®

INA116

8

FIGURE 2. Optional Trimming of Output Offset Voltage.

INPUT BIAS CURRENT RETURN PATH

Input circuitry must provide an input bias current path for

proper operation. Figure 3 shows resistors R1 and R2 to

provide an input current path. Without these resistors, the

inputs would eventually float to a potential that exceeds the

common-mode range of the INA116 and the input amplifiers

would saturate. Because of its exceedingly low input bias

current, improperly biased inputs may operate normally for

a period of time after power is first applied, or operate

intermittently.

CIRCUIT BOARD LAYOUT AND ASSEMBLY

Careful circuit board layout and assembly techniques are

required to achieve the exceptionally low input bias current

performance of the INA116. Guard terminals adjacent to

both inputs make it easy to properly guard the critical input

terminal layout. Since traces are not required to run between

device pins, this layout is easily accomplished, even with the

surface mount package. The guards should completely en-

circle their respective input connections—see Figure 4. Both

sides of the circuit board should be guarded, even if only one

side has an input terminal conductor. Route any time-

varying signals away from the input terminals. Solder mask

should not cover the input and guard traces since this can

increase leakage.

FIGURE 3. Providing An Input Bias Current Path.

FIGURE 4. Circuit Board Guard Layout.

After assembly, the circuit board should be cleaned. Com-

mercial solvents should be chosen according to the soldering

method and flux used. Solvents should be cleaned and

replaced often. Solvent cleaning should be followed by a de-

ionized water rinse and 85

°C bake out.

Sockets can be used, but select and evaluate them carefully

for best results. Use caution when installing the INA116 in

a socket. Careless handling can contaminate the plastic near

the input pins, dramatically increasing leakage current.

A proven low leakage current assembly method is to bend

the input pins outward so they do not contact the circuit

board. Input connections are made in air and soldered

directly to the input pin. This technique is often not practical

or production-worthy. It is, however, a useful technique for

evaluation and testing and provides a benchmark with which

to compare other wiring techniques. The circuit board guard-

ing techniques discussed normally reduce leakage to accept-

able levels.

A solid mechanical assembly is required for good results.

Nearby plastic parts can be especially troublesome since a

static charge can develop and the slightest motion or vibra-

tion will couple charge to the inputs. Place a Faraday shield

around the whole amplifier and input connection assembly

to eliminate stray fields.

INA116

100M

Ω

R

1

100M

Ω

R

2

V

O

V

O

INA116

100M

Ω

R

2

100M

Ω

R

1

100M

Ω

100M

Ω

Polarizing

Voltage

Capacitive

Sensor

Crystal or

Ceramic

Transducer

Guard Top and

Bottom of Circuit Board.

INA116

V

IN

V

IN

R

G

–

+

10k

Ω(1)

V

O

OPA131

Ref

±10mV

Adjustment Range

100

Ω(1)

100

Ω(1)

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

NOTE: (1) For wider trim range required

in high gains, scale resistor values larger