®
INA
INA121
121
INA
121
FET-Input, Low Power
INSTRUMENTATION AMPLIFIER
FEATURES
DESCRIPTION
● LOW BIAS CURRENT: ±4pA
● LOW QUIESCENT CURRENT: ±450µA
The INA121 is a FET-input, low power instrumentation amplifier offering excellent accuracy. Its versatile
three-op amp design and very small size make it ideal
for a variety of general purpose applications. Low bias
current (±4pA) allows use with high impedance
sources.
● LOW INPUT OFFSET VOLTAGE: ±200µV
● LOW INPUT OFFSET DRIFT: ±2µV/°C
● LOW INPUT NOISE:
20nV/√Hz at f = 1kHz (G =100)
● HIGH CMR: 106dB
● WIDE SUPPLY RANGE: ±2.25V to ±18V
● LOW NONLINEARITY ERROR: 0.001% max
● INPUT PROTECTION TO ±40V
● 8-PIN DIP AND SO-8 SURFACE MOUNT
APPLICATIONS
Gain can be set from 1V to 10,000V/V with a single
external resistor. Internal input protection can withstand up to ±40V without damage.
The INA121 is laser-trimmed for very low offset
voltage (±200µV), low offset drift (±2µV/°C), and
high common-mode rejection (106dB at G = 100). It
operates on power supplies as low as ±2.25V (+4.5V),
allowing use in battery operated and single 5V systems. Quiescent current is only 450µA.
Package options include 8-pin plastic DIP and SO-8
surface mount. All are specified for the –40°C to
+85°C industrial temperature range.
● LOW-LEVEL TRANSDUCER AMPLIFIERS
Bridge, RTD, Thermocouple
● PHYSIOLOGICAL AMPLIFIERS
ECG, EEG, EMG, Respiratory
● HIGH IMPEDANCE TRANSDUCERS
● CAPACITIVE SENSORS
● MULTI-CHANNEL DATA ACQUISITION
● PORTABLE, BATTERY OPERATED SYSTEMS
● GENERAL PURPOSE INSTRUMENTATION
V+
7
INA121
2
–
VIN
Over-Voltage
Protection
A1
40kΩ
1
G=1+
40kΩ
50kΩ
RG
25kΩ
A3
RG
8
+
VIN
3
6
VO
25kΩ
Over-Voltage
Protection
5
A2
40kΩ
Ref
40kΩ
4
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
®
©1997 Burr-Brown Corporation
PDS-1412A
1
INA121
Printed in U.S.A. May, 1998
SPECIFICATIONS: VS = ±15V
At TA = +25°C, VS = ±15V, RL = 10kΩ, and IA reference = 0V, unless otherwise noted.
INA121P, U
PARAMETER
CONDITIONS
INPUT
Offset Voltage, RTI
vs Temperature
vs Power Supply
Long-Term Stability
Impedance, Differential
Common-Mode
Input Voltage Range
Safe Input Voltage
Common-Mode Rejection
VS = ±2.25V to ±18V
VO = 0V
VCM = –12.5V to 13.5V
G=1
G = 10
G = 100
G = 1000
BIAS CURRENT
vs Temperature
Offset Current
vs Temperature
VCM = 0V
NOISE, RTI
Voltage Noise: f = 10Hz
f = 100Hz
f = 1kHz
f = 0.1Hz to 10Hz
Current Noise: f = 1kHz
RS = 0Ω
G = 100
G = 100
G = 100
G = 100
GAIN
Gain Equation
Range of Gain
Gain Error
MIN
TYP
INA121PA, UA
MAX
MIN
86
100
106
106
±4
See Typical Curve
±0.5
See Typical Curve
72
85
90
±50
Nonlinearity
OUTPUT
Voltage: Positive
Negative
Positive
Negative
Capacitance Load Drive
