ADA4255ACPZ-R7 datasheet(25/64 Pages) AD

25 / 64 page

Data Sheet

ADA4255

THEORY OF OPERATION

analog.com

Rev. 0 | 25 of 64

PROGRAMMABLE GAIN INSTRUMENTATION

AMPLIFIER

The ADA4255 is a direct current mode instrumentation amplifier

implemented with zero drift amplifiers. The ADA4255 topology en-

sures precision operation over temperature. Refer to the simplified

architecture shown in Figure 85 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 a positive voltage is applied to the ADA4255 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 a 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= V+IN−V−IN×ROUT×2

RIN

(1)

where:

V+IN is the positive input voltage.

V−IN is the negative input voltage.

The overall gain of the ADA4255 amplifier is 2 × ROUT/RIN. The dif-

ferent 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 ADA4255 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 ADA4255 uses a proprietary, zero drift

architecture to ensure low offset voltage, offset voltage drift, and 1/f

noise.

Figure 85. Simplified ADA4255 Programmable Gain Instrumentation Amplifier Topology

ADA4255ACPZ-R7 データシート(PDF) 25 Page - Analog Devices

ADA4255ACPZ-R7 データシート(HTML) 25 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

データシートダウンロード