Datasheet INA125UA-2K5, INA125UA, INA125U-2K5, INA125PA, INA125P Datasheet (Burr Brown)

©1997 Burr-Brown Corporation PDS-1361B Printed in U.S.A., February, 1998

®

INA125

INSTRUMENTATION AMPLIFIER

With Precision Voltage Reference

FEATURES

● LOW QUIESCENT CURRENT: 460µA

● PRECISION VOLTAGE REFERENCE:

1.24V, 2.5V, 5V or 10V

● SLEEP MODE

● LOW OFFSET VOLTAGE: 250

µV max

● LOW OFFSET DRIFT: 2

µV/°C max

● LOW INPUT BIAS CURRENT: 20nA max

● HIGH CMR: 100dB min

● LOW NOISE: 38nV/√Hz at f = 1kHz

● INPUT PROTECTION TO

±40V

● WIDE SUPPLY RANGE

Single Supply: 2.7V to 36V
Dual Supply:

±1.35V to ±18V

● 16-PIN DIP AND SO-16 SOIC PACKAGES

APPLICATIONS

● PRESSURE AND TEMPERATURE BRIDGE

AMPLIFIERS

● INDUSTRIAL PROCESS CONTROL

● FACTORY AUTOMATION

● MULTI-CHANNEL DATA ACQUISITION

● BATTERY OPERATED SYSTEMS

● GENERAL PURPOSE INSTRUMENTATION

DESCRIPTION

The INA125 is a low power, high accuracy instrumen­tation amplifier with a precision voltage reference. It
provides complete bridge excitation and precision dif­ferential-input amplification on a single integrated
circuit.

A single external resistor sets any gain from 4 to
10,000. The INA125 is laser-trimmed for low offset
voltage (250µV), low offset drift (2µV/°C), and high
common-mode rejection (100dB at G = 100). It oper­ates on single (+2.7V to +36V) or dual (±1.35V to
±18V) supplies.

The voltage reference is externally adjustable with pin­selectable voltages of 2.5V, 5V, or 10V, allowing use
with a variety of transducers. The reference voltage is
accurate to ±0.5% (max) with ±35ppm/°C drift (max).
Sleep mode allows shutdown and duty cycle operation
to save power.

The INA125 is available in 16-pin plastic DIP and
SO-16 surface-mount packages and is specified for
the –40°C to +85°C industrial temperature range.

A

1

Ref

Amp

10V

A

2

30kΩ

10kΩ

10kΩ

30kΩ

Bandgap

V

REF

13

12

1

14

15

16

4

6

9

10

11

IA

REF

5

8

7

R

G

Sense

R

R

2R

4R

INA125

V

REF

COM

V

REF

BG

V

REF

2.5

V

REF

5

V

REF

10

V

REF

Out

V

IN

V+

+

V

IN

–

2

SLEEP

3

V–

VO = (VIN – VIN) G

G = 4 + 60kΩ

+

–

R

G

VO

INA125

INA125

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]

2

®

INA125

SPECIFICATIONS: VS = ±15V

At TA = +25°C, VS = ±15V, IA common = 0V, V

REF

common = 0V, and RL = 10kΩ, unless otherwise noted.

INA125P, U INA125PA, UA

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.

PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
INPUT

Offset Voltage, RTI

Initial ±50 ±250 ✻ ±500 µV

vs Temperature ±0.25 ±2 ✻ ±5 µV/°C
vs Power Supply V

S

= ±1.35V to ±18V, G = 4 ±3 ±20 ✻ ±50 µV/V

Long-Term Stability ±0.2 ✻ µV/mo

Impedance, Differential 10

11

|| 2 ✻ Ω || pF

Common-Mode 10

11

|| 9 ✻ Ω || pF
Safe Input Voltage ±40 ✻ V
Input Voltage Range See Text ✻
Common-Mode Rejection V

