Burr-Brown INA2128 Dual, low power instrumentation amplifier Datasheet

INA
®
INA2128
212
INA
8
212
8
Dual, Low Power
INSTRUMENTATION AMPLIFIER
FEATURES
DESCRIPTION
● LOW OFFSET VOLTAGE: 50µV max
The INA2128 is a dual, low power, general purpose
instrumentation amplifier offering excellent accuracy.
Its versatile 3-op amp design and small size make it
ideal for a wide range of applications. Current-feedback
input circuitry provides wide bandwidth even at high
gain (200kHz at G = 100).
● LOW DRIFT: 0.5µV/°C max
● LOW INPUT BIAS CURRENT: 5nA max
● HIGH CMR: 120dB min
● INPUTS PROTECTED TO ±40V
● WIDE SUPPLY RANGE: ±2.25V to ±18V
A single external resistor sets any gain from 1 to 10,000.
Internal input protection can withstand up to ±40V
without damage.
● LOW QUIESCENT CURRENT: 700µA / IA
● 16-PIN PLASTIC DIP, SOL-16
The INA2128 is laser trimmed for very low offset
voltage (50µV), drift (0.5µV/°C) and high commonmode rejection (120dB at G ≥ 100). It operates with
power supplies as low as ±2.25V, and quiescent current
is only 700µA per IA—ideal for battery operated and
multiple-channel systems.
APPLICATIONS
● SENSOR AMPLIFIER
THERMOCOUPLE, RTD, BRIDGE
● MEDICAL INSTRUMENTATION
● MULTIPLE-CHANNEL SYSTEMS
The INA2128 is available in 16-pin plastic DIP, and
SOL-16 surface-mount packages, specified for the
–40°C to +85°C temperature range.
● BATTERY OPERATED EQUIPMENT
V+
9
–
VINA
1
Over-Voltage
Protection
INA2128
7
A1A
40kΩ
3
40kΩ
25kΩ
A3A
RGA
4
2
Over-Voltage
Protection
–
16
Over-Voltage
Protection
VINB
14
5
A2A
40kΩ
40kΩ
40kΩ
40kΩ
13
15
GB = 1 +
25kΩ
A3B
+
RefA
10
A1B
RGB
VINB
VOA
25kΩ
+
VINA
6
GA = 1 + 50kΩ
RGA
11
50kΩ
RGB
VOB
25kΩ
Over-Voltage
Protection
12
A2B
40kΩ
RefB
40kΩ
8
V–
International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 • Twx: 910-952-1111
Internet: http://www.burr-brown.com/ • FAXLine: (800) 548-6133 (US/Canada Only) • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
©
1994 Burr-Brown Corporation
PDS-1243C
Printed in U.S.A. January, 1996
SPECIFICATIONS
At TA = +25°C, VS = ±15V, RL = 10kΩ, unless otherwise noted.
INA2128P, U
PARAMETER
CONDITIONS
INPUT
Offset Voltage, RTI
Initial
TA = +25°C
vs Temperature
TA = TMIN to TMAX
vs Power Supply
VS = ±2.25V to ±18V
Long-Term Stability
Impedance, Differential
Common-Mode
VO = 0V
Common-Mode Voltage Range(1)
Safe Input Voltage
Common-Mode Rejection
VCM = ±13V, ∆RS = 1kΩ
G=1
G=10
G=100
G=1000
TYP
MAX
±50 ±500/G
±0.5 ± 20/G
±1 ±100/G
(V+) – 2
(V–) + 2
±10 ±100/G
±0.2 ± 2/G
±0.2 ±20/G
±0.1 ±3/G
1010 || 2
1011 || 9
(V+) – 1.4
(V–) + 1.7
80
100
120
120
86
106
125
130
±2
±30
±1
±30
BIAS CURRENT
vs Temperature
Offset Current
vs Temperature
NOISE VOLTAGE, RTI
f = 10Hz
f = 100Hz
f = 1kHz
fB = 0.1Hz to 10Hz
Noise Current
f=10Hz
f=1kHz
fB = 0.1Hz to 10Hz
INA2128PA, UA
MIN
✻
✻
±40
73
93
110
110
±5
±5
G=1
G=10
G=100
G=1000
G=1
VO = ±13.6V, G=1
G=10
G=100
G=1000
OUTPUT
Voltage: Positive
Negative
Load Capacitance Stability
Short-Circuit Current
RL = 10kΩ
RL = 10kΩ
(V+) – 1.4
(V–) + 1.4
MAX
±25 ±100/G ±125 ±1000/G
