®

9

INA116

INPUT CONNECTIONS

Some applications must make high impedance input connec-

tions to external sensors or input connectors. To assure low

leakage, the input should be guarded all the way to the signal

source—see Figure 5. Coaxial cable can be used with the

shield driven by the guard. A separate connection is required

to provide a ground reference at the signal source. Triaxial

cable may reduce noise pickup and provides the ground

reference at the source. Drive the inner shield at guard

potential and ground the outer shield. Two separate guarded

lines are required if both the inverting and non-inverting

inputs are brought to the source.

The guard drive output current is limited to approximately

+2mA/–50

µA. For slow input signals the internal guard

output can directly drive a cable shield. With fast input

signals, however, the guard may not provide sufficient

output current to rapidly charge the cable capacitance. An op

amp buffer may be required as shown in Figure 6.

FIGURE 7. pH or Ion Measurement System.

FIGURE 6. Buffered Guard Drive.

V

IN

1M

Ω

+

V

IN

–

Two coaxial cables and ground

Two triaxial cables

V

High-Z

Source

1M

Ω

V

High-Z

Source

V

IN

+

V

IN

–

FIGURE 5. Input Cable Guarding Circuits.

OPA131

Cable

V

IN

Op amp buffer helps

guard cables with

fast input signals—

see text.

Circuit Board

Guard

150

Ω

INA116

6

3

1

16

5.62k

Ω

13

8

9

11

V

O

G = 10

+15

–15

Solution

Ground

Sample

Electrode

Reference

Electrode