Detailed Description

The MAX5420/MAX5421 are digitally programmable pre-

cision voltage dividers for programmable-gain amplifiers.

Operationally, these devices consist of digitally selectable

precision resistor-dividers providing noninverting gains of

1, 2, 4, and 8 for PGA applications (see Functional

Diagram and Figure 1).

The MAX5420/MAX5421 achieve divider ratio accuracy of

0.025% (MAX5420A/MAX5421A), 0.09% (MAX5420B/

MAX5421B) or 0.5% (MAX5420C/MAX5421C).

The end-to-end resistance from H to L is 15k

Ω. The

impedance seen at W is designed to be the same 8k

Ω

for divider ratios 2, 4, and 8, ensuring excellent imped-

ance matching and constant switching times. In gain of

+1 configuration, H is internally connected to W with a

typical resistance of 500

Ω.

Matching Resistor (MAX5421)

The MAX5421 includes a matching resistor to compen-

sate for offset voltage due to input bias current of the

op amp. The resistance from MATCH_H to MATCH_L,

equals the resistance seen at W for gains of 2, 4, and 8.

In the gain of +1, an internal switch connects

MATCH_H and MATCH_L. This internal switch matches

the impedance of the switch between H and W.

Digital Interface Operation

The MAX5420/MAX5421 feature a simple two-bit paral-

lel programming interface. D1 and D0 program the

divider ratio setting according to the Logic Control

Truth Table (Table 1). The digital interface is CMOS/TTL

logic compatible.

Applications Information

Programmable-Gain Amplifier

The MAX5420/MAX5421 are ideally suited for high-pre-

cision PGA applications. The typical application circuit

of Figure 3 uses the MAX5421 with matching resistor to

compensate for voltage offset due to op amp input bias

currents. Use the MAX5420 with an ultra-low input bias

current op amp such as the MAX4237 (Figure 4).

Power Supplies and Bypassing

The MAX5420/MAX5421 operate from dual ±5V sup-

plies, or a single +5V supply. For dual supplies, bypass

Ratio

RB

RA

=+

1

_

_

Digitally Programmable Precision

Voltage Divider for PGAs

6

_______________________________________________________________________________________

DIGITAL INPUTS

D1

D0

DIVIDER

RATIO

00

1

01

2

10

4

11

8

Table 1. Logic Control Truth Table

PIN

MAX5420

MAX5421

NAME

FUNCTION

11

Positive Supply Voltage

22

Negative Supply Voltage

3

3

GND

Ground

4, 5

4, 5

D0, D1

Digital Control Inputs. Drive D0 and D1 logic high or logic low to set the divider ratio.

See Logic Control Truth Table.

6

8

L

Low Terminal of Resistor-Divider

7

9

W

Resistor-Divider Output

8

10

H

High Terminal of Resistor-Divider

—

6

MATCH_L

Matching Resistor Low Terminal

—

7

MATCH_H

Matching Resistor High Terminal

Pin Description