ADA4254ARUZ-R7 datasheet(23/59 Pages) AD

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Data Sheet

ADA4254

Rev. B | Page 23 of 59

THEORY OF OPERATION

PROGRAMMABLE GAIN INSTRUMENTATION

AMPLIFIER

The ADA4254 is a direct current mode instrumentation amplifier

implemented with zero drift amplifiers. The ADA4254 topology

ensures precision operation over temperature. Refer to the

simplified architecture shown in Figure 79 to understand the

following circuit description.

The input multiplexer connects the inputs to Amplifier A3 and

Amplifier A7, which are configured to replicate these input voltages

on the RIN input resistor. The A1, A2, A5, and A6 amplifiers are

configured to replicate the internal reference voltage, VREF, on R1,

R2, R5, and R6, creating four nominally equal dc bias currents in

the drains of M1, M2, M5, and M6. Amplifier A4 and Amplifier

A8 are configured to replicate the currents in R3 and R7 in the

drains of M4 and M8, respectively, forming current mirrors.

When positive voltage is applied to the ADA4254 inputs, a

proportional current is conducted by RIN. The drain currents of

M3 and M4 increase by this amount, and the drain currents of

M7 and M8 reduce by this amount. This portion of the amplifier

operates as a transconductance with differential output, each

having a gain of 1/RIN. Output amplifier A9 is configured as a

transimpedance amplifier with gain of ROUT. A9 provides a

common-mode level shift to the output and produces the

differential output voltage (VOUT, DIFF) as follows:

VOUT, DIFF = ()

OUT







where:

V+IN is the positive input voltage.

V−IN is the negative input voltage.

The overall gain of the ADA4254 amplifier is 2 × ROUT/RIN. The

different gain settings are achieved by internally switching in

different values for ROUT and RIN.

The value of RIN can be set to 12 different values via the G3 to

G0 bits, resulting in 12 binary weighted input gains. The value

of ROUT can also be set to three different values via G4 and G5,

resulting in three output scaling gains. Table 6 shows the 36

possible gain configurations, making the ADA4254 versatile

when interfacing with a wide selection of sensors and ADCs.

Table 6. Possible Gain Settings

Input Gain

Output Scaling Gain (V/V)

1.25

1.375

0.0625

0.078125

0.085938

0.125

0.15625

0.171875

0.25

0.3125

0.34375

0.5

0.625

0.6875

1.25

1.375

2.5

2.75

5.5

128

160

176

Each amplifier used in the ADA4254 uses a proprietary, zero

drift architecture to ensure very low offset voltage, offset

voltage drift, and 1/f noise.

EMI

FILTER

VREF

+IN

–IN

(V+IN – V–IN) × ROUT

VOCM –

VOCM +

RIN

VSSH

INPUT TRANSCONDUCTANCE AMPLIFIER

OUTPUT TRANSIMPEDANCE AMPLIFIER

VDDH

RIN

(V+IN – V–IN) × ROUT

RIN

V+IN – V–IN

RIN

V+IN – V–IN

RIN

V+IN – V–IN

RIN

+IN1

+IN2

–IN1

–IN2

+OUT

–OUT

VOCM

AVDD

AVSS

ROUT

Figure 79. Simplified ADA4254 Programmable Gain Instrumentation Amplifier Topology

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