Short-Circuit Current
FREQUENCY RESPONSE
Bandwidth, –3dB
±0.01
±0.03
±0.05
±0.5
±1
±25
±0.05
±0.4
±0.5
VO = –14V to 13.5V
G=1
G = 10
G = 100
G = 1000
±0.0002
±0.0015
±0.0015
±0.002
±0.001
±0.005
±0.005
Overload Recovery
G=1
G = 10
G = 100
G = 1000
VO = ±10V, G ≤ 10
G = 1 to 10
G = 100
G = 1000
50% Input Overload
POWER SUPPLY
Voltage Range
Quiescent Current
IO = 0V
Slew Rate
Settling Time, 0.01%
(V+)–1.5
(V–)+1
TEMPERATURE RANGE
Specification
Operating
Storage
Thermal Resistance, θJA
8-Lead DIP
SO-8 Surface Mount
✻
±10
±100
(V+)–0.9
(V–)+0.15
(V+)–0.9
(V–)+0.25
1000
±14
✻
✻
600
300
50
5
0.7
20
35
260
5
±2.25
±15
±450
–40
–55
–55
100
150
2
pA
pA
nV/√Hz
nV/√Hz
nV/√Hz
µVp-p
fA/√Hz
±18
±525
✻
85
125
125
✻
✻
✻
✻
✻
✻
✻
✻
✻
±0.1
±0.5
±0.7
✻
✻
✻
✻
±0.002
±0.008
±0.008
V/V
V/V
✻
✻
%
%
%
%
ppm/°C
ppm/°C
%
%
%
%
of
of
of
of
FSR
FSR
FSR
FSR
✻
✻
✻
✻
✻
✻
V
V
V
V
pF
mA
✻
✻
✻
✻
✻
✻
✻
✻
✻
kHz
kHz
kHz
kHz
V/µs
µs
µs
µs
µs
✻
✻
✻
✻
NOTE: (1) Temperature coefficient of the “Internal Resistor” in the gain equation. Does not include TCR of gain-setting resistor, RG.
INA121
✻
✻
✻ Specification same as INA121P, U.
®
V
✻
10,000
VO = –14V to 13.5V
G=1
G = 10
G = 100
G = 1000
G=1
G>1
RL = 100kΩ
RL = 100kΩ
RL = 10kΩ
RL = 10kΩ
µV
µV/°C
µV/V
µV/mo
Ω || pF
Ω || pF
dB
dB
dB
dB
✻
✻
✻
✻
✻
1 + (50kΩ/RG)
Gain vs Temperature(1)
UNITS
✻
✻
✻
✻
✻
✻
✻
✻
30
21
20
1
1
1
MAX
±300±200/G ±1000±1000/G
✻
±15±20/G
✻
✻
✻
✻
✻
✻
✻
±200±200/G ±500±500/G
±2±2/G
±5±20/G
±5±20/G
±50±150/G
±0.5
1012 || 1
1012 || 12
See Text and Typical Curves
±40
78
91
96
TYP
✻
✻
V
µA
✻
✻
✻
°C
°C
°C
°C/W
°C/W
ELECTROSTATIC
DISCHARGE SENSITIVITY
PIN CONFIGURATION
Top View
8-Pin DIP and SO-8
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.
Top View
RG
1
8
RG
V–IN
2
7
V+
+
IN
3
6
VO
V–
4
5
Ref
V
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.
ABSOLUTE MAXIMUM RATINGS(1)
Supply Voltage .................................................................................. ±18V
Analog Input Voltage Range ............................................................. ±40V
Output Short-Circuit (to ground) .............................................. Continuous
Operating Temperature ................................................. –55°C to +125°C
Storage Temperature ..................................................... –55°C to +125°C
Junction Temperature .................................................................... +150°C
Lead Temperature (soldering, 10s) ............................................... +300°C
NOTE: (1) Stresses above these ratings may cause permanent damage.
Exposure to absolute maximum conditions for extended periods may degrade
device reliability.