±0.2 ± 5/G
±1 ± 20/G
✻
±2 ±200/G
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
UNITS
µV
µV/°C
µV/V
µV/mo
Ω || pF
Ω || pF
V
V
V
dB
dB
dB
dB
±10
±10
nA
pA/°C
nA
pA/°C
10
8
8
0.2
✻
✻
✻
✻
nV/√Hz
nV/√Hz
nV/√Hz
µVp-p
0.9
0.3
30
✻
✻
✻
pA/√Hz
pA/√Hz
pAp-p
✻
1 + (50kΩ/RG)
1
TYP
✻
✻
✻
✻
G = 1000, RS = 0Ω
GAIN
Gain Equation
Range of Gain
Gain Error
Gain vs Temperature(2)
50kΩ Resistance(2, 3)
Nonlinearity
MIN
±0.01
±0.02
±0.05
±0.5
±1
±25
±0.0001
±0.0003
±0.0005
±0.001
10000
±0.024
±0.4
±0.5
±1
±10
±100
±0.001
±0.002
±0.002
(Note 4)
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
(V+) – 0.9
(V–) + 0.8
1000
+6/–15
✻
±0.1
±0.5
±0.7
±2
✻
✻
±0.002
±0.004
±0.004
✻
V/V
V/V
%
%
%
%
ppm/°C
ppm/°C
% of FSR
% of FSR
% of FSR
% of FSR
✻
✻
✻
✻
V
V
pF
mA
✻
✻
✻
✻
✻
✻
✻
✻
✻
✻
MHz
kHz
kHz
kHz
V/µs
µs
µs
µs
µs
µs
FREQUENCY RESPONSE
Bandwidth, –3dB
Overload Recovery
G=1
G=10
G=100
G=1000
VO = ±10V, G=10
G=1
G=10
G=100
G=1000
50% Overdrive
POWER SUPPLY
Voltage Range
Current, Total
VIN = 0V
Slew Rate
Settling Time, 0.01%
1.3
700
200
20
4
7
7
9
80
4
±2.25
TEMPERATURE RANGE
Specification
Operating
θJA
±15
±1.4
–40
–40
80
±18
±1.5
✻
85
125
✻
✻
✻
✻
✻
✻
✻
V
mA
✻
✻
°C
°C
°C/W
✻ Specification same as INA2128P, U.
NOTE: (1) Input common-mode range varies with output voltage—see typical curves. (2) Guaranteed by wafer test. (3) Temperature coefficient of the “50kΩ” term in
the gain equation. (4) Nonlinearity measurements in G = 1000 are dominated by noise. Typical nonlinearity is ±0.001%.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
®
INA2128
2
ELECTROSTATIC
DISCHARGE SENSITIVITY
PIN CONFIGURATION
Top View
DIP
SOL-16
–
VINA
1
–
16 VINB
+
VINA
2
+
15 VINB
RGA
3
14 RGB
RGA
4
13 RGB
RefA
5
12 RefB
VOA
6
11 VOB
SenseA
7
10 SenseB
V–
8
9
This integrated circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degradation
to complete device failure. Precision integrated circuits may
be more susceptible to damage because very small parametric
changes could cause the device not to meet its published
specifications.
V+
ORDERING INFORMATION
ABSOLUTE MAXIMUM RATINGS
Supply Voltage .................................................................................. ±18V
Analog Input Voltage Range ............................................................. ±40V
Output Short-Circuit (to ground) .............................................. Continuous
Operating Temperature ................................................. –40°C to +125°C
Storage Temperature ..................................................... –40°C to +125°C
Junction Temperature .................................................................... +150°C
Lead Temperature (soldering, 10s) ............................................... +300°C
PRODUCT
PACKAGE
PACKAGE
DRAWING
NUMBER(1)
INA2128PA
INA2128P
INA2128UA
INA2128U
16-Pin Plastic DIP
16-Pin Plastic DIP
SOL-16 Surface-Mount
SOL-16 Surface-Mount
180
180
211
211
TEMPERATURE
RANGE
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix C of Burr-Brown IC Data Book.
