ETC AB-025

®
BOOST INSTRUMENT AMP CMR
WITH COMMON-MODE DRIVEN SUPPLIES
By R. Mark Stitt
RFB1, RFB2, and RG) is used ahead of the difference amplifier.
The low output-impedance of the of the gain stage preserves
difference amplifier resistor matching and maintains the
CMR of the difference amplifier. The input amplifiers also
provide high input impedance and additional gain.
Ever-increasing demands are being placed on instrumentation amplifier (IA) performance. When standard IAs can not
deliver the required performance, consider this enhanced
version. Dramatic performance improvements can be
achieved by operating the input amplifiers of a classical
three-op-amp IA from common-mode driven sub-regulated
power supplies.
When designing a high CMR instrumentation amplifier, it is
important to use a differential input, differential output
amplifier using a single gain-set resistor (see Figure 1A). In
the Figure 1A circuit, CMR is independent of resistor
matching. Resistor mismatches degrade CMR in the two
gain-set-resistor differential in/out amplifier (see Figure 1B).
Instrumentation amplifiers are designed to amplify lowlevel differential signals while rejecting unwanted commonmode signals. One of the most important specifications is
common-mode rejection (CMR)—the ability to reject common mode signals. AC CMR is especially important since
the common-mode signals are inevitably dynamic—commonly ranging from 60Hz power-line interference to switching-power-supply noise at tens to hundreds of kHz. With
common-mode driven sub-regulated supplies, both the AC
and DC CMR of the IA can be dramatically improved.
Improved AC and DC power supply noise rejection is an
added bonus.
To understand why CMR is independent of resistor matching in the single gain-set resistor amplifier, consider the
Figure 1A circuit. With a common-mode input signal, and
no differential input signal, the voltage between VN and VP
does not change. Therefore the voltage across RG remains
constant and, since no current flows in the op amp inputs,
there is no current change in RFB1 or RFB2, and the differential
output voltage, V1-V2, does not change. Ideally then, with a
perfect difference amplifier, the common-mode gain is zero
and the CMRR is ∞.
At the high gains often required, input offset voltage drift
can also be a critical specification. In some applications, the
low input offset voltage drift of chopper stabilized op amps
might provide the best solution. But, since many of these
chopper stabilized op amps are built using low voltage
CMOS processes, they can not be operated on standard
±15V power supplies. Operating the chopper stabilized op
amps from common-mode-driven, sub-regulated ±5V supplies allows them to be used without restriction in ±15V
systems.
A1
5
8
Difference Amplifier
RFB1
V1
R1
RG
V2
R3
9
R2
A3
RFB2
VO
R4
11
The difference amplifier consists of op amp A3 and ratio
matched resistors R1 through R4. If the resistor ratios R2/R1
exactly match R4/R3 the difference amplifier will amplify
differential signals by a gain of R2/R1 while rejecting common-mode signals. The CMR of the difference amplifier
will almost certainly be limited by resistor mismatch when
a high-performance op amp is used for A3. A unity-gain
difference amplifier requires a difficult 0.01% resistor match
for CMR of 86dB.
A2
VP
Mismatches in gain-set resistors RFB1 and RFB2 do not degrade IA
CMR.
GAIN = (1 + [RFB1 + RFB2]/RG) (R2/R1)
If RFB1 = RFB2 = RFB,
GAIN = (1 + [2 • RFB]/RG) (R2/R1)
Since the slightest input source impedance mismatch would
degrade the resistor matching of the difference amplifier, a
differential input, differential output gain-stage (A1, A2,
1991 Burr-Brown Corporation
4
VN
THE THREE OP AMP IA
To understand how the technique works, first consider the
operation of the three op amp IA shown in Figure 1A. The
design consists of an input gain stage driving a difference
amplifier.
©
2
FIGURE 1A. The Three Op-Amp Instrumentation Amplifier.
AB-025
1
Printed in U.S.A. June, 1997
amplifier. That is why IA data sheets usually specify one
CMR (e.g. 80dB) at gain = 1 and a much higher CMR at
higher gains (e.g. 100dB at gain = 1000).
