Datasheet INA121U, INA121PA, INA121P, INA121UA-2K5, INA121UA Datasheet (Burr Brown)

1

®

INA121

International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: [phone] • Twx: 910-952-1111

Internet: http://www.burr-brown.com/ • FAXLine: [phone] (US/Canada Only) • Cable: BBRCORP • Telex: 066-6491 • FAX: [phone] • Immediate Product Info: [phone]

®

INA121

INA121

INA121

©1997 Burr-Brown Corporation PDS-1412A Printed in U.S.A. May, 1998

FEATURES

● LOW BIAS CURRENT: ±4pA

● LOW QUIESCENT CURRENT:

±450µA

● 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

FET-Input, Low Power

INSTRUMENTATION AMPLIFIER

DESCRIPTION

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.

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.

APPLICATIONS

● 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

A

1

A

2

A

3

6

40kΩ40kΩ

40kΩ40kΩ

7

4

3

8

1

2

V

IN

V

IN

R

G

V+

V–

INA121

Ref

V

O

G = 1 +

50kΩ

R

G

–

+

5

Over-Voltage

Protection

25kΩ

25kΩ

Over-Voltage

Protection

2

®

INA121

SPECIFICATIONS: VS = ±15V

At TA = +25°C, VS = ±15V, RL = 10kΩ, and IA reference = 0V, unless otherwise noted.

✻ Specification same as INA121P, U.
NOTE: (1) Temperature coefficient of the “Internal Resistor” in the gain equation. Does not include TCR of gain-setting resistor, R

G

.

INA121P, U INA121PA, UA
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
INPUT

Offset Voltage, RTI ±200±200/G ±500±500/G ±300±200/G

±1000±1000/G

µV

vs Temperature ±2±2/G ±5±20/G ✻ ±15±20/G µV/°C
vs Power Supply V

S

= ±2.25V to ±18V ±5±20/G ±50±150/G ✻✻µV/V

Long-Term Stability ±0.5 ✻ µV/mo

Impedance, Differential 10

12

|| 1 ✻ Ω || pF

Common-Mode V

O

= 0V 10

12

|| 12 ✻ Ω || pF
Input Voltage Range See Text and Typical Curves ✻
Safe Input Voltage ±40 ✻ V
Common-Mode Rejection V

CM

= –12.5V to 13.5V

G = 1 78 86 72 ✻ dB

G = 10 91 100 85 ✻ dB

G = 100 96 106 90 ✻ dB

G = 1000 106 ✻ dB

BIAS CURRENT V

CM

= 0V ±4 ±50 ✻✻pA

vs Temperature See Typical Curve ✻

Offset Current ±0.5 ✻ pA

vs Temperature See Typical Curve ✻

NOISE, RTI R

S

= 0Ω

Voltage Noise: f = 10Hz G = 100 30 ✻ nV/√Hz

f = 100Hz G = 100 21 ✻ nV/√Hz
f = 1kHz G = 100 20 ✻ nV/√Hz
f

= 0.1Hz to 10Hz G = 100 1 ✻ µVp-p

Current Noise: f = 1kHz 1 ✻ fA/√Hz

GAIN

Gain Equation 1 + (50kΩ/R

G

) ✻ V/V
Range of Gain 1 10,000 ✻✻V/V
Gain Error V

O

= –14V to 13.5V

G = 1 ±0.01 ±0.05 ✻ ±0.1 %

G = 10 ±0.03 ±0.4 ✻ ±0.5 %

G = 100 ±0.05 ±0.5 ✻ ±0.7 %

G = 1000 ±0.5 ✻ %

Gain vs Temperature

(1)

