U.S. patent application number 12/476132 was filed with the patent office on 2010-12-02 for differential low frequency noise suppression technique for a common mode feedback circuit.
Invention is credited to Ronald J. Lipka.
Application Number | 20100301936 12/476132 |
Document ID | / |
Family ID | 43065866 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100301936 |
Kind Code |
A1 |
Lipka; Ronald J. |
December 2, 2010 |
DIFFERENTIAL LOW FREQUENCY NOISE SUPPRESSION TECHNIQUE FOR A COMMON
MODE FEEDBACK CIRCUIT
Abstract
Disclosed is a common mode feedback circuit for a differential
amplifier that eliminates the effects of low frequency noise. A
modulator is placed in a common mode feedback loop that modulates
the feedback loop signal at a predetermined frequency to up-convert
the low frequency noise. The predetermined frequency may be
selected to be above the operating range of the downstream
circuitry. In addition, a low pass filter can be used to eliminate
the up-converted noise.
Inventors: |
Lipka; Ronald J.; (Fort
Collins, CO) |
Correspondence
Address: |
COCHRAN FREUND & YOUNG LLC;LSI CORPORATION
2026 CARIBOU DRIVE, SUITE 201
FORT COLLINS
CO
80525
US
|
Family ID: |
43065866 |
Appl. No.: |
12/476132 |
Filed: |
June 1, 2009 |
Current U.S.
Class: |
330/149 |
Current CPC
Class: |
H03F 2203/45418
20130101; H03F 2200/171 20130101; H03F 3/45946 20130101; H03F
2203/45082 20130101; H03F 3/45475 20130101; H03F 2203/45424
20130101 |
Class at
Publication: |
330/149 |
International
Class: |
H03F 1/26 20060101
H03F001/26 |
Claims
1. A method of up-converting low frequency differential mode noise
generated in a common mode feedback circuit comprising: generating
differential voltage signals at an output of said differential
amplifier, said output having a positive voltage output and a
negative voltage output; connecting a first side of a first
resistor to said positive voltage output of said differential
amplifier; connecting a first side of a second resistor to said
negative voltage output of said differential amplifier; providing a
transconductance amplifier that has a positive input and a negative
input and that generates a common mode output current signal on
both a first output and a second output; applying a common mode
voltage of said differential amplifier to said negative input of
said transconductance amplifier by connecting said negative input
of said transconductance amplifier to said second side of said
first resistor and a second side of said second resistor;
up-converting said low frequency noise by modulating said common
mode output current signal at a predetermined frequency which
up-converts said low frequency noise to said predetermined
frequency.
2. The method of claim 1 wherein said process of providing a
transconductance amplifier further comprises: providing a
transconductance amplifier that compares said common mode voltage
to a reference voltage to maintain a voltage level of said common
mode voltage.
3. The method of claim 2 wherein said process of up-converting said
low frequency noise comprises: modulating said common mode output
current signal generated by said transconductance amplifier with
four transmission gates that swap polarity of said first output and
said second output of said transconductance amplifier at said
predetermined frequency and with a duty cycle of approximately
fifty percent.
4. The method of claim 3 further comprising: low pass filtering
said differential voltage signals generated by said differential
amplifier to remove said low frequency noise that has been
up-converted.
5. A common mode feedback circuit that up-converts low frequency
noise to a predetermined frequency comprising: a differential
amplifier that generates differential voltage signals on a positive
output and a negative output; a first resistor having a first side
that is connected to said positive output of said differential
amplifier; a second resistor having a first side that is connected
to said negative output of said differential amplifier; a
transconductance amplifier that generates a common mode output
signal on both a first output and a second output, said
transconductance amplifier having negative input that is connected
to a second side of said first resistor and a second side of said
second resistor so that a common mode voltage of said difference
voltage signals of said differential amplifier is connected to said
negative input of said transconductance amplifier, said
transconductance amplifier having a positive input that is
connected to a reference voltage so that said common mode voltage
is compared to said reference voltage and said common mode output
current signal of said transconductance amplifier is representative
of the difference between said common mode voltage and said
reference voltage; a modulator having an input that is connected to
said first output and said second output of said transconductance
amplifier that modulates said common mode output current signal
generated by said transconductance amplifier at said predetermined
frequency and up-converts noise on said first output and said
second output of said transconductance amplifier to said
predetermined frequency.