CM

= –10.7V to +10.2V

G = 4 78 84 72 ✻ dB

G = 10 86 94 80 ✻ dB
G = 100 100 114 90 ✻ dB
G = 500 100 114 90 ✻ dB

BIAS CURRENT V

CM

= 0V 10 25 ✻ 50 nA

vs Temperature ±60 ✻ pA/°C

Offset Current ±0.5 ±2.5 ✻ ±5nA

vs Temperature ±0.5 ✻ pA/°C

NOISE, RTI R

S

= 0Ω

Voltage Noise, f = 10Hz 40 ✻ nV/√Hz

f = 100Hz 38 ✻ nV/√Hz
f = 1kHz 38 ✻ nV/√Hz
f

= 0.1Hz to 10Hz 0.8 ✻ µVp-p

Current Noise, f = 10Hz 170 ✻ fA/√Hz

f

= 1kHz 56 ✻ fA/√Hz

f = 0.1Hz to 10Hz 5 ✻ pAp-p

GAIN

Gain Equation

4 + 60kΩ/R

G

✻ V/V
Range of Gain 4 10,000 ✻✻V/V
Gain Error V

O

= –14V to +13.3V

G = 4 ± 0.01 ±0.075 ✻ ±0.1 %

G = 10 ±0.03 ±0.3 ✻ ±0.5 %
G = 100 ±0.05 ±0.5 ✻ ±1%
G = 500 ±0.1 ✻ %

Gain vs Temperature

G = 4 ±1 ±15 ✻✻ppm/°C

G > 4

(1)

±25 ±100 ✻✻ppm/°C

Nonlinearity V

O

= –14V to +13.3V

G = 4 ±0.0004 ±0.002 ✻ ± 0.004 % of FS

G = 10 ±0.0004 ±0.002 ✻ ± 0.004 % of FS
G = 100 ±0.001 ±0.01 ✻✻% of FS
G = 500 ±0.002 ✻ % of FS

OUTPUT

Voltage: Positive (V+)–1.7 (V+)–0.9 ✻✻ V

Negative (V–)+1 (V–)+0.4 ✻✻ V
Load Capacitance Stability 1000 ✻ pF
Short-Circuit Current –9/+12 ✻ mA

VOLTAGE REFERENCE V

REF

= +2.5V, +5V, +10V

Accuracy I

L

= 0 ±0.15 ±0.5 ✻ ±1%

vs Temperature I

L

= 0 ±18 ±35 ✻ ±100 ppm/°C

vs Power Supply, V+ V+ = (V

REF

+ 1.25V) to +36V ±20 ±50 ✻ ±100 ppm/V

vs Load I

L

= 0 to 5mA 3 75 ✻✻ppm/mA

Dropout Voltage, (V+) – V

REF

(2)

Ref Load = 2kΩ 1.25 1 ✻✻ V

Bandgap Voltage Reference 1.24 ✻ V

Accuracy I

L

= 0 ±0.5 ✻ %

vs Temperature I

L

= 0 ±18 ✻ ppm/°C

3

®

INA125

INA125P, U INA125PA, UA

PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
FREQUENCY RESPONSE

Bandwidth, –3dB G = 4 150 ✻ kHz

G = 10 45 ✻ kHz
G = 100 4.5 ✻ kHz
G = 500 0.9 ✻ kHz

Slew Rate G = 4, 10V Step 0.2 ✻ V/µs
Settling Time, 0.01% G = 4, 10V Step 60 ✻ µs

G = 10, 10V Step 83 ✻ µs
G = 100, 10V Step 375 ✻ µs
G = 500, 10V Step 1700 ✻ µs

Overload Recovery 50% Overdrive 5 ✻ µs

POWER SUPPLY

Specified Operating Voltage ±15 ✻ V
Specified Voltage Range ±1.35 ±18 ✻✻V
Quiescent Current, Positive I

O

= I

REF

= 0mA 460 525 ✻✻ µA

Negative I

O

= I

REF

= 0mA –280 –325 ✻✻ µA

Reference Ground Current

(3)

180 ✻ µA

Sleep Current (V

SLEEP

≤ 100mV) RL = 10kΩ, Ref Load = 2kΩ±1±25 ✻✻ µA

SLEEP MODE PIN

(4)

VIH (Logic high input voltage) +2.7 V+ ✻✻V
V

IL

(Logic low input voltage) 0 +0.1 ✻✻V

I

IH

(Logic high input current) 15 ✻ µA

I

IL

(Logic low input current) 0 ✻ µA

Wake-up Time

(5)

150 ✻ µs

TEMPERATURE RANGE

Specification Range –40 +85 ✻✻°C
Operation Range –55 +125 ✻✻°C
Storage Range –55 +125 ✻✻°C
Thermal Resistance,

θ

JA

16-Pin DIP 80 ✻ °C/W
SO-16 Surface-Mount 100 ✻ °C/W

✻ Specification same as INA125P, U.
NOTES: (1) Temperature coefficient of the "Internal Resistor" in the gain equation. Does not include TCR of gain-setting resistor, R

G

. (2) Dropout voltage is the

positive supply voltage minus the reference voltage that produces a 1% decrease in reference voltage. (3) V

REF

COM pin. (4) Voltage measured with respect to

Reference Common. Logic low input selects Sleep mode. (5) IA and Reference, see Typical Performance Curves.