PACKAGE/ORDERING INFORMATION
PRODUCT
PACKAGE
PACKAGE
DRAWING
NUMBER(1)
Single
INA121P
INA121PA
INA121U
"
INA121UA
"
8-Pin DIP
8-Pin DIP
SO-8 Surface-Mount
"
SO-8 Surface-Mount
"
006
006
182
"
182
"
SPECIFIED
TEMPERATURE
RANGE
PACKAGE
MARKING
ORDERING
NUMBER(2)
TRANSPORT
MEDIA
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
"
–40°C to +85°C
"
INA121P
INA121PA
INA121U
"
INA121UA
"
INA121P
INA121PA
INA121U
INA121U/2K5
INA121UA
INA121UA/2K5
Rails
Rails
Rails
Tape and Reel
Rails
Tape and Reel
NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Models with a slash (/) are
available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “INA121U/2K5” will get a single
2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to Appendix B of Burr-Brown IC Data Book.
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.
®
3
INA121
TYPICAL PERFORMANCE CURVES
At TA = +25°C, VS = ±15V, unless otherwise noted.
COMMON-MODE REJECTION
vs FREQUENCY
GAIN vs FREQUENCY
120
60
Gain (dB)
40
Common-Mode Rejection (dB)
50
G = 1000V/V
G = 100V/V
30
20
G = 10V/V
10
0
G = 1V/V
100
G = 1000V/V
80
G = 100V/V
60
40
G = 10V/V
20
G = 1V/V
–10
0
–20
1k
10k
100k
1M
10
10M
10k
100k
POSITIVE POWER SUPPLY REJECTION
vs FREQUENCY
NEGATIVE POWER SUPPLY REJECTION
vs FREQUENCY
1M
120
Power Supply Rejection (dB)
G = 1000V/V
100
G = 1000V/V
80
G = 100V/V
60
G = 10V/V
40
20
G = 1V/V
0
G = 100V/V
100
G = 10V/V
80
G = 1V/V
60
40
20
0
10
100
1k
10k
100k
1M
10
100
1k
10k
100k
Frequency (Hz)
Frequency (Hz)
INPUT COMMON-MODE RANGE
vs OUTPUT VOLTAGE, VS = ±15V
INPUT COMMON-MODE RANGE
vs OUTPUT VOLTAGE, VS = ±5V, ±2.5V
1M
5
15
G ≥ 10
4
10
5
VO
–
+
VD/2
0
+15V
+
VD/2
Ref
–
+
VCM
Common-Mode Voltage (V)
Common-Mode Voltage (V)
1k
Frequency (Hz)
120
Power Supply Rejection (dB)
100
Frequency (Hz)
–15V
–5
–10
G=1
G ≥ 10
–15
–15
–10
–5
0
5
10
G=1
2
G ≥ 10
1
G=1
0
–1
–2
–3
VS = ±5V
VS = ±2.5V
–4
–5
–5
15
–4
–3
–2
–1
0
1
Output Voltage (V)
Output Voltage (V)
®
INA121
3
4
2
3
4
5
TYPICAL PERFORMANCE CURVES
(CONT)
At TA = +25°C, VS = ±15V, unless otherwise noted.
INPUT BIAS CURRENT
vs COMMON-MODE INPUT VOLTAGE
10k
1m
1k
100µ
Input Bias Current (A)
Bias Current (pA)
INPUT BIAS CURRENT vs TEMPERATURE
100
10
IB
1
IOS
0.1
0.01
–75
10µ
10p
1p
–10µ
–100µ
–50
–25
0
25
50
75
100
125
Temperature (°C)
–1m
–20
–15
–10
–5
0
5
10
15
20
Common-Mode Voltage (V)
SETTLING TIME vs GAIN
INPUT OVER-VOLTAGE V/I CHARACTERISTICS
1000
1
0.8
G = 1V/V
Flat region represents
normal linear operation.