6. The circuit of claim 5 wherein said modulator comprises: four
transmission gates that swap polarity of said first output and said
second output of said transconductance amplifier at said
predetermined frequency with a duty cycle of approximately fifty
percent.
7. The circuit of claim 6 further comprising: a low pass filter
connected to said differential voltage signals generated by said
differential amplifier to filter out said low frequency noise.
Description
BACKGROUND OF THE INVENTION
[0001] Common mode noise generated by a differential amplifier can
be eliminated by subtracting the differential signals. The derived
signal is substantially devoid of induced differential noise. As
such, differential amplifiers are a useful analog circuit for
eliminating differential noise.
SUMMARY OF THE INVENTION
[0002] An embodiment of the present invention may therefore
comprise a method of up-converting low frequency differential mode
noise generated in a common mode feedback circuit comprising:
generating differential voltage signals at an output of the
differential amplifier, the output having a positive voltage output
and a negative voltage output; connecting a first side of a first
resistor to the positive voltage output of the differential
amplifier; connecting a first side of a second resistor to the
negative voltage output of the differential amplifier; providing a
transconductance amplifier that has a positive input and a negative
input and that generates a common mode output current signal on
both a first output and a second output; applying a common mode
voltage of the differential amplifier to the negative input of the
transconductance amplifier by connecting the negative input of the
transconductance amplifier to the second side of the first resistor
and a second side of the second resistor; up-converting the low
frequency noise by modulating the common mode output current signal
at a predetermined frequency which up-converts the low frequency
noise to the predetermined frequency.
[0003] An embodiment of the present invention may further comprise
a common mode feedback circuit that up-converts low frequency noise
to a predetermined frequency comprising: a differential amplifier
that generates differential voltage signals on a positive output
and a negative output; a first resistor having a first side that is
connected to the positive output of the differential amplifier; a
second resistor having a first side that is connected to the
negative output of the differential amplifier; a transconductance
amplifier that generates a common mode output signal on both a
first output and a second output, the transconductance amplifier
having negative input that is connected to a second side of the
first resistor and a second side of the second resistor so that a
common mode voltage of the difference voltage signals of the
differential amplifier is connected to the negative input of the
transconductance amplifier, the transconductance amplifier having a
positive input that is connected to a reference voltage so that the
common mode voltage is compared to the reference voltage and the
common mode output current signal of the transconductance amplifier
is representative of the difference between the common mode voltage
and the reference voltage; a modulator having an input that is
connected to the first output and the second output of the
transconductance amplifier that modulates the common mode output
current signal generated by the transconductance amplifier at the
predetermined frequency and up-converts noise on the first output
and the second output of the transconductance amplifier to the
predetermined frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is an overall block diagram of an embodiment of a
differential noise suppressor for a common mode feedback circuit of
a differential amplifier.
[0005] FIG. 2 is a schematic diagram of the modulator circuit
illustrated in FIG. 1.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0006] FIG. 1 is a schematic block diagram of a differential noise
suppressor for a common mode feedback circuit 100 that is used with
a differential amplifier 102. As shown in FIG. 1, inputs 104, 106
are applied to differential amplifier 102. Differential amplifier
102 generates a differential output on a positive output voltage
line 108 and a negative output voltage line 110. The outputs 108,
110 of differential amplifier 102 are connected to a feedback
circuit that comprises resistors 112, 114, transconductance
amplifier 120, and modulator 126. The feedback loop controls the
common mode voltage level of outputs 108, 110 of differential
amplifier 102. Resistors 112, 114 have the same resistance, so that
node 116 comprises the common mode voltage, since node 116 is a
summing node. The common mode voltage is applied to the negative
input of the transconductance amplifier 120, while a reference
voltage 118 is applied to the positive input of the
transconductance amplifier 120. The outputs 122, 124 of the
transconductance amplifier 120 comprise a common mode signal that
is duplicated on outputs 122, 124 that is representative of the
difference between the common mode voltage 116 and the reference
voltage 118.
[0007] Various types of noise can be present on the outputs 108,
110 of the differential amplifier 102. For example, common mode
noise, which appears on both output 108 and output 110 can be
eliminated by generating a difference signal from outputs 108, 110.
