7 Detailed Description

7.1 Overview

The INA630 is a monolithic precision instrumentation amplifier that incorporates an indirect current-feedback

architecture. The block diagram in Figure 7-1 provides an overview of the functionality of this architecture. The

differential input signal (VDM) is converted by the transconductance amplifier (gm1) into an input current (IIN).

The common-mode voltage (VCM) is thereby directly rejected on the inputs. An additional transconductance

amplifier (gm2) converts the feedback voltage across R1 (VFB – VREF) into a feedback current (IFB). IFB is then

subtracted from the input current IIN. The integrator amplifier (gm3) converts the differential current back to an

output voltage (VOUT). If VDM is far different then feedback voltage, IOUT increases and thus VOUT increases.

When the input differential voltage and the feedback voltage are the same, generating IIN and IREF to be the

same, the differential current IOUT is zero and VOUT is stabilized.

An accurate output voltage is dependent on the differential current IOUT, therefore the matching of the two

transconductances gm1 and gm2 primarily defines the linearity and accuracy of this architecture. In the INA630,

gain is set externally by the ratio of resistors R1 and R2. Unlike a traditional INA, in the indirect current feedback

architecture, the input common-mode voltage is rejected by the first transconductance amplifier (gm1) and the

output swing is not limited by the input common-mode voltage.

The precision performance of the INA630 is optimized for maximum differential input voltages less then ±125mV.

When this limit is exceeded, the differential voltage protection scheme shown in Input Protection limit the input

current to a safe level while ensuring the outputs to stay within the rails.

7.2 Functional Block Diagram

R2

R1

+

–

GND

+

–

VDM/2

VDM/2

+

VOUT

VFB

VREF

VINP

VINN

gm3

VCM

VS

VS

VS+

IFB

IIN

IOUT

VS

VS+

VS

VS+

VS

VS+

VS

VS+

R4

R5

R3

R6

Figure 7-1. Detailed Schematic of INA630

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INA630

SBOSAI8A – MARCH 2025 – REVISED JULY 2025

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