VN
A1
Most high-performance op amps have better CMR than is
available from difference amplifiers. Be careful when selecting an input op amp though; the venerable “741” op amp has
a minimum high-grade CMR of 80dB, and the world’s most
popular op amp(1), the LM324, has a min high-grade CMR of
only 70dB. High performance bipolar input op amps have
the best CMR. The OPA177 has a min CMR of 130dB. FET
input op amps usually don’t offer quite as much performance. The Burr-Brown OPA627 comes the closest with a
min CMR of 106dB.
Difference Amplifier
RG1
RFB1
V1
R1
R2
A3
RG2
RFB2
V2
R3
VO
R4
LIMITING FACTORS IN IA PERFORMANCE
The DC CMR of a standard IA can be improved by driving
the power supply connections of the input op amps from
sub-regulated power supplies referenced to the IA commonmode input voltage. Op amp CMR is limited by device
mismatch and thermal feedback that occurs as the op amp
inputs change relative to its power supplies. If the power
supply rails are varied to track the common mode input
signal, there is no variation of the inputs relative to the
power-supply rails, errors which degrade CMR are largely
eliminated, and CMR can be substantially improved.
A2
VP
NOTE: Mismatches between gain-set resistor pairs, RFB1/RG1 and
RFB2/RG2 do degrade IA CMR.
FIGURE 1B. The Wrong Way to Make a Three Op-Amp
Instrumentation Amplifier.
In the Figure 1B circuit, CMR does depend on resistor
matching. Common-mode signals will cause different common-mode currents to flow through RG1 and RG2 if their
values are not matched. Then, if the ratio of RFB1/RG1 is not
exactly equal to the ratio of RFB2/RG2, there will be commonmode gain and the CMRR of the instrumentation amplifier
will be degraded.
The AC CMR of the IA is limited by the AC response of the
input amplifiers. The outputs of the input amplifiers in the
IA follow the common mode input signal. As the frequency
of the common-mode signal increases, the loop gain of the
input op amps diminishes, and CMR falls off.
For large common-mode signals, the slew rate of the input
op amps can limit the ability of the IA to function altogether.
This will happen when the maximum rate of change of the
common-mode signal exceeds the slew rate limit of the op
amp. For a sine wave, the maximum rate of change occurs
at the zero crossing and can be derived as follows:
Mathematically, for the two circuits:
GDIFF = (V1 – V2)/(VN – VP)
GCM = (V1 – V2)/VCM
For the single gain-set resistor circuit, Figure 1A:
GDIFF = (RFB1 + RFB2 + RG)/RG
V = VP • sin(2 • π • f • t)
dV/dt = 2 • π • f • VP • cos(2 • π • f • t)
If RFB1 = RFB2 = RFB, this becomes the familiar
GDIFF = 1 + (2 • RFB/RG)
GCM = 0
At t = 0,
dV/dt = 2 • π • f • VP
Slew rate limit = 2 • π • fMAX • VP
For the two gain-set resistor circuit, Figure 1B:
GDIFF =
VN • (1 + (RFB1/RG1)) – VP • (1 + (RFB2/RG2))
Where:
VN – VP
V = common-mode voltage vs time (t)
VP = peak common-mode voltage
Slew rate limit = maximum dV/dt
fMAX = maximum common-mode frequency at amplitude
VP beyond which standard IA fails to function due
to slew-rate limit of input op amp.
GCM = (RFB1/RG1) – (RFB2/RG2)
(The CMRR of the Figure 1B circuit does depend on buffer
amplifier resistor matching.)
Where:
GDIFF = Differential gain of the IA (V/V)
GCM = Common-mode gain of the IA (V/V)
As with DC CMR, AC CMR can be improved by driving the
power supply connections of the input op amps from common-mode referenced sub-regulated supplies. Since neither
the inputs nor the output of the amplifier change relative to
See Figures 1A and 1B for VS and RS.
Common-mode rejection ratio is the ratio of differential gain
to common-mode gain. Adding gain ahead of the difference
amplifier increases the CMR of the IA so long as the op
amps in the gain stage have better CMR than the difference
(1) According to its designer: Frederiksen, Thomas M., Intuitive IC Op
Amps, National Semiconductor’s Technology Series, 1984, back cover.
2
the power supply rails, nothing within the amplifier moves
in response to the common-mode signal. No current flows in
the phase compensation capacitors and the phase compensation is therefore defeated for common-mode response.