0.4
G = 1000V/V
Settling Time (µs)
Input Current (mA)
0.6
0.2
0
–0.2
+15V
–0.4
G = 1V/V
–0.6
0.1%
G = 1000V/V
VIN
–0.8
IIN
–15V
10
–1
–50 –40 –30
–20 –10
0
10
20
30
40
1
50
100
1000
Gain (V/V)
QUIESCENT CURRENT AND SLEW RATE
vs TEMPERATURE
SHORT-CIRCUIT CURRENT
vs TEMPERATURE
1.4
475
1.2
IQ
450
1
425
0.8
SR
400
±15
0.6
375
–50
–25
0
25
50
75
100
Short-Circuit Current (µA)
500
–75
10
Input Voltage (V)
Slew Rate (V/µs)
Quiescent Current (µA)
0.01%
100
±13
–ISC
±12
±11
±10
–75
0.4
125
Temperature (°C)
+ISC
±14
–50
–25
0
25
50
75
100
125
Temperature (°C)
®
5
INA121
TYPICAL PERFORMANCE CURVES
(CONT)
At TA = +25°C, VS = ±15V, unless otherwise noted.
OUTPUT VOLTAGE SWING vs OUTPUT CURRENT
MAXIMUM OUTPUT VOLTAGE vs FREQUENCY
V+
+85°C
(V+) –0.6
+25°C
(V+) –0.9
–40°C, –55°C
(V+) –1.2
+125°C
(V+) –1.5
(V–) +1.5
+125°C
(V–) +1.2
+85°C
(V–) +0.9
+25°C
(V–) +0.6
–40°C, –55°C
(V–) +0.3
(V–)
±2
±4
±6
±8
±10
25
G=1
20
G = 1000
15
10
5
100
10k
INPUT OFFSET VOLTAGE WARM-UP
INPUT OFFSET VOLTAGE DRIFT
PRODUCTION DISTRIBUTION
18
8
16
6
14
4
2
0
–2
–4
8
6
4
2
100
0
500
400
300
200
Typical production
distribution of
packaged units.
10
–8
–10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Time (µs)
Offset Voltage Drift (µV/°C)
INPUT-REFERRED NOISE VOLTAGE
vs FREQUENCY
VOLTAGE NOISE 0.1 TO 10Hz
INPUT-REFERRED, G ≥ 100
1000
G=1
100
0.5µV
G = 10
G = 100
10
G = 1000
(BW Limit)
1
10
100
1k
10k
Frequency (Hz)
1s /div
®
INA121
1M
12
–6
1
100k
Frequency (Hz)
10
0
1k
Output Current (mA)
Percent of Units (%)
Offset Voltage Change (µV)
G = 10 to 100
0
0
Voltage Noise (nV/√Hz)
Output Voltage Swing (V)
(V+) –0.3
Peak-to-Peak Output Voltage (Vp-p)
30
6
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
50mV/div
50mV/div
G = 10
G = 1000
10µs/div
100µs/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
100µs/div
100µs/div
®
7
INA121
APPLICATION INFORMATION
The 50kΩ term in Equation 1 comes from the sum of the two
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 INA121.
Figure 1 shows the basic connections required for operation
of the INA121. Applications with noisy or high impedance
power supplies may require decoupling capacitors close to
the device pins as shown.
The output is referred to the output reference (Ref) terminal
which is normally grounded. This must be a low-impedance
connection to assure good common-mode rejection. A resistance of 8Ω in series with the 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 (possibly an unstable gain error) in gains of approximately 100 or
greater.
SETTING THE GAIN
Gain of the INA121 is set by connecting a single external
resistor, RG, connected between pins 1 and 8:
DYNAMIC PERFORMANCE
The typical performance curve “Gain vs Frequency” shows
that, despite its low quiescent current, the INA121 achieves
wide bandwidth, even at high gain. This is due to the
current-feedback topology of the INA121. Settling time also
remains excellent at high gain.
(1)
50kΩ
G = 1+
RG
Commonly used gains and resistor values are shown in
Figure 1.