®
3
INA2128
TYPICAL PERFORMANCE CURVES
At TA = +25°C, VS = ±15V, unless otherwise noted.
COMMON-MODE REJECTION vs FREQUENCY
GAIN vs FREQUENCY
140
60
G = 1000V/V
G = 100V/V
G = 1000V/V
Common-Mode Rejection (dB)
50
40
Gain (dB)
G = 100V/V
30
20
G = 10V/V
10
0
G = 1V/V
–10
1k
10k
100k
1M
G = 1V/V
80
60
40
20
10
10M
100
1k
100k
10k
Frequency (Hz)
Frequency (Hz)
POSITIVE POWER SUPPLY REJECTION
vs FREQUENCY
NEGATIVE POWER SUPPLY REJECTION
vs FREQUENCY
140
120
120
Power Supply Rejection (dB)
140
G = 1000V/V
100
G = 100V/V
80
60
G = 10V/V
40
G = 1V/V
1M
G = 1000V/V
G = 100V/V
100
80
60
40
G = 10V/V
20
20
0
0
G = 1V/V
10
15
100
1k
10k
100k
10
1M
100k
INPUT COMMON-MODE RANGE
vs OUTPUT VOLTAGE, VS = ±5, ±2.5V
G=1
G=1
VD/2
VD/2
+
+15V
–
VO
+
–
Ref
+
VCM
–15V
–10
3
2
G=1
0
5
10
1
0
G=1
–1
–2
–3
–5
–5
15
Output Voltage (V)
VS = ±5V
VS = ±2.5V
–4
–3
–2
–1
0
1
Output Voltage (V)
®
INA2128
G=1
G ≥ 10
–4
–5
G ≥ 10
G ≥ 10
4
5
0
1M
5
G ≥ 10
–10
10k
INPUT COMMON-MODE RANGE
vs OUTPUT VOLTAGE, VS = ±15V
10
–15
–15
1k
Frequency (Hz)
G ≥ 10
–5
100
Frequency (Hz)
Common-Mode Voltage (V)
Power Supply Rejection (dB)
G = 10V/V
100
0
–20
Common-Mode Voltage (V)
120
4
2
3
4
5
TYPICAL PERFORMANCE CURVES
(CONT)
At TA = +25°C, VS = ±15V, unless otherwise noted.
CROSSTALK vs FREQUENCY
INPUT- REFERRED NOISE vs FREQUENCY
120
G = 10V/V
Crosstalk (dB)
100
G = 1V/V
G = 1000V/V
80
G = 100V/V
60
40
20
0
G = 1V/V
10
100
G = 10V/V
1
10
G = 100, 1000V/V
Current Noise
0.1
1
10
100
1k
100k
10k
1M
1
10
QUIESCENT CURRENT and SLEW RATE
vs TEMPERATURE
Quiescent Current (µA)
0.1%
1.7
6
1.6
5
1.5
1
1.4
3
IQ
1.3
2
10
100
–75
1000
–50
–25
Gain (V/V)
INPUT OVER-VOLTAGE V/I CHARACTERISTICS
8
Input Current (mA)
Flat region represents
normal linear operation.