G = 1 ±1 ±10 ✻✻ppm/°C
G > 1 ±25 ±100 ✻✻ppm/°C

Nonlinearity V

O

= –14V to 13.5V

G = 1 ±0.0002 ±0.001 ✻ ± 0.002 % of FSR

G = 10 ±0.0015 ±0.005 ✻ ± 0.008 % of FSR

G = 100 ±0.0015 ±0.005 ✻ ±0.008 % of FSR

G = 1000 ±0.002 ✻ % of FSR

OUTPUT

Voltage: Positive R

L

= 100kΩ (V+)–0.9 ✻ V

Negative R

L

= 100kΩ (V–)+0.15 ✻ V

Positive R

L

= 10kΩ (V+)–1.5 (V+)–0.9 ✻✻ V

Negative R

L

= 10kΩ (V–)+1 (V–)+0.25 ✻✻ V
Capacitance Load Drive 1000 ✻ pF
Short-Circuit Current ±14 ✻ mA

FREQUENCY RESPONSE

Bandwidth, –3dB G = 1 600 ✻ kHz

G = 10 300 ✻ kHz

G = 100 50 ✻ kHz

G = 1000 5 ✻ kHz

Slew Rate V

O

= ±10V, G ≤ 10 0.7 ✻ V/µs

Settling Time, 0.01% G = 1 to 10 20 ✻ µs

G = 100 35 ✻ µs

G = 1000 260 ✻ µs

Overload Recovery 50% Input Overload 5 ✻ µ s

POWER SUPPLY

Voltage Range ±2.25 ±15 ±18 ✻✻ ✻ V
Quiescent Current I

O

= 0V ±450 ±525 ✻✻µA

TEMPERATURE RANGE

Specification –40 85 ✻✻°C
Operating –55 125 ✻✻°C
Storage –55 125 ✻✻°C
Thermal Resistance,

θ

JA

8-Lead DIP 100 ✻ °C/W
SO-8 Surface Mount 150 ✻ °C/W

3

®

INA121

PIN CONFIGURATION

ELECTROSTATIC
DISCHARGE SENSITIVITY

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.

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.

ABSOLUTE MAXIMUM RATINGS

(1)

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.

Top View 8-Pin DIP and SO-8

R

G

V

–

IN

V

+

IN

V–

R

G

V+
V

O

Ref

1
2
3
4

8
7
6
5

Top View

PACKAGE SPECIFIED
DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT

PRODUCT PACKAGE NUMBER(1) RANGE MARKING NUMBER(2) MEDIA
Single

INA121P 8-Pin DIP 006 –40°C to +85°C INA121P INA121P Rails
INA121PA 8-Pin DIP 006 –40°C to +85°C INA121PA INA121PA Rails
INA121U SO-8 Surface-Mount 182 –40°C to +85°C INA121U INA121U Rails

" " " " " INA121U/2K5 Tape and Reel

INA121UA SO-8 Surface-Mount 182 –40°C to +85°C INA121UA INA121UA Rails

" " " " " INA121UA/2K5 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.

PACKAGE/ORDERING INFORMATION

4

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INA121

TYPICAL PERFORMANCE CURVES

At TA = +25°C, VS = ±15V, unless otherwise noted.

NEGATIVE POWER SUPPLY REJECTION

vs FREQUENCY

Frequency (Hz)

Power Supply Rejection (dB)

10 100 10k 100k 1M1k

120

100

80

60

40

20

0

G = 1000V/V

G = 1V/V

G = 10V/V

G = 100V/V

INPUT COMMON-MODE RANGE

vs OUTPUT VOLTAGE, V

S

= ±15V

Output Voltage (V)

Common-Mode Voltage (V)

–15 –10 0 5 15–5

15

10

5

0

–5

–10

–15

10

V

D/2

+
–

+
–

+

V

CM

V

O

V

D/2

Ref

–15V

+15V

G = 1
G ≥ 10

POSITIVE POWER SUPPLY REJECTION

vs FREQUENCY

Frequency (Hz)

Power Supply Rejection (dB)

10 100 10k 100k 1M1k

120

100

80

60

40

20

0

G = 1000V/V

G = 1V/V

G = 10V/V

G = 100V/V

COMMON-MODE REJECTION

vs FREQUENCY

Frequency (Hz)

