Precision Resistor Network for

Programmable Instrumentation Amplifiers

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5

Detailed Description

The MAX5426 is a precision resistor network with low

temperature drift and high accuracy that performs the

same function as a precision resistor array and CMOS

switches. Operationally, this device consists of fixed

resistors and digitally controlled variable resistors that

provide differential gains of 1, 2, 4, and 8 (see

Functional Diagram). The MAX5426 provides gains

accurate to 0.025% (MAX5426A), 0.09% (MAX5426B)

or 0.5% (MAX5426C).

The MAX5426 is ideal for programmable instrumenta-

tion amplifiers. An offset pin is available to apply a DC

offset voltage to the output of the differential amplifier.

Pin CM is the common-mode input voltage and can be

buffered and connected to the common-mode input of

the instrumentation amplifier (usually the shield of the

input cable to reduce the effects of cable capacitance

and leakage).

Digital Interface Operation

The MAX5426 features a simple two-bit parallel pro-

gramming interface. D1 and D0 program the gain set-

ting according to the Logic-Control Truth Table (see

Table 1). The digital interface is CMOS/TTL logic com-

patible.

Timing Diagram

Figure 3 shows the timing diagram of MAX5426 for two

cases. In case 1, the differential input changes are at

OUT1 and OUT2, while the voltage settling is observed

as the time for the output voltage (from the change in

the input) to reach (and stay) within 0.02% of its final

value.

In case 2, the differential inputs (OUT1 and OUT2) are

at constant voltages, while D1 and D0 are varied (for

example from 01 to 10) to make a change in the gain.

No op amps are used in these cases.

Pin Description

PIN

NAME

FUNCTION

2

GND

Ground

4

FB2

First Stage Positive Input Terminal Resistor. Connect to the inverting terminal of the second input buffer

(see Figure 1).

5

OUT2

First Stage Positive Output Terminal Resistor. Connect to the output terminal of the second input buffer.

6

OFFSET

Second Stage Offset Terminal. Connect to a DC voltage to offset the output of the differential amplifier.

7

INDIF-

Second Stage Negative Input Terminal Resistor. Connect to the inverting input terminal of the

differential op amp.

8

INDIF+

Second Stage Positive Input Terminal Resistor. Connect to the noninverting input terminal of the

differential op amp.

9

OUT

Second Stage Output Terminal, Final Output Terminal

10

OUT1

First Stage Negative Output Terminal of Resistor. Connect to the output terminal of the first input buffer.

11

FB1

First Stage Negative Input Terminal of Resistor. Connect to the inverting input terminal of the first input

buffer.

12

CM

Common-Mode Voltage. CM is the input common-mode voltage of the instrumentation amplifier.

Typically varies ±1% of input common-mode voltage.

13, 14

D0, D1

Digital Inputs. See Table 1.

DIGITAL INPUTS

D1

D0

GAIN

00

1

01

2

10

4

11

8

Table 1. Logic-Control Truth Table