INA116P Burr-Brown Corporation, INA116P Datasheet - Page 9

• Manufacturer Part Number: INA116P
• Description: Ultra Low Input Bias Current INSTRUMENTATION AMPLIFIER
• Manufacturer: Burr-Brown Corporation

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
FIGURE 5. Input Cable Guarding Circuits.
FIGURE 6. Buffered Guard Drive.
Op amp buffer helps
guard cables with
fast input signals—
see text.
V
IN
Source
Source
High-Z
High-Z
V
V
Cable
1M
1M
150
OPA131
Circuit Board
Guard
Two coaxial cables and ground
Two triaxial cables
9
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 A. 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.
5.62k
Reference
Electrode
Electrode
Sample
Solution
Ground
16
INA116
3
1
6
INA116
+15
–15
13
8
V
V
V
V
IN
IN
IN
IN
–
+
–
+
9
G = 10
11
V
O
®