Offset Voltage Change (µV)
4
G = 1000V/V
1
G = 1V/V
0
+15V
G = 1V/V
1/2
INA2128
–2
–3
VIN
25
50
75
100
IIN
6
4
2
0
–2
–4
–6
–8
–15V
–5
–10
–50 –40 –30
–20 –10
0
10
1
125
OFFSET VOLTAGE WARM-UP
10
G = 1000V/V
0
Temperature (°C)
5
–4
4
Slew Rate
1.2
1
–1
10k
SETTLING TIME vs GAIN
10
2
1k
Frequency (Hz)
0.01%
3
100
Frequency (Hz)
100
Settling Time (µs)
100
Input Bias Current Noise (pA/√ Hz)
G = 100V/V
1k
Slew Rate (V/µs)
G = 1000V/V
Input-Referred Voltage Noise (nV/√ Hz)
140
20
30
40
50
0
Input Voltage (V)
10
20
30
40
50
Time (ms)
®
5
INA2128
TYPICAL PERFORMANCE CURVES
(CONT)
At TA = +25°C, VS = ±15V, unless otherwise noted.
OUTPUT VOLTAGE SWING
vs OUTPUT CURRENT
INPUT BIAS CURRENT vs TEMPERATURE
(V+)
2
1
Output Voltage (V)
Input Bias Current (nA)
(V+)–0.4
IB
IOS
0
Typical IB and IOS
Range ±2nA at 25°C
–1
(V+)–0.8
(V+)–1.2
(V+)+1.2
(V–)+0.8
(V–)+0.4
–2
V–
–50
–25
0
25
50
75
100
0
125
4
SHORT-CIRCUIT OUTPUT CURRENT
vs TEMPERATURE
Short Circuit Current (mA)
14
+25°C +85°C
(V+)–0.8
–40°C
RL = 10kΩ
(V–)+1.2
+25°C
–40°C
(V–)+0.4
3
OUTPUT VOLTAGE SWING
vs POWER SUPPLY VOLTAGE
(V+)–0.4
(V–)+0.8
2
Output Current (mA)
16
(V+)–1.2
1
Temperature (°C)
V+
+85°C
–40°C
+85°C
–ISC
12
10
8
6
4
+ISC
2
V–
0
0
5
10
15
20
–75
–25
0
25
50
75
100
Temperature (°C)
MAXIMUM OUTPUT VOLTAGE vs FREQUENCY
TOTAL HARMONIC DISTORTION + NOISE
vs FREQUENCY
125
1
G = 10, 100
25
–50
Power Supply Voltage (V)
30
VO = 1Vrms
500kHz Measurement
Bandwidth
G=1
G=1
RL = 10kΩ
G = 1000
20
THD+N (%)
Peak-to-Peak Output Voltage (Vpp)
Output Voltage Swing (V)
–75
15
10
0.1
G = 100, RL = 100kΩ
0.01
G = 1, RL = 100kΩ
5
Dashed Portion
is noise limited.
0
1k
10k
100k
0.001
100
1M
Frequency (Hz)
10k
Frequency (Hz)
®
INA2128
1k
6
G = 10V/V
RL = 100kΩ
100k
TYPICAL PERFORMANCE CURVES
(CONT)
At TA = +25°C, VS = ±15V, unless otherwise noted.
SMALL-SIGNAL STEP RESPONSE
(G = 1, 10)
SMALL-SIGNAL STEP RESPONSE
(G = 100, 1000)
G=1
G = 100
20mV/div
20mV/div
G = 10
G = 1000
5µs/div
20µs/div
LARGE-SIGNAL STEP RESPONSE
(G = 1, 10)
LARGE-SIGNAL STEP RESPONSE
(G = 100, 1000)
G=1
G = 100
5V/div
5V/div
G = 10
G = 1000
5µs/div
5µs/div
VOLTAGE NOISE 0.1 to 10Hz
INPUT-REFERRED, G ≥ 100
0.1µV/div
1s/div
®
7
INA2128
APPLICATION INFORMATION
Figure 1 shows the basic connections required for operation
of the INA2128. Applications with noisy or high impedance
power supplies may require decoupling capacitors close to
the device pins as shown.
internal feedback resistors of A1 and A2. These on-chip
metal film resistors are laser trimmed to accurate absolute
values. The accuracy and temperature coefficient of these
resistors are included in the gain accuracy and drift specifications of the INA2128.
The output is referred to the output reference (Ref) terminals
(RefA and RefB) which are normally grounded. These must
be low-impedance connections to assure good commonmode rejection. A resistance of 8Ω in series with a Ref pin
will cause a typical device to degrade to approximately
80dB CMR (G = 1).