SPECIFICATIONS: VS = ±15V (CONT)

At TA = +25°C, VS = ±15V, IA common = 0V, V

REF

common = 0V, and RL = 10kΩ, unless otherwise noted.

INA125P, U INA125PA, UA

PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
INPUT

Offset Voltage, RTI

Initial ±75 ±500 ✻ ±750 µV

vs Temperature ±0.25 ✻ µV/°C
vs Power Supply V

S

= +2.7V to +36V 3 20 ✻ 50 µV/V
Input Voltage Range See Text ✻
Common-Mode Rejection V

CM

= +1.1V to +3.6V

G = 4 78 84 72 ✻ dB

G = 10 86 94 80 ✻ dB
G = 100 100 114 90 ✻ dB
G = 500 100 114 90 ✻ dB

GAIN

Gain Error V

O

= +0.3V to +3.8V

G = 4 ± 0.01 ✻ %

OUTPUT

Voltage, Positive (V+)–1.2 (V+)–0.8 ✻✻ V

Negative (V–)+0.3 (V–)+0.15 ✻✻ V

POWER SUPPLY

Specified Operating Voltage +5 ✻ V
Operating Voltage Range +2.7 +36 ✻✻V
Quiescent Current I

O

= I

REF

= 0mA 460 525 ✻✻ µA

Sleep Current (V

SLEEP

≤ 100mV) RL = 10kΩ, Ref Load = 2kΩ±1±25 ✻✻ µA

✻ Specification same as INA125P, U.

SPECIFICATIONS: VS = +5V

At TA = +25°C, VS = +5V, IA common at VS/2, V

REF

common = VS/2, VCM = VS/2, and RL = 10kΩ to VS/2, unless otherwise noted.

4

®

INA125

PIN CONFIGURATION

Top View 16-Pin DIP, SO-16

Power Supply Voltage, V+ to V– ........................................................36V

Input Signal Voltage .......................................................................... ±40V

Output Short Circuit ................................................................. Continuous

Operating Temperature ................................................. –55°C to +125°C

Storage Temperature ..................................................... –55° C to +125°C

Lead Temperature (soldering, 10s)............................................... +300°C

NOTE: Stresses above these ratings may cause permanent damage.

ABSOLUTE MAXIMUM RATINGS

(1)

V+

SLEEP

V–

V

REF

OUT

IA

REF

V

IN

V

IN

R

G

V

REF

10

V

REF

5

V

REF

2.5

V

REF

BG

V

REF

COM
Sense
V

O

R

G

1
2
3
4
5
6
7
8

16
15
14
13
12
11
10

9

–

+

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.

PACKAGE INFORMATION

PACKAGE DRAWING

PRODUCT PACKAGE NUMBER

(1)

INA125PA 16-Pin Plastic DIP 180
INA125P 16-Pin Plastic DIP 180

INA125UA SO-16 Surface-Mount 265
INA125U SO-16 Surface-Mount 265

NOTES: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix C of Burr-Brown IC Data Book.

5

®

INA125

TYPICAL PERFORMANCE CURVES

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

GAIN vs FREQUENCY

60

50

40

30

20

10

0

Gain (dB)

Frequency (Hz)

1 10 100 1k 10k 100k 1M

G = 500

G = 100

G = 10

G = 4

COMMON-MODE REJECTION vs FREQUENCY

120

100

80

60

40

20

0

Common-Mode Rejection (dB)

Frequency (Hz)

1 10 100 1k 10k 100k 1M

G = 100, 500

G = 4

G = 10

G = 500

G = 100

INPUT COMMON-MODE VOLTAGE

vs OUTPUT VOLTAGE, V

S

= ±5V

Output Voltage (V)

Input Common-Mode Voltage (V)