An INA106 gain-of-10 difference amplifier is used for the
difference amplifier. The INA106 contains a precision op
amp and ratio matched resistors R1 through R4 pretrimmed
for 100dB min CMR. No critical resistor matching by the
user is required to build a precision IA using this approach.
THE BOOSTED IA
The common-mode signal driving the subregulated supplies
is derived from resistor divider network, R5, R6. The network
is driven from the IA inputs through unity-gain connected op
amps A4 and A5. These buffer amplifiers persevere the IA’s
high input impedance. In some applications the impedance
of the R5, R6 network connected directly to the IA inputs is
acceptable and buffer amplifiers A4 and A5 can be deleted as
shown in Figure 3. The signal at the R5, R6 connection of the
resistor divider is the average or common-mode voltage of
the two IA inputs.
The complete circuit for the enhanced IA is shown in Figure
2. In addition to the three op amp IA, it contains a buffered
common-mode voltage generator, and ±5V subregulated
power supplies.
VN
A1
OPA177
Gain-of-10
Difference Amplifier
The negative subregulator consists of A6, R7, C1, and a
100µA current source (1/2 of Burr-Brown REF200). Since
no current flows in the op amp input, 100µA flows through
the 50kΩ resistor, R7 forcing a –5V drop from the op amp
input to its output. The op amp forces the negative input to
be at the same potential as its positive input. The result is a
a –5V floating voltage reference relative to the op amp
noninverting input terminal.
INA106
2
RFB1
5kΩ
R1
R2
10kΩ
100kΩ
RG
100kΩ
6
A3
5kΩ
RFB2
3
A2
OPA177
5
R3
R4
10kΩ
100kΩ
VO
1
The positive subregulator is the same as the negative
subregulation except for the polarity of the current source
connection.
+15V
The outputs of the positive and negative subregulators are
connected to the power supplies of the input op amps A1 and
A2 only. All other op amps are connected to ±15V power
supplies.
VP
100µA
1 2 REF200
C1 0.01µF
R7
14
COMMON MODE RANGE OF BOOSTED IA
The common-mode input range of the boosted IA is limited
by the subregulated supply voltage. The outputs of the
subregulator amplifiers, A6 and A7 must swing the commonmode voltage plus the subregulator voltage. The smaller the
subregulator voltage, the better the common-mode input
range. A subregulator voltage of ±5V was chosen because it
is low enough to give good input common-mode range while
it is high enough to allow full performance from almost any
op amp.
50kΩ
OPA404
A4
A6
R5
2kΩ
14
OPA404
VCM –5V
to –VS of
A1 & A2
VCM
R6
2kΩ
A7
A5
14
OPA404
14
OPA404
VCM +5V
to +VS of
A1 & A2
COMMON MODE RANGE OF BOOSTED IA
IS AS GOOD AS STANDARD IA
The common-mode input range of the boosted instrumentation amplifier is as good as that of most integrated circuit
IAs. It might seem that the subregulated supplies would
reduce the IA’s common-mode range. But because the
boosted IA uses a gain-of-10 difference amplifier rather than
a unity gain difference amplifier its common mode range is
not limited by the input amplifiers. The common-mode input
range of both the boosted IA and the standard IA is about
±7V.
R8
50kΩ
C2 0.01µF
100µA
1 2 REF200
–15V
NOTE: Driving the power supplies of the input amplifiers of a three op
amp IA from common-mode driven supplies can dramatically improve
both AC and DC CMR.
That’s right. With a 10V output, the common mode input
range of a standard IA is only about ±7V, not ±10V as many
have been incorrectly led to believe.
FIGURE 2. Boosted Instrumentation Amplifier.
3
VN
A1
OPA177
Gain-of-10
Difference Amplifier
INA106
R5
10kΩ
2
RFB1
5kΩ
VCM
R1
R2
10kΩ
100kΩ
RG
100Ω
5kΩ
RFB2
R6
10kΩ
5
6
A3
3
R3
R4
10kΩ
100kΩ
VO
1
A2
OPA177
VP
+15V
100µA
1 2 REF200
C1 0.01µF
R7
50kΩ
A6
12
OPA2107
A7
NOTE: If high input impedance is not needed,
the common-mode buffers (A4 and A5 from
Figure 2) can be eliminated.
