9

®

INA115

FIGURE 2. Switched-Gain Instrumentation Amplifier (minimum components).

SWITCHED GAIN

Figure 2 shows a circuit for digital selection of four gains.

Multiplexer “on” resistance does not significantly affect gain.

The resistor values required for some commonly used gain

steps are shown. This circuit uses the internal 25k

Ω feedback

resistors, so the resistor values shown cannot be scaled to a

different impedance level.

Figure 3 shows an alternative switchable gain configuration.

This circuit does not use the internal 25k

Ω feedback resistors,

so the nominal values shown can be scaled to other impedance

levels. This circuit is ideal for use with a precision resistor

network to achieve excellent gain accuracy and lowest gain

drift.

NOISE PERFORMANCE

The INA115 provides very low noise in most applications. For

differential source impedances less than 1k

Ω, the INA103

may provide lower noise. For source impedances greater than

50k

Ω,theINA111FET-InputInstrumentationAmplifiermay

provide lower noise.

Low frequency noise of the INA115 is approximately

0.4

µVp-p measured from 0.1 to 10Hz. This is approximately

one-tenth the noise of “low noise” chopper-stabilized ampli-

fiers.

OFFSET TRIMMING

The INA115 is laser trimmed for very low offset voltage and

drift. Most applications require no external offset adjustment.

Figure 4 shows an optional circuit for trimming the output

offset voltage. The voltage applied to Ref terminal is summed

at the output. Low impedance must be maintained at this node

to assure good common-mode rejection. This is achieved by

buffering the trim voltage with an op amp as shown.

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA115 is extremely high—

Ω. However, a path must be provided for

the input bias current of both inputs. This input bias current is

typically less than

±1nA (it can be either polarity due to

cancellation circuitry). 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

if the INA115 is to operate properly. Figure 5 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 INA115 and the input

amplifiers will saturate. If the differential source resistance is

low, a bias current return path can be connected to one input

(see thermocouple example in Figure 5). With higher source

impedance, using two resistors provides a balanced input with

possible advantages of lower input offset voltage due bias

current and better common-mode rejection.

A

0

A

1

Gain

LL

1

HL

LH

H

H

Highest

R

1

R

2

R

3

R

4

GAIN STEPS

(

Ω)(Ω)(Ω)(Ω)

1, 10, 100, 1000

2.5k

55.6

500

2.5k

1, 2, 4, 8

12.5k

12.5k

12.5k

12.5k

1, 2, 5, 10

15k

10k

10k

15k

0, +3, +6, +9dB

17.7k

60.3k

25k

17.7k

A

1

A

2

A

3

12

11

10

25k

Ω

25k

Ω

25k

Ω

25k

Ω

5

4

V

IN

V

IN

INA115

V

O

–

+

Over-Voltage

Protection

25k

Ω

25k

Ω

Over-Voltage

Protection

Feedback

15

14

3

2

8

1

HI-509

–15V

+15V

16

1

2

14

3

15

8

9

4

5

6

7

10

11

12

13

R

1

R

2

R

3

R

4

A

1

A

0

Enable