8 Detailed Description

8.1 Overview

The INA823 is a monolithic precision instrumentation amplifier that incorporates a current-feedback input stage

and a four-resistor difference-amplifier output stage. One of the features of an instrumentation amplifier (IA)

is that the gain is set by placing an external resistor across the RG pins, as described in Section 8.3.1. The

three-op-amp IA topology in the INA823 limits the maximum input voltage applied to the input terminal. The

maximum input voltage depends on the common-mode voltage, differential voltage, gain, and the reference

voltage; for more information, see Section 8.3.2. The INA823 also features protection at each input by two

junction field-effect transistors (JFETs) that provide a low series resistance under normal signal conditions,

and preserve excellent noise performance. When excessive voltage is applied, these transistors limit the input

current, as described in Section 8.3.3.

The INA823 is developed for medical-sector applications such as infusion pumps (see Section 9.2.1), and

industrial applications such as programmable logic controllers (see Section 9.2.2)

The schematic in Figure 8-1 shows how the INA823 operates. A differential input voltage is buffered by the input

A

3

50 k

50 k

50 k

50 k

OUT

REF

V+

V

Overvoltage

Protection

A

2

V

50 k

A

1

50 k

(External)

V+

V

Overvoltage

Protection

+IN

V+

V

V+

V

V+

V

V+

V

IN

A Out = V

+ V

+ 0.125 V

V /2

G

+

1

CM

BE

D

A Out = V

+ V

+ 0.125 V

V /2

G

2

CM

BE

D

Output Swing Range A , A , (V+)

0.1 V to (V ) + 0.1 V

1

2

V = G

(V

V

) + V

O

IN+

IN

REF

Linear Input Range A = (V+)

0.9 V to (V ) + 0.1 V

3

V /2

D

V /2

D

Figure 8-1. Detailed Schematic

8.2 Functional Block Diagram

─IN

Current-

Feedback

Input Stage

Current-

Feedback

Input Stage

+IN

Difference

Amplifier

OUT

REF

External Gain Set

Resistor (Optional)

Overvoltage

Protection

Overvoltage

Protection

Input Bias

Current

Cancellation

Input Bias

Current

Cancellation

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INA823

SBOSA75B – JULY 2021 – REVISED NOVEMBER 2021

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