–5 –4 5–3 –2 –1 0 1 2 3 4

5
4
3
2
1

0
–1
–2
–3
–4
–5

Limited by A

2

output swing—see text

Limited by A

2

output swing—see text

VS = ±5V

VS = +5V

IA

REF

= 0V

POSITIVE POWER SUPPLY REJECTION

vs FREQUENCY

140

120

100

80

60

40

20

Power Supply Rejection (dB)

Frequency (Hz)

1 10 100 1k 10k 100k 1M

G = 4

G = 10

G = 500

G = 100

NEGATIVE POWER SUPPLY REJECTION

vs FREQUENCY

120

100

80

60

40

20

0

Power Supply Rejection (dB)

Frequency (Hz)

1 10 100 1k 10k 100k 1M

G = 4

G = 10

G = 100

G = 500

INPUT COMMON-MODE VOLTAGE

vs OUTPUT VOLTAGE, V

S

= ±15V

Output Voltage (V)

Input 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

IA

REF

–15V

+15V

+

Limited by A

2

output swing—see text

Limited by A

2

output swing—see text

6

®

INA125

TYPICAL PERFORMANCE CURVES (CONT)

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

SLEW RATE vs TEMPERATURE

Temperature (°C)

Slew Rate (V/µs)

0.30

0.25

0.20

0.15

0.10

0.05

0

–75 –50 –25 0 25 50 75 100 125

INPUT BIAS AND OFFSET CURRENT

vs TEMPERATURE

Temperature (°C)

Input Bias and Offset Current (nA)

16
14
12
10

8
6
4
2
0

–75 –50 –25 0 25 50 75 100 125

I

B

I

OS

INPUT-REFERRED VOLTAGE AND CURRENT NOISE

vs FREQUENCY

Frequency (Hz)

Input-Referred Voltage Noise (nV/√Hz)

1 10010 1k 10k

1k

100

10

1

1k

100

10

1

100k

Input Bias Current Noise (fA/√Hz)

Voltage Noise

Current Noise

INPUT-REFERRED OFFSET VOLTAGE

vs SLEEP TURN-ON TIME

Time From Turn-On (µs)

Offset Voltage Change (µV)

0 25050 100 150 200

100

80
60
40
20

0
–20
–40
–60
–80

–100

G = 100

SETTLING TIME vs GAIN

Gain (V/V)

Settling Time (µs)

1 10010 1k

10k

1k

100

10

0.1%

0.01%

QUIESCENT CURRENT AND SLEEP CURRENT

vs TEMPERATURE

Temperature (°C)

Quiescent and Sleep Current (µA)

550
500
450
400
350
300
250
200
150
100

50

0

–50

–75 –50 –25 0 25 50 75 100 125

–I

SLEEP

–I

Q

+I

Q

±I

SLEEP

+I

SLEEP

V

SLEEP

= 100mV

V

SLEEP

= 0V

7

®

INA125

TYPICAL PERFORMANCE CURVES (CONT)

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

200mV/div

5V/div

100µs/div

100µs/div

200nV/div

1µs/div

INPUT-REFERRED NOISE, 0.1Hz to 10Hz

SMALL-SIGNAL RESPONSE

LARGE-SIGNAL RESPONSE

INPUT BIAS CURRENT

vs INPUT OVERLOAD VOLTAGE

Overload Voltage (V)

Input Bias Current (µA)

–40 400

200
160
120

80
40

0
–40
–80

–120
–160
–200

All Gains

OUTPUT VOLTAGE SWING

vs OUTPUT CURRENT

0 ±2±4±6±8±10

Output Current (mA)

Output Voltage (V)

+125°C

V+
(V+)–1
(V+)–2
(V+)–3
(V+)–4
(V+)–5

(V–)+5
(V–)+4
(V–)+3
(V–)+2
(V–)+1

V–

+75°C

–55°C

+125°C

–55°C

+25°C

+75°C

+25°C

DELTA VOS vs REFERENCE CURRENT

Reference Current (mA)

Delta V

OS

, RTI (µV)

25

20

15

10

5

0

–5

–8 –6 –4 –2 0 2 4 6 8

Sourcing

Sinking

G = 4

G = 100

G = 4

G = 100

8

®

INA125

TYPICAL PERFORMANCE CURVES (CONT)

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

REFERENCE TURN-ON SETTLING TIME

Time From Power Supply Turn-On (µs)

Reference Error (%)