12
OPA2107
VCM –5V
to –VS of
A1 & A 2
VCM +5V
to +VS of
A1 & A 2
R8
50kΩ
C2 0.01µF
100µA
1 2 REF200
–15V
FIGURE 3. Simplified Boosted Instrumentation Amplifier.
The common-mode swing of a standard IA is limited by the
output swing of the input amplifiers. The common mode
range of the boosted IA is limited by the output swing of the
subregulator amplifiers.
is not critical for good CMR. Also, The more gain placed
ahead of the difference amplifier, the better the IA CMR.
To compare the limits on input common-mode range, assume the op amps used can all swing to within 3V of their
power supply rails (i.e. they can swing to ±12V when
operating on ±15V power supplies).
Standard IAs use unity gain difference amplifiers for practical reasons. Since standard IAs are designed for general
applications, they must be adjustable to unity gain. Because
it would be difficult for the user to maintain the resistor ratio
matching necessary for good difference amplifier CMR, a
fixed unity gain difference amplifier is provided. Gain adjustment is made with the input amplifiers, where matching
In a standard IA, using a unity gain difference amplifier, the
input amplifiers must provide a differential 10V output for
a 10V IA output. With the input amplifiers in equal gains,
each must deliver one half of the 10V differential signal.
4
VN
A1
OPA177
R5
10kΩ
RFB1
5kΩ
R2
10kΩ
100kΩ
A3
OPA445
RG
100Ω
5kΩ
RFB2
R6
10kΩ
VO
R3
R4
10kΩ
97.6kΩ
5kΩ
CMR
Adj
A2
OPA177
+40V
VP
100µA 100µA
VCM
R1
REF200
C1 0.01µF
R7
50kΩ
A6
OPA445
A7
OPA445
NOTE: You can make a high-voltage IA with the
boosted IA by using high-voltage op amps for A3, A6,
and A7 while using high performance, signal level op
amps such as the OPA177 for A1 and A2.
VCM –5V
to –VS of
A1 & A2
VCM +5V
to +VS of
A1 & A2
R8
50kΩ
100µA 100µA
C2 0.01µF
REF200
–40V
FIGURE 4. High Voltage Instrumentation Amplifier.
With a common-mode input of 7V one input amplifier must
deliver 7V common-mode plus 5V differential—its 12V
swing limit.
In the boosted IA, using a gain-fo-10 difference amplifier,
the buffer amplifiers must provide a differential output of
only 1V for a 10V IA output. With the input amplifiers in
equal gains, each must deliver one half of the 1V differential
signal. With a common-mode input of 7V, one input amplifier must deliver 7V common-mode plus 0.5V differential
for a total of 7.5V at its output which is no problem since the
VS is 12V (5V subregulated + 7V common-mode).
The boosted IA also has a ±7V common-mode input limit.
The subregulators are set at ±5V from the input commonmode signal. With a 7V common mode input, one of the
subregulator outputs is at its 12V swing limit.
5
–150
–140
CMR (dB)
–130
Boosted IA
–120
–110
–100
–90
–80
Standard IA
–70
10
100
1k
10k
100k
1M
Frequency (Hz)
NOTE: Both the standard and boosted IA use OPA177s for input
amplifiers. The overall gain of the IA is 1000V/V (input section gain = 100,
difference gain = 10). The boosted IA gives more than 100/1 CMR
improvement. Noise limits the DC CMR to slightly greater than 140dB
referred to a +9dBm common-mode input signal.
Input signal is ±2.5V, 2kHz. Output shows approximately 56dB CMR.
FIGURE 5B. Common-Mode Input and Output of Standard
Gain = 1000V/V IA Using LTC1050 Chopper
Stabilized Op Amp.
FIGURE 5. CMR vs Frequency Comparison between Standard Three Op Amp IA and Boosted IA.
SUBREGULATION IMPROVES PRECISION
Lower power dissipation in the input op amps due to reduced
power supplies can improve performance by reducing thermally induced low-frequency noise. In all semiconductor
packages thermocouples are formed at various conductor
interfaces. Matched-seal metal, side-braze ceramic, cerdip,
and many plastic packages use Kovar leads. Significant
thermocouples are formed between the lead plating and the
Kovar. Thermocouples are also formed between the leads
and the solder connections to the printed circuit.