Common-Mode Rejection (dB)

10 100 10k 100k 1M1k

120

100

80

60

40

20

0

G = 1000V/V

G = 1V/V

G = 10V/V

G = 100V/V

INPUT COMMON-MODE RANGE

vs OUTPUT VOLTAGE, V

S

= ±5V, ±2.5V

Output Voltage (V)

Common-Mode Voltage (V)

–5

5
4
3
2
1

0
–1
–2
–3
–4
–5

–4 –3 –2 –1 0 1 2 3 4 5

VS = ±5V
V

S

= ±2.5V

G = 1

G ≥ 10

G = 1

G ≥ 10

GAIN vs FREQUENCY

Frequency (Hz)

Gain (dB)

1k 10k 1M 10M100k

60
50
40
30
20
10

0
–10
–20

G = 1000V/V

G = 100V/V

G = 10V/V

G = 1V/V

5

®

INA121

INPUT BIAS CURRENT vs TEMPERATURE

Temperature (°C)

Bias Current (pA)

10k

1k

100

10

1

0.1

0.01
–75 –50 –25 0 25 50 75 100 125

I

B

I

OS

TYPICAL PERFORMANCE CURVES (CONT)

At TA = +25°C, VS = ±15V, unless otherwise noted.

INPUT OVER-VOLTAGE V/I CHARACTERISTICS

1

0.8

0.6

0.4

0.2
0

–0.2
–0.4
–0.6
–0.8

–1

Input Current (mA)

Input Voltage (V)

–50 –40 –30 –20 –10 10 20 30 40050

G = 1V/V

G = 1000V/V

G = 1V/V

Flat region represents
normal linear operation.

V

IN

I

IN

–15V

+15V

G = 1000V/V

QUIESCENT CURRENT AND SLEW RATE

vs TEMPERATURE

500

475

450

425

400

375

–75 –50 –25 0 25 50 75 100 125

Temperature (°C)

Quiescent Current (µA)

1.4

1.2

1

0.8

0.6

0.4

Slew Rate (V/µs)

SR

I

Q

SHORT-CIRCUIT CURRENT

vs TEMPERATURE

±15

±14

±13

±12

±11

±10

–75 –50 –25 0 25 50 75 100 125

Temperature (°C)

Short-Circuit Current (µA)

+I

SC

–I

SC

SETTLING TIME vs GAIN

1000

100

10

1 10 100 1000

Gain (V/V)

Settling Time (µs)

0.01%

0.1%

INPUT BIAS CURRENT

vs COMMON-MODE INPUT VOLTAGE

1m

100µ

10µ

10p

1p

–10µ

–100µ

–1m

–20 –15 –10 –5 0 5 10 15 20

Common-Mode Voltage (V)

Input Bias Current (A)

6

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INA121

TYPICAL PERFORMANCE CURVES (CONT)

At TA = +25°C, VS = ±15V, unless otherwise noted.

0.5µV

VOLTAGE NOISE 0.1 TO 10Hz

INPUT-REFERRED, G ≥ 100

1s/div

OUTPUT VOLTAGE SWING vs OUTPUT CURRENT

V+
(V+) –0.3
(V+) –0.6
(V+) –0.9
(V+) –1.2
(V+) –1.5
(V–) +1.5
(V–) +1.2
(V–) +0.9
(V–) +0.6
(V–) +0.3

(V–)

0 ±2 ±4 ±6 ±8 ±10

Output Current (mA)

Output Voltage Swing (V)

+125°C

+125°C

+85°C

+85°C

–40°C, –55°C

–40°C, –55°C

+25°C

+25°C

INPUT OFFSET VOLTAGE WARM-UP

10

8
6
4
2

0
–2
–4
–6
–8

–10

0

100

200

300

400

500

Time (µs)

Offset Voltage Change (µV)

INPUT OFFSET VOLTAGE DRIFT

PRODUCTION DISTRIBUTION

Percent of Units (%)

Offset Voltage Drift (µV/°C)

18
16
14
12
10

8
6
4
2
0

Typical production
distribution of
packaged units.

