether the offset needs to be corrected.
As the variable trimming circuit 202 has been fully tested in step
402, a later adjustment to the trimming current that uses current
branches not originally used will provide the proper trimming
current. Without such complete testing of the variable trimming
circuit 202, later adjustments may rely on malfunctioning current
branches and so would not produce the proper trimming current.
Accordingly, it is understood that the method 400 may end and later
be restarted at a particular step (e.g., step 410 or 404) or that a
relatively long period of time may elapse before step 410 is
executed.
[0029] Although not shown, in some embodiments the variable
trimming circuit 202 may be retested as well. Such functionality
may be available or may be hidden based on packaging, available
interfaces (e.g., whether the test node is accessible), and/or
other factors.
[0030] Referring to FIG. 5, one embodiment of a system 500 within
which the circuit 200 of FIG. 2 may be used is provided. The system
500 includes an integrated circuit (IC) package 502 that provides,
in the present example, ALS functionality, although the IC package
may be configured to provide any optical sensing functionality. The
IC package 502 includes the circuit 200, although not all
components are shown (e.g., the photodiode 102 of FIG. 1). For
example, the op-amp 104 and the variable trimming circuit 202 are
illustrated along with one or more registers 504 that are coupled
to the variable trimming circuit 202. The registers 504 may be used
to store configuration variables for the variable trimming circuit
202, such as which current branches are to be used for trimming the
op-amp 104. A feedback loop 506 may be provided to aid in
identifying the offset during testing. It is understood that
packaging material (e.g., a plastic) may encapsulate some or all of
the IC package 502, although pins and the photodiode 102 may be
mounted above or otherwise not covered by the packaging
material.
[0031] The system 500 may further include a microcontroller unit
(MCU) 508 or other processer that is coupled to the IC package 502.
In the present example, the MCU 508 is coupled to the variable
control circuitry 202 via lines 510 and 512, which may be clock and
data lines respectively. In other embodiments, the MCU 508 may be
coupled directly to the registers 504.
[0032] In operation, a user may interact with the MCU 508 to test
the offset of the op-amp 104, set a new trimming current via the
variable trimming circuitry 202, and/or otherwise configure and use
the functionality provided by the IC package 502.
[0033] It will be appreciated by those skilled in the art having
the benefit of this disclosure that this circuit and method for
dynamically controlling op-amp offset for photodetector
applications provides improved functionality. It should be
understood that the drawings and detailed description herein are to
be regarded in an illustrative rather than a restrictive manner,
and are not intended to be limiting to the particular forms and
examples disclosed. On the contrary, included are any further
modifications, changes, rearrangements, substitutions,
alternatives, design choices, and embodiments apparent to those of
ordinary skill in the art, without departing from the spirit and
scope hereof, as defined by the following claims. Thus, it is
intended that the following claims be interpreted to embrace all
such further modifications, changes, rearrangements, substitutions,
alternatives, design choices, and embodiments.
* * * * *