05010 20 30 40

15
12

9
6
4

0
–3
–6
–9

–12
–15

V

REF

= 10V

V

REF

= 5V

V

REF

= 2.5V

REFERENCE VOLTAGE DEVIATION

vs TEMPERATURE

Temperature (°C)

Reference Voltage Deviation (ppm)

–75 125–50 –25 0 25 50 75 100

50

0

–50

–100

–150

–200

V

REF

= VBG, 2.5V, 5V, or 10V

INPUT-REFERRED OFFSET VOLTAGE

PRODUCTION DISTRIBUTION, V

S

= ±15V

Percent of Amplifiers (%)

Input-Referred Offset Voltage (µV)

30

25

20

15

10

5

0

–500

–450

–400

–350

–300

–250

–200

–150

–100

–50

0

50

100

150

200

250

300

350

400

450

500

Typical production

distribution of

packaged units.

0.02%

0.1%

0.02%

0.1%

INPUT-REFERRED OFFSET VOLTAGE

PRODUCTION DISTRIBUTION, V

S

= +5V

Percent of Amplifiers (%)

Input-Referred Offset Voltage (µV)

35

30

25

20

15

10

5

0

–750

–675

–600

–525

–450

–375

–300

–225

–150

–75

0

75

150

225

300

375

450

525

600

675

750

Typical production

distribution of

packaged units.

0.02%

0.05%

0.1%

0.1%

INPUT-REFERRED OFFSET VOLTAGE DRIFT

PRODUCTION DISTRIBUTION

Percent of Amplifiers (%)

Input-Referred Offset Voltage Drift (µV/°C)

90
80
70
60
50
40
30
20
10

0

±0.25

±0.50

±0.75

±1.00

±1.25

±1.50

±1.75

±2.00

±2.25

±2.50

±2.75

±3.00

±3.25

±3.50

±3.75

±4.00

Typical production

distribution of packaged units.

V

S

= ±15V or +5V

VOLTAGE REFERENCE DRIFT

PRODUCTION DISTRIBUTION

Percent of Amplifiers (%)

Voltage Reference Drift (ppm/°C)

100

90
80
70
60
50
40
30
20
10

0

10

20

30

40

50

60

70

80

90

100

Typical production

distribution of packaged units.

0.3% 0.2%

0.05%

9

®

INA125

2µV/div

10µs/div1µs/div

0.1Hz to 10Hz REFERENCE NOISE
V

REF

= 2.5V, CL = 100pF

TYPICAL PERFORMANCE CURVES (CONT)

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

REFERENCE TRANSIENT RESPONSE

V

REF

= 2.5V, CL = 100pF

1mA/div

50mV/div

NEGATIVE REFERENCE AC LINE REJECTION

vs FREQUENCY

Frequency (Hz)

Negative AC Line Rejection (dB)

11M10 100 1k 10k 100k

120

100

80

60

40

20

0

V

REF

= 2.5V

V

REF

= 5V

V

REF

= 10V

Reference

Output

POSITIVE REFERENCE AC LINE REJECTION

vs FREQUENCY

Frequency (Hz)

Positive AC Line Rejection (dB)

11M10 100 1k 10k 100k

120

100

80

60

40

20

0

V

REF

= 2.5V

V

REF

= 5V

V

REF

= 10V

C = 0.01µF

C = 0.1µF

Capacitor connected between

V

REF

OUT and V

REF

COM.

+1mA
0mA

–1mA

10

®

INA125

APPLICATION INFORMATION

Figure 1 shows the basic connections required for operation
of the INA125. 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 instrumentation amplifier refer­ence (IA

REF

) terminal which is normally grounded. This

must be a low impedance connection to assure good com­mon-mode rejection. A resistance of 12Ω in series with the
IA

REF

pin will cause a typical device to degrade to approxi-

mately 80dB CMR (G = 4).
Connecting V

REF

OUT (pin 4) to one of the four available

reference voltage pins (V

REF

BG, V

REF

2.5, V

REF

5, or V

REF

10)

provides an accurate voltage source for bridge applications.

For example, in Figure 1 V

REF

OUT is connected to V

REF

10
thus supplying 10V to the bridge. It is recommended that
V

REF

OUT be connected to one of the reference voltage pins
even when the reference is not being utilized to avoid
saturating the reference amplifier. Driving the SLEEP pin
LOW puts the INA125 in a shutdown mode.