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

MAXIMUM OUTPUT VOLTAGE vs FREQUENCY

30

25

20

15

10

5

0

100 1k 10k 100k 1M

Frequency (Hz)

Peak-to-Peak Output Voltage (Vp-p)

G = 1000

G = 10 to 100

G = 1

INPUT-REFERRED NOISE VOLTAGE

vs FREQUENCY

Frequency (Hz)

Voltage Noise (nV/√Hz)

1 10 1k 10k100

1000

100

10

1

G = 1

G = 10

G = 1000

(BW Limit)

G = 100

7

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INA121

SMALL-SIGNAL STEP RESPONSE

(G = 1, 10)

TYPICAL PERFORMANCE CURVES (CONT)

At TA = +25°C, VS = ±15V, unless otherwise noted.

SMALL-SIGNAL STEP RESPONSE

(G = 100, 1000)

LARGE-SIGNAL STEP RESPONSE

(G = 1, 10)

LARGE-SIGNAL STEP RESPONSE

(G = 100, 1000)

10µs/div

G = 10

50mV/div

G = 1

100µs/div

G = 100

50mV/div

G = 1000

100µs/div

G = 10

5V/div

G = 1

G = 1000

5V/div

G = 100

100µs/div

8

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INA121

APPLICATION INFORMATION

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).

SETTING THE GAIN

Gain of the INA121 is set by connecting a single external
resistor, R

G

, connected between pins 1 and 8:

Commonly used gains and resistor values are shown in
Figure 1.

(1)

G = 1+

50 kΩ

R

G

The 50kΩ term in Equation 1 comes from the sum of the two
internal feedback resistors of A

1

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.

The stability and temperature drift of the external gain
setting resistor, R

G

, 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.

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.

A

1

A

2

A

3

6

40kΩ40kΩ

40kΩ40kΩ

7

4

3

8

1

2

V

IN

V

IN

R

G

V+

V–

INA121

G = 1 +

50kΩ

R

G

–

+

5

Over-Voltage

Protection

25kΩ

25kΩ

Over-Voltage

Protection

Load

V

O

= G • (VIN – VIN)

+

–

0.1µF

0.1µF

+

–

V

O

R

G

Also drawn in simplified form:

INA121

Ref

V

O

V

IN

–

V

IN

+

Ref

DESIRED R

G

NEAREST 1% R

G

GAIN (Ω )(Ω)

1NC NC
2 50.00k 49.9k

5 12.50k 12.4k
10 5.556k 5.62k
20 2.632k 2.61k
50 1.02k 1.02k

100 505.1 511
200 251.3 249

500 100.2 100
1000 50.05 49.9
2000 25.01 24.9
5000 10.00 10

10000 5.001 4.99

NC: No Connection.

FIGURE 1. Basic Connections.

9

®

INA121

INA121

1MΩ1MΩ

INA121

10kΩ

Thermocouple

INA121

Center-tap provides
bias current return.

INA121

Bridge resistance provides
bias current return.

Crystal or

Ceramic

Transducer

BridgeV

REF

The INA121 provides excellent rejection of high frequency

common-mode signals. The typical performance curve,

“Common-Mode Rejection vs Frequency” shows this be-

havior. If the inputs are not properly balanced, however,

common-mode signals can be converted to differential sig-

nals. Run the V

IN

and V

IN

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.

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.

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.

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.

+

–

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA121 is extremely high—
approximately 10

12

Ω. 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.

INA121

V

IN

V

IN

R

G

–

+

10kΩ

(1)

V

O

OPA277

Ref

±10mV

Adjustment Range

100Ω

(1)

100Ω

(1)

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

NOTE: (1) For wider trim range required
in high gains, scale resistor values larger

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 A

1

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 3. Providing an Input Common-Mode Current Path.