However, other types of noise may randomly occur on one or the
other of the outputs 108, 110. A typical type of noise that occurs
is low frequency noise resulting from the use of certain types of
components, such as CMOS components. This type of noise is
typically referred to as 1/f noise. 1/f noise typically increases
in amplitude for lower frequency signals. Hence, in low frequency
applications of a differential amplifier, such as differential
amplifier 102, 1/f noise can be a factor. The differential 1/f
noise that is generated by the differential amplifier 102 cannot be
removed by generating a difference signal from the outputs 108,
110. In addition, transconductance amplifier 120 may also generate
1/f low frequency noise, which is injected into the outputs 108,
110 of the differential amplifier 102. The differential amp 102 by
itself removes common mode noise only. Embodiments of the present
invention function to suppress the 1/f differential mode noise
being generated from the common mode feedback circuit.
[0008] As also shown in FIG. 1, modulator 126 modulates the common
mode signal at outputs 122, 124 of the transconductance amplifier
120 at a predetermined frequency. The modulated current signals
128, 130 are applied to outputs 108, 110, respectively, of the
differential amplifier 102 to complete the feedback circuit. In
this fashion, the differential output signals 108, 110 are
controlled to produce a common mode voltage at node 116 that is
equal to the reference voltage 118.
[0009] Modulator 126, illustrated in FIG. 1, has a modulation
frequency that up-converts the 1/f low frequency noise to a higher
frequency that is equal to the modulation frequency plus the
frequency of the low frequency noise. The 1/f noise is double
sideband (i.e. a 1/f noise at a single frequency has a component at
+1/f and -1/f) so the modulation operation produces signals at the
modulation frequency + and - the noise frequency. In this fashion,
the low frequency noise can be band-shifted to a higher frequency,
so that the low frequency noise does not affect a particular
application that is within a lower frequency band of interest. In
addition, an optional low pass filter 132 can be used to filter out
the noise and only pass the lower frequency signal. This may be
particularly useful in audio circuits that have a frequency range
from just several Hertz up to 20 KHz. In this manner, low frequency
1/f noise from the common mode feedback circuit can be virtually
eliminated from an audio or other low frequency circuit. Hence, in
one embodiment, the modulation frequency may be a frequency above
20 KHz, which is the top of the audio range. Similarly, the low
pass filter 132 in the described embodiment may have a cutoff
frequency of approximately 20 KHz, so that the low pass filter
passes all signals below the 20 KHz cutoff.
[0010] One of the problems with using a modulator 126, as
illustrated in FIG. 1, is that switching noise may be induced in
the modulated output signals 128, 130 at high switching
frequencies. Hence, application of the feedback circuit 100,
illustrated in FIG. 1, may be less useful at high frequencies
unless quiet switching devices and/or low pass filter 132 can be
used.
[0011] FIG. 2 is a schematic illustration of the modulator 126 that
is illustrated in FIG. 1. As shown in FIG. 2, transmission gates
132, 134, 136, 138 are controlled by a modulation signal that
switches the transmission gates 132, 138 to either a connected
position or a disconnected position. During a first time period,
transmission gate 132 and transmission gate 138 are connected,
while transmission gate 134 and transmission gate 136 are
disconnected. During a second equal time period, transmission gates
134, 136 are connected, while transmission gates 132, 138 are
disconnected. This results in the common mode output current signal
122, 124 from the transconductance amplifier 120 being
alternatively connected to outputs 108, 110 at a fifty percent duty
cycle. Again, the modulation frequency is sufficiently high to
up-convert the low frequency noise to a frequency that does not
impact the circuits downstream from the differential amplifier
102.
[0012] Hence, the common mode feedback loop, illustrated in the
embodiments of FIGS. 1 and 2, up-converts low frequency noise to a
frequency that does not affect downstream circuitry. In addition,
the low frequency noise can be filtered by a low pass filter to
eliminate the noise from the output of the differential amplifier.
This process results in the generation of a highly desirable output
signal with little or no 1/f noise from the common mode feedback
circuit.
[0013] The foregoing description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and other modifications and variations may be
possible in light of the above teachings. The embodiment was chosen
and described in order to best explain the principles of the
invention and its practical application to thereby enable others
skilled in the art to best utilize the invention in various
embodiments and various modifications as are suited to the
particular use contemplated. It is intended that the appended
claims be construed to include other alternative embodiments of the
invention except insofar as limited by the prior art.
* * * * *