The stability and temperature drift of the external gain
setting resistor, RG, also affects gain. RG’s contribution to
gain accuracy and drift can be directly inferred from the gain
equation (1). Low resistor values required for high gain can
make wiring resistance important. Sockets add to the wiring
resistance which will contribute additional gain error in
gains of approximately 100 or greater.
The INA2128 has a separate output sense feedback connections SenseA and SenseB. These must be connected to their
respective output terminals for proper operation. The output
sense connection can be used to sense the output voltage
directly at the load for best accuracy.
DYNAMIC PERFORMANCE
The typical performance curve “Gain vs Frequency” shows
that despite its low quiescent current, the INA2128 achieves
wide bandwidth, even at high gain. This is due to its currentfeedback topology. Settling time also remains excellent at
high gain—see “Settling Time vs Gain.”
SETTING THE GAIN
Gain of the INA2128 is set by connecting a single external
resistor, RG, connected as shown:
G = 1+
50kΩ
RG
NOISE PERFORMANCE
(1)
The INA2128 provides very low noise in most applications.
Low frequency noise is approximately 0.2µVp-p measured
from 0.1 to 10Hz (G ≥ 100). This provides dramatically
improved noise when compared to state-of-the-art chopperstabilized amplifiers.
Commonly used gains and resistor values are shown in
Figure 1.
The 50kΩ term in equation 1 comes from the sum of the two
V+
0.1µF
Pin numbers for
Channel B shown
in parenthesis.
–
VIN
1
(16)
9
INA2128
Over-Voltage
Protection
40kΩ
3
RG
(Ω)
1
2
5
10
20
50
100
200
500
1000
2000
5000
10000
NC
50.00k
12.50k
5.556k
2.632k
1.02k
505.1
251.3
100.2
50.05
25.01
10.00
5.001
NC
49.9k
12.4k
5.62k
2.61k
1.02k
511
249
100
49.9
24.9
10
4.99
6
(11)
A3
+
4
25kΩ
Load VO
(13)
+
VIN
2
(15)
–
Ref
A2
Over-Voltage
Protection
40kΩ
8
40kΩ
5
(12)
0.1µF
V–
Also drawn in simplified form:
–
VIN
NC: No Connection.
RG
INA2128
Ref
+
VIN
FIGURE 1. Basic Connections.
®
INA2128
G=1+
RG
NEAREST 1% RG
(Ω)
40kΩ
25kΩ
(14)
DESIRED
GAIN
7 Sense
+
–
(10)
)
VO = G • (VIN – VIN
A1
8
VO
NOTE: If channel is unused,
connect inputs to ground, sense
to VO, and leave Ref open-circuit.
50kΩ
RG
OFFSET TRIMMING
The INA2128 is laser trimmed for low offset voltage and
offset voltage drift. Most applications require no external
offset adjustment. Figure 2 shows an optional circuit for
trimming the output offset voltage. The voltage applied to
Ref terminal is summed with the output. The op amp buffer
provides low impedance at the Ref terminal to preserve good
common-mode rejection.
Microphone,
Hydrophone
etc.
1/2
INA2128
47kΩ
47kΩ
1/2
INA2128
Thermocouple
–
VIN
RG
+
VIN
V+
1/2
INA2128
10kΩ
VO
100µA
1/2 REF200
Ref
OPA177
10kΩ
100Ω
(For other
channel)
1/2
INA2128
±10mV
Adjustment Range
100Ω
Center-tap provides
bias current return.
100µA
1/2 REF200
FIGURE 3. Providing an Input Common-Mode Current Path.
V–
INPUT BIAS CURRENT RETURN PATH
voltage swing of amplifiers A1 and A2. So the linear common-mode input range is related to the output voltage of the
complete amplifier. This behavior also depends on supply
voltage—see performance curves “Input Common-Mode
Range vs Output Voltage.”
The input impedance of the INA2128 is extremely high—
approximately 1010Ω. However, a path must be provided for
the input bias current of both inputs. This input bias current
is approximately ±2nA. High input impedance means that
this input bias current changes very little with varying input
voltage.