V+
0.1µF
7
–
VIN
DESIRED
GAIN
RG
(Ω)
NEAREST 1% 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
2
INA121
Over-Voltage
Protection
A1
40kΩ
1
25kΩ
G=1+
A3
RG
50kΩ
RG
6
+
8
25kΩ
Load VO
–
+
VIN
3
5
A2
Over-Voltage
Protection
40kΩ
4
NC: No Connection.
V–
Also drawn in simplified form:
–
VIN
RG
INA121
Ref
+
VIN
FIGURE 1. Basic Connections.
®
INA121
+
–
)
VO = G • (VIN – VIN
40kΩ
8
VO
0.1µF
40kΩ
Ref
The INA121 provides excellent rejection of high frequency
common-mode signals. The typical performance curve,
“Common-Mode Rejection vs Frequency” shows this behavior. If the inputs are not properly balanced, however,
common-mode signals can be converted to differential sig–
+
nals. Run the VIN
and VIN connections directly adjacent each
other, from the source signal all the way to the input pins. If
possible use a ground plane under both input traces. Avoid
running other potentially noisy lines near the inputs.
Input circuitry must provide a path for this input bias current
if the INA121 is to operate properly. Figure 3 shows various
provisions for an input bias current path. Without a bias
current return path, the inputs will float to a potential which
exceeds the common-mode range of the INA121 and the
input amplifiers will saturate.
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 resistors provides a balanced input with possible
advantages of lower input offset voltage due to bias current
and better high-frequency common-mode rejection.
NOISE AND ACCURACY PERFORMANCE
The INA121’s FET input circuitry provides low input bias
current and high speed. It achieves lower noise and higher
accuracy with high impedance sources. With source impedances of 2kΩ to 50kΩ the INA114, INA128, or INA129 may
provide lower offset voltage and drift. For very low source
impedance (≤1kΩ), the INA103 may provide improved
accuracy and lower noise. At very high source impedances
(> 1MΩ) the INA116 is recommended.
Crystal or
Ceramic
Transducer
INA121
1MΩ
1MΩ
OFFSET TRIMMING
The INA121 is laser trimmed for low offset voltage and
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 at the output. The op amp buffer provides low
impedance at the Ref terminal to preserve good commonmode rejection. Trim circuits with higher source impedance
should be buffered with an op amp follower circuit to assure
low impedance on the Ref pin.
Thermocouple
INA121
10kΩ
INA121
–
VIN
RG
+
VIN
V+
VO
INA121
Center-tap provides
bias current return.
100µA
1/2 REF200
Ref
INA121
100Ω(1)
OPA277
±10mV
Adjustment Range
VREF
Bridge
10kΩ(1)
Bridge resistance provides
bias current return.
100Ω(1)
FIGURE 3. Providing an Input Common-Mode Current Path.
100µA
1/2 REF200
NOTE: (1) For wider trim range required
in high gains, scale resistor values larger
V–
INPUT COMMON-MODE RANGE
The linear input voltage range of the input circuitry of the
INA121 is from approximately 1.2V below the positive
supply voltage to 2.1V above the negative supply. A differential input voltage causes the output voltage to increase.
The linear input range, however, will be limited by the
output 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 typical performance curve “Input Common-Mode Range vs Output Voltage”.
FIGURE 2. Optional Trimming of Output Offset Voltage.
INPUT BIAS CURRENT RETURN PATH
The input impedance of the INA121 is extremely high—
approximately 1012Ω. However, a path must be provided for
the input bias current of both inputs. This input bias current
is typically 4pA. High input impedance means that this input
bias current changes very little with varying input voltage.
®
9
INA121
A combination of common-mode and differential input
voltage can cause the output of A1 or A2 to saturate. Figure
4 shows the output voltage swing of A1 and A2 expressed in
terms of a common-mode and differential input voltages.
For applications where input common-mode range must be
maximized, limit the output voltage swing by connecting the
INA121 in a lower gain (see performance curve “Input
Common-Mode Voltage Range vs Output Voltage”). If
necessary, add gain after the INA121 to increase the voltage
swing.
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 common-mode 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.