SETTING THE GAIN

Gain of the INA125 is set by connecting a single external
resistor, R

G

, between pins 8 and 9:

(1)

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

DESIRED GAIN RGNEAREST 1%

(V/V) (Ω)R

G

VALUE (Ω)

4NCNC

5 60k 60.4k
10 10k 10k
20 3750 3740
50 1304 1300

100 625 619
200 306 309

500 121 121
1000 60 60.4
2000 30 30.1

10000 6 6.04

NC: No Connection.

FIGURE 1. Basic Connections.

G =4+

60kΩ

R

G

A

1

Ref

Amp

10V

A

2

30kΩ

10kΩ

10kΩ

30kΩ

Bandgap

V

REF

13

12

1

14

15

16

4

6

9

10

11

IA

REF

V

O

5

8

7

R

G

Load

Sense

+

–

R

(2)

R

2R

4R

INA125

V

REF

COM

V

REF

BG

V

REF

2.5

V

REF

5

V

REF

10

V

REF

Out

V

IN

0.1µF

V+

+

V

IN

–

2

SLEEP

(1)

3

0.1µF

V–

VO = (VIN – VIN) G

G = 4 + 60kΩ

+–

R

G

NOTE: (1) SLEEP pin should be connected
to V+ if shutdown function is not being used.
(2) Nominal value of R is 21kΩ, ±25%.

11

®

INA125

The 60kΩ term in equation 1 comes from the internal metal
film resistors which 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 INA125.

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.

OFFSET TRIMMING

The INA125 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
the IA

REF

terminal is added to the output signal. The op amp

buffer is used to provide low impedance at the IA

REF

terminal to preserve good common-mode rejection.

FIGURE 2. Optional Trimming of Output Offset Voltage.

10kΩ

OPA237

±10mV

Adjustment Range

100Ω

100Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

R

G

INA125

IA

REF

V

O

V

IN

–

V

IN

+

INPUT BIAS CURRENT RETURN

The input impedance of the INA125 is extremely high—
approximately 10

11

Ω. However, a path must be provided for

the input bias current of both inputs. This input bias current
flows out of the device and is approximately 10nA. High
input impedance means that this input bias current changes
very little with varying input 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, 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 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 input common-mode range of the INA125 is shown in
the typical performance curves. The common-mode range is
limited on the negative side by the output voltage swing of
A

2

, an internal circuit node that cannot be measured on an

external pin. The output voltage of A2 can be expressed as:

V

02

= 1.3VIN – (VIN – VIN) (10kΩ/RG)

(voltages referred to IA

REF

terminal, pin 5)

The internal op amp A

2

is identical to A1. Its output swing
is limited to approximately 0.8V from the positive supply
and 0.25V from the negative supply. When the input com­mon-mode range is exceeded (A

2

’s output is saturated), A

1

can still be in linear operation, responding to changes in the
non-inverting input voltage. The output voltage, however,
will be invalid.

PRECISION VOLTAGE REFERENCE

The on-board precision voltage reference provides an accu­rate voltage source for bridge and other transducer applica­tions or ratiometric conversion with analog-to-digital con­verters. A reference output of 2.5V, 5V or 10V is available
by connecting V

REF

OUT (pin 4) to one of the V

REF

pins

(V

REF

2.5, V

REF

5, or V

REF

10). Reference voltages are laser­trimmed for low inital error and low temperature drift.
Connecting V

REF

OUT to V

REF

BG (pin 13) produces the

bandgap reference voltage (1.24V ±0.5%) at the reference
output.

Positive supply voltage must be 1.25V above the desired
reference voltage. For example, with V+ = 2.7V, only the

1.24V reference (V

REF

BG) can be used. If using dual sup-

plies V

REF

COM can be connected to V–, increasing the

–

+

FIGURE 3. Providing an Input Common-Mode Current Path.

47kΩ47kΩ

10kΩ

Microphone,
Hydrophone

etc.

Thermocouple

Center-tap provides

bias current return.

INA125

INA125

INA125

–

12

®

INA125

amount of supply voltage headroom available to the refer­ence. Approximately 180µA flows out of the V

REF

COM
terminal, therefore, it is recommended that it be connected
through a low impedance path to sensor common to avoid
possible ground loop problems.

Reference noise is proportional to the reference voltage
selected. With V

REF

= 2.5V, 0.1Hz to 10Hz peak-to-peak

noise is approximately 9µVp-p. Noise increases to 36µVp-p
for the 10V reference. Output drive capability of the voltage
reference is improved by connecting a transistor as shown in
Figure 4. The external transistor also serves to remove power
from the INA125.

