Datasheet

®
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/–50µA. 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
1MΩ
+
V
IN
–
Two coaxial cables and ground
Two triaxial cables
V
High-Z
Source
1MΩ
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
150Ω
INA116
6
3
1
16
5.62kΩ
13
8
9
11
V
O
G = 10
+15
–15
Solution
Ground
Sample
Electrode
Reference
Electrode