If thermal gradients are properly matched (at the amplifier
inputs) the thermocouple errors will cancel. In practice,
mismatches occur. Even under laboratory conditions, the
error produced can be several tenths of microvolts—well
above the levels achievable with low-noise amplifiers. At
the output of a high-gain amplifier, the error will appear as
low frequency noise or short-term input offset error.
Input signal is ±2.5V, 2kHz. Output shows improvement over standard
IA to approximately 82dB CMR.
FIGURE 5C. Common-Mode Input and Output of Boosted
Gain = 1000V/V IA Using LTC1050 Chopper
Stabilized Op Amp.
In signal op amp packages, much of the heat is conducted
away through the leads. The resultant thermal gradient
between the package and the printed circuit can be a major
source of error. Air currents cool one lead more than another, resulting in mismatched thermal gradients. Operating
the op amp on ±5V supplies (reduced from ±15V supplies)
decreases quiescent power dissipation and associated temperature rise by three-to-one, providing a commensurate
reduction in thermally induced errors.
amp in the boosted circuit. Overall gain of the IA is set at
1000V/V. The OPA177 is an improved version of the
industry standard OP 07. It offers 10µV max VOS and 0.1µV/
°C max VOS/dT. The OPA404 is used for speed and bias
current. The FET inputs of the OPA404 do not add loading
at the input of the IA. The speed is high as compared to the
OPA177 giving a good improvement of CMR vs Frequency.
The CMR plots were made using an HP4194A gain-phase
analyzer with an input signal to the IA of +9dBm. As you
can see, CMR vs Frequency is boosted dramatically. At
2kHz, for example, the CMR of the standard IA is ≈ 80dB
while the CMR of the boosted IA is more than 120dB—
more than a 100-to-1 improvement!
PERFORMANCE OF
BOOSTED IA vs STANDARD IA
A performance comparison between the standard IA and the
boosted IA in Figure 2 is shown in Figure 5. Amplifiers used
for A1 and A2 are OPA177; A3 is an INA106 gain-of-10
difference amplifier; and A4 to A7 are an OPA404 quad op
6
VN
A1
OPA128
Gain-of-10
Difference Amplifier
INA106
R1
R2
10kΩ
100kΩ
2
RFB1
5kΩ
RG
100Ω
5
6
A3
5kΩ
RFB2
R3
R4
10kΩ
100kΩ
3
VO
1
A2
OPA128
VP
+15V
100µA
1 2 REF200
C1 0.01µF
R7
80.6kΩ
R6
10kΩ
R5
10kΩ
A6
12
OPA2107
VCM –8V
to –VS of
A1 & A 2
3.3V
1N4684
A7
3.3V
1N4684
12
OPA2107
VCM +8V
to +VS of
A1 & A 2
R8
80.6kΩ
NOTE: You can derive the common-mode drive signal for
the subregulated supplies from the outputs of the input
amplifiers, but it can create problems such as latch-up,
instability, and reduced performance.
C2 0.01µF
100µA
1 2 REF200
–15V
FIGURE 6. Boosted Electrometer Instrumentation Amplifier.
Another dramatic comparison is shown in the scope photos of
the same IAs using LTC1050 chopper stabilized op amps for
A1 and A2. When VOS/dT is critical, chopper stabilized op
amps may be the best choice—they offer 5µV max VOS over
temperature. As you can see, with a ±2.5V, 2kHz input signal,
CMR is limited to ≈56dB by chopper noise. With the boosted
circuit, CMR is a respectable ≈82dB.
actual performance boost will depend on matching and
parasitics in the devices selected.
The limit for CMR performance in the boosted IA is the
difference amplifier. The more gain added ahead of the difference amplifier, the better the potential for improvement. For
example, with a gain of 100V/V ahead of the difference
amplifier an improvement in CMR of 40dB is possible. The
HIGH-VOLTAGE IA
High voltage IAs can also be easily implemented using the
boosted IA configuration. Standard precision signal level op
amps can be used for the input amplifiers while the HV chores
are taken care of by the other (less critical) op amps. The
Of course, CMR vs Frequency depends on the dynamic
performance of all amplifiers. Improvement in dynamic CMR
will be most dramatic when the speed of the amplifiers used
for A4 to A7 is much higher than the speed of A1 and A2.