Internal resistors that set the voltage reference output are
ratio-trimmed for accurate output voltages (±0.5% max). The
absolute resistance values, however, may vary ±25%. Adjust­ment of the reference output voltage with an external resistor
is not recommended because the required resistor value is
uncertain.

SHUTDOWN

The INA125 has a shutdown option. When the SLEEP pin
is LOW (100mV or less), the supply current drops to
approximately 1µA and output impedance becomes approxi­mately 80kΩ. Best performance is achieved with CMOS
logic. To maintain low sleep current at high temperatures,
V

SLEEP

should be as close to 0V as possible. This should not
be a problem if using CMOS logic unless the CMOS gate is
driving other currents. Refer to the typical performance
curve, “Sleep Current vs Temperature.”

A transition region exists when V

SLEEP

is between 400mV

and 2.7V (with respect to V

REF

COM) where the output is
unpredictable. Operation in this region is not recommended.
The INA125 achieves high accuracy quickly following wake­up (V

SLEEP

≥ 2.7V). See the typical performance curve

“Input-Referred Offset Voltage vs Sleep Turn-on Time.” If
shutdown is not being used, connect the SLEEP pin to V+.

LOW VOLTAGE OPERATION

The INA125 can be operated on power supplies as low as
±1.35V. Performance remains excellent with power sup­plies ranging from ±1.35V 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 ensure that the com­mon-mode voltage remains within its linear range. See
“Input Common-Mode Voltage Range.” As previously men­tioned, when using the on-board reference with low supply
voltages, it may be necessary to connect V

REF

COM to V– to

ensure V

S

– V

REF

≥ 1.25V.

SINGLE SUPPLY OPERATION

The INA125 can be used on single power supplies of +2.7V
to +36V. Figure 5 shows a basic single supply circuit. The
IA

REF

, V

REF

COM, and V– terminals are connected to ground.
Zero differential input voltage will demand an output volt­age of 0V (ground). When the load is referred to ground as
shown, actual output voltage swing is limited to approxi­mately 150mV above ground. The typical performance curve
“Output Voltage Swing vs Output Current” shows how the
output swing varies with output current.

With single supply operation, careful attention should be
paid to input common-mode range, output voltage swing of
both op amps, and the voltage applied to the IA

REF

terminal.

V

IN+

and V

IN–

must both be 1V above ground for linear
operation. You cannot, for instance, connect the inverting
input to ground and measure a voltage connected to the non­inverting input.

FIGURE 5. Single Supply Bridge Amplifier.

1000Ω

+3V

R

L

1.5V – ∆V

1.5V + ∆V

+3V

R

G

INA125

V

O

3

5

12

Ref

Amp

to load

(transducer)

V+

Bandgap

V

REF

13

12

14

15

16

4

INA125

V

REF

COM

V

REF

BG

V

REF

2.5

V

REF

5

V

REF

10

V

REF

Out

TIP29C

10V

FIGURE 4. Reference Current Boost.

13

®

INA125

INPUT PROTECTION

The inputs of the INA125 are individually protected for
voltage 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

FIGURE 6. Psuedoground Bridge Measurement, 5V Single Supply.

excessive noise. If the input is overloaded, the protection
circuitry limits the input current to a safe value of approxi­mately 120µA to 190µA. The typical performance curve
“Input Bias Current vs Input Overload Voltage” shows this
input current limit behavior. The inputs are protected even if
the power supplies are disconnected or turned off.

A

1

Ref

Amp

2.5V

A

2

30kΩ

10kΩ

10kΩ

30kΩ

Bandgap

V

REF

13

12

14

15

16

4

6

9

10

11

IA

REF

5

2.5V

(1)

(Psuedoground)

8

7

R

G

Load

Sense

INA125

V

REF

COM

V

REF

BG

V

REF

2.5

V

REF

5

V

REF

10

V

IN

+

V

IN

–

1

3

+5V

2

SLEEP

+

–

VO = +2.5V +

[

(

V

IN

– V

IN

) (

4 +

)

]

+–

60kΩ

R

G

NOTE: (1) “Psuedoground” is at +2.5V above actual ground.
This provides a precision reference voltage for succeeding
single-supply op amp stages.