INPUT FILTERING
The INA121’s FET input allows use of an R/C input filter
without creating large offsets due to input bias current.
Figure 5 shows proper implementation of this input filter to
preserve the INA121’s excellent high frequency commonmode rejection. Mismatch of the common-mode input time
constant (R1C1 and R2C 2), either from stray capacitance or
mismatched values, causes a high frequency common-mode
signal to be converted to a differential signal. This degrades
common-mode rejection. The differential input capacitor,
C 3, reduces the bandwidth and mitigates the effects of
mismatch in C1 and C 2. Make C 3 much larger than C1 and
C 2. If properly matched, C1 and C2 also improve ac CMR.
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 A3 will be near 0V even though both
inputs are overloaded.
LOW VOLTAGE OPERATION
The INA121 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
VCM –
V+
G • VD
2
INA121
A1
40kΩ
VD
2
40kΩ
G=1+
25kΩ
A3
RG
50kΩ
RG
VO = G • V D
25kΩ
VD
2
A2
40kΩ
VCM
VCM +
G • VD
2
40kΩ
V–
FIGURE 4. Voltage Swing of A1 and A2.
f−3 d B =
C1
–
R1
1
C1 


4 π R1  C 3 +
2 
+10V
G = 500
VIN
Bridge
INA121
C3
R2
+
VO
RG
100Ω
INA121
Ref
VIN
C2
R1 = R2
C1 = C2
C3 ≈ 10C1
Ref
FET input allows use
of large resistors and
small capacitors.
FIGURE 5. Input Low-Pass Filter.
FIGURE 6. Bridge Transducer Amplifier.
®
INA121
10
VO
±6V to ±18V
Isolated Power
C1
V+
V–
±15V
RG
C2
VO
INA121
Ref
–
VIN
R1
R2
INA121
1
2πR1C1
fc =
+
VO
ISO124
Ref
VIN
NOTE: To preserve good low frequency CMR,
make R1 = R2 and C1 = C2.
Isolated
Common
FIGURE 7. High-Pass Input Filter.
FIGURE 8. Galvanically Isolated Instrumentation
Amplifier.
VIN
OPA277
–
VIN
+
RG
INA121
Ref
C1
50nF
VO
C1
0.1µF
R1
1MΩ
R1
10kΩ
INA121
1
f–3dB =
2πR1C1
OPA277
RG
R2
Ref
IL =
= 1.59Hz
Make G ≤ 10 where G = 1 + 50k
RG
FIGURE 9. AC-Coupled Instrumentation Amplifier.
VIN
G • R2
Load
FIGURE 10. Voltage Controlled Current Source.
VAC
R1
R2
C1
C2
Null
RG
Transducer
INA121
VO
Ref
FIGURE 11. Capacitive Bridge Transducer Circuit.
®
11
INA121
+5V
VREF
Channel 1
VIN
+
–
+In
MPC800
MUX
Channel 8
VIN
12 Bits Out
Serial
ADS7816
INA121
RG
–In
+
–
Ref
FIGURE 12. Multiplexed-Input Data Acquisition System.
–
VIN
22.1kΩ
22.1kΩ
+
VIN
511Ω
VO
INA121
Ref
100Ω
NOTE: Driving the shield minimizes CMR degradation
due to unequally distributed capacitance on the input
line. The shield is driven at approximately 1V below
the common-mode input voltage.
OPA130
For G = 100
RG = 511Ω // 2(22.1kΩ)
effective RG = 505Ω
FIGURE 13. Shield Driver Circuit.
RG = 5.6kΩ
2.8kΩ
G = 10
LA
RA
RG/2
INA121
VO
Ref
2.8kΩ
390kΩ
Low bias current
allows use with high
electrode impedances.
1/2
OPA2131
RL
VG
10kΩ
390kΩ
FIGURE 14. ECG Amplifier With Right-Leg Drive.
®
INA121
12
1/2
OPA2131
VG
NOTE: Due to the INA121’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.