Input-overload can produce an output voltage that appears
normal. For example, if an input overload condition drives
both input amplifiers to their positive output swing limit, the
difference voltage measured by the output amplifier will be
near zero. The output of the INA2128 will be near 0V even
though both inputs are overloaded.
FIGURE 2. Optional Trimming of Output Offset Voltage.
Input circuitry must provide a path for this input bias current
for proper operation. Figure 3 shows various provisions for
an input bias current path. Without a bias current path, the
inputs will float to a potential which exceeds the commonmode range of the INA2128 and the input amplifiers will
saturate.
LOW VOLTAGE OPERATION
The INA2128 can be operated on power supplies as low as
±2.25V. Performance remains excellent with power supplies ranging from ±2.25V to ±18V. Most parameters vary
only slightly throughout this supply voltage range—see
typical performance curves. Operation at very low supply
voltage requires careful attention to assure that the input
voltages remain within their linear range. Voltage swing
requirements of internal nodes limit the input commonmode range with low power supply voltage. Typical performance curves, “Input Common-Mode Range vs Output
Voltage” show the range of linear operation for ±15V, ±5V,
and ±2.5V supplies.
If the differential source resistance is low, the bias current
return path can be connected to one input (see the thermocouple example in Figure 3). With higher source impedance,
using two equal resistors provides a balanced input with
possible advantages of lower input offset voltage due to bias
current and better high-frequency common-mode rejection.
INPUT COMMON-MODE RANGE
The linear input voltage range of the input circuitry of the
INA2128 is from approximately 1.4V below the positive
supply voltage to 1.7V above the negative supply. As a
differential input voltage causes the output voltage increase,
however, the linear input range will be limited by the output
®
9
INA2128
INPUT PROTECTION
The inputs of the INA2128 are individually protected for
voltages up to ±40V. For example, a condition of –40V on
one input and +40V on the other input will not cause
damage. Internal circuitry on each input provides low series
impedance under normal signal conditions. To provide
equivalent protection, series input resistors would contribute
excessive noise. If the input is overloaded, the protection
circuitry limits the input current to a safe value of approximately 1.5 to 5mA. The typical performance curve “Input
Bias Current vs Common-Mode Input Voltage” shows this
input current limit behavior. The inputs are protected even if
the power supplies are disconnected or turned off.
CHANNEL CROSSTALK
The two channels of the INA2128 are completely independent, including all bias circuitry. At DC and low frequency
there is virtually no signal coupling between channels.
Crosstalk increases with frequency and is dependent on
circuit gain, source impedance and signal characteristics.
As source impedance increases, careful circuit layout will
help achieve lowest channel crosstalk. Most crossstalk is
produced by capacitive coupling of signals from one channel
to the input section of the other channel. To minimize
coupling, separate the input traces as far as practical from
any signals associated with the opposite channel. A grounded
guard trace surrounding the inputs helps reduce stray coupling between channels. Run the differential inputs of each
channel parallel to each other or directly adjacent on top and
bottom side of a circuit board. Stray coupling then tends to
produce a common-mode signal which is rejected by the
IA’s input.
VEX
X-axis
X-axis
VO
1/2
INA2128
V1
VO = GA (V2 – V1) + GB (V4 – V3)
1/2
INA2128
RGA
VEX
Ref
V2
V3
Y-axis
Y-axis
VO
1/2
INA2128
1/2
INA2128
RGB
Ref
V4
FIGURE 5. Sum of Differences Amplifier.
FIGURE 4. Two-Axis Bridge Amplifier.
RG = 5.6kΩ
2.8kΩ
LA
RA
RG/2
1/2
INA2128
VO
Ref
2.8kΩ
390kΩ
1/2
OPA2604
RL
VG
1/2
OPA2604
10kΩ
390kΩ
FIGURE 6. ECG Amplifier With Right-Leg Drive.
®
INA2128
10
G = 10
VG
NOTE: Due to the INA2128’s current-feedback
topology, VG is approximately 0.7V less than
the common-mode input voltage. This DC offset
in this guard potential is satisfactory for many
guarding applications.