FIGURE 2. Optional Trimming of Output Offset Voltage.

10

®

INA121

INA121

R

G

100Ω

V

O

+10V

Bridge

G = 500

Ref

A

1

A

2

A

3

40kΩ40kΩ

40kΩ40kΩ

R

G

V+

V–

INA121

V

O

= G • V

D

G = 1 +

50kΩ

R

G

25kΩ

25kΩ

VCM –

G • V

D

2

V

D

2

V

D

2

V

CM

VCM +

G • V

D

2

A combination of common-mode and differential input
voltage can cause the output of A

1

or A2 to saturate. Figure

4 shows the output voltage swing of A

1

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.

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 A

3

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

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 (R

1C1

and R2C2), 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,
C3, reduces the bandwidth and mitigates the effects of
mismatch in C

1

and C2. Make C3 much larger than C1 and

C

2

. If properly matched, C1 and C2 also improve ac CMR.

FIGURE 4. Voltage Swing of A1 and A2.

FIGURE 5. Input Low-Pass Filter.

FIGURE 6. Bridge Transducer Amplifier.

f

−3dB

=

1

4πR

1C3

+

C

1

2











INA121

V

O

V

IN

–

V

IN

+

R

1

R

2

C

3

C

1

C2R1 = R

2

C1 = C

2

C3 ≈ 10C

1

FET input allows use
of large resistors and
small capacitors.

Ref

11

®

INA121

INA121

Transducer

C

1

C

2

Null

R

2

R

1

R

G

V

O

Ref

V

AC

INA121

OPA277

C

1

50nF

R

G

R

2

R

G

Make G ≤ 10 where G = 1 +

50k

Load

V

IN

G • R

2

IL =

R

1

10kΩ

V

IN

Ref

INA121

R

G

V

O

C

1

0.1µF

OPA277

Ref

R

1

1MΩ

f

–3dB

=

1

2πR

1C1

= 1.59Hz

V

IN

+

–

INA121 ISO124

±6V to ±18V

Isolated Power

±15V

Ref

V

IN

–

V

IN

+

V

O

Isolated

Common

V+ V–

FIGURE 8. Galvanically Isolated Instrumentation

Amplifier.

FIGURE 7. High-Pass Input Filter.

FIGURE 10. Voltage Controlled Current Source.FIGURE 9. AC-Coupled Instrumentation Amplifier.

INA121

C

1

C

2

R

1

R

2

V

O

2πR1C

1

1

f

c

=

NOTE: To preserve good low frequency CMR,
make R

1

= R2 and C1 = C2.

R

G

Ref

FIGURE 11. Capacitive Bridge Transducer Circuit.

12

®

INA121

INA121

R

G

/2

R

G

= 5.6kΩ

V

O

LA

RL

RA

10kΩ

Ref

NOTE: Due to the INA121’s current-feedback
topology, V

G

is approximately 0.7V less than
the common-mode input voltage. This DC offset
in this guard potential is satisfactory for many
guarding applications.

Low bias current
allows use with high
electrode impedances.

G = 10

2.8kΩ

V

G

V

G

2.8kΩ

1/2

OPA2131

390kΩ

390kΩ

1/2

OPA2131

FIGURE 12. Multiplexed-Input Data Acquisition System.

FIGURE 13. Shield Driver Circuit.

INA121

V

IN

–

V

IN

+

OPA130

511Ω

22.1kΩ

22.1kΩ
Ref

V

O

For G = 100
R

G

= 511Ω // 2(22.1kΩ)

effective R

G

= 505Ω

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.

FIGURE 14. ECG Amplifier With Right-Leg Drive.

INA121

V

IN

+
–

V

IN

+
–

Channel 8

Channel 1

MPC800

MUX

ADS7816

+5V

V

REF

+In
–In

12 Bits Out

Serial

R

G

Ref