7
simple modifications to the Figure 3 circuit are shown in
Figure 4. OPA445 op amps are used for A6 and A7 and for the
difference amplifier, A3. To boost the voltage rating of the
current sources used in the subregulated supplies, two REF200
current source sections are placed in series. If 1% resistors are
used for difference resistors R1-R4, a pot may be required to
adjust CMR as shown. The resulting IA will provide outstanding performance on power supplies up to ±45V.
pins from the op amp output can cancel the op amp phase
compensation and cause the op amp to oscillate. Driving the
input op amps power supplies through the two-to-one R5, R6
divider does not completely cancel the compensation, but it
does reduce amplifier phase margin significantly. Phase shift
through A6 and A7 further reduces phase margin.
You might think that significantly reduced phase margin
would be acceptable if the input amplifiers are used in high
gain. But high gain in the differential input/output amplifier
depends on virtual grounds at both ends of RG. At the unity
gain frequency of the op amp, where instability occurs, loop
gain disappears and the op amps no longer approximate the
ideal. At best, the op amps operate in a noise gain of two—
the op amps must be stable in a gain of two. That’s why you
can get in trouble trying to use decompensated op amps in the
front-end of an IA.
BOOSTED ELECTROMETER IA
Electrometer amps depend on the lowest possible input bias
current. Connecting the input drive circuitry to the IA inputs
as shown in Figure 2 may result in too much bias current. In
this case you may want to take the common-mode drive
signal from the outputs of the input stage as shown in Figure
6.
Taking the common-mode drive from the outputs of the input
stage may seem like a good idea, but it creates problems—
latch-up, instability, and reduced performance.
Using the faster OPA2107 for A6 and A7 along with the
OPA128 electrometer op amp for A1 and A2 results in good
stability. The improvement of CMR is shown in the CMR vs
Frequency plot, Figure 7. The plot compares a standard IA
(using the OPA128 and an INA106) to the Figure 2 and Figure
6 boosted IA circuits. The boosted circuits give a 30dB (better
than 30/1) improvement in CMR up to about 1kHz. Beyond
1kHz, the CMR vs Frequency of the Figure 6 circuit begins to
fall-off. At 10kHz, the Figure 6 circuit only offers about a sixto-one improvement.
Driving the supplies of the input amplifiers from their outputs
can cause latch-up. Many amplifiers exhibit input phase anomalies when their inputs are overloaded or overdirven relative to
their power supplies. Unless precautions are taken when
deriving the power supplies from the amplifier outputs, these
anomalies will result in latch-up conditions. The back-to-back
3.3V zener diodes connected to the common-mode node
(where R5 and R6 connect) prevents latch-up by keeping the
common-mode drive point within 4V of ground. Also, the
common-mode driven supplies are increased from 5V to 8V.
In combination, this keeps 4V minimum on the power supplies of the input amps eliminating the latch-up condition
when using the op amps shown.
LAYOUT
To get the best performance from the boosted IA, use a good
printed circuit layout. For best CMR, keep the signal-path
circuitry symmetrical. A printed circuit layout of the complete boosted IA circuit, Figure 2, is shown in Figure 8. It
produced the excellent results shown in this bulletin. Notice
that good signal-path symmetry is achieved even though a
single-sided layout is used.
The disadvantage of the clamp circuitry is reduced commonmode input range. The input common-mode range is limited to
the clamp voltage of approximately 4V.
Driving the supplies of the input amplifiers from their outputs
can also cause insatiably. Driving an op amp’s power supply
–160
–150
–140
Boosted IA
CMR (dB)
–130
–120
Electrometer IA
–110
–100
–90
Standard IA
–80
–70
–60
10
100
1k
10k
Frequency (Hz)
NOTE: The boosted electrometer IA circuit adds CMR boost, but the
boost starts to fall off beyond about 1kHz.
Best performance depends on signal-path symmetry. Notice that excellent symmetry can be maintained even with single-sided board layout.
Extra pads at RFB allow stacking of feedback capacitors.
FIGURE 7. CMR vs Frequency Comparison between Standard Three Op Amp IA, Boosted IA, and
Boosted Electrometer IA.
FIGURE 8. Printed Circuit Layout of Figure 2 Boosted IA.
8