®

PGA202/203

9

INPUT CHARACTERISTICS

Because the PGA202/203 have FET inputs, the bias currents

drawn through input source resistors have a negligible effect

on DC accuracy. The picoamp currents produce no more

than microvolts through megohm sources. The inputs are

also internally diode clamped to the supplies. Thus, input

filtering and input series protection are easily achievable.

A return path for the input bias currents must always be

provided to prevent the charging of any stray capacitance.

Otherwise, the amplifier could wander and saturate. A 1M

Ω

to 10M

Ω resistor from the input to common will return

floating sources such as thermocouples and AC-coupled

inputs (see Applications Section, Figures 8 and 9.)

DYNAMIC PERFORMANCE

The PGA202 and the PGA203 are fast-settling FET input

programmable gain instrumentation amplifiers. Careful at-

tention to minimize stray capacitance is necessary to achieve

specified performance. High source resistance will interact

with the input capacitance to reduce speed and overall

bandwidth. Also, to maintain stability, avoid capacitance

from the output to the input or the offset adjust pins.

Applications with balanced source impedance will provide

the best performance. In some applications, mismatched

source impedances may be required. If the impedance in the

negative input exceeds that in the positive input, stray

capacitance from the output will create a net negative feed-

back and improve the stability of the circuit. If, however, the

impedance in the positive input is greater, then the feedback

due to stray capacitance will be positive and instability may

result. The degree of positive feedback will, of course,

depend on the source impedance imbalance as well as the

board layout and the operating gain. The addition of a small

bypass capacitor of about 5 to 50pF directly across the input

terminals of the PGIA will generally eliminate any instabil-

ity arising from these stray capacitances. CMR errors due to

the source imbalance will also be reduced by the addition of

this capacitor.

The PGA202 and the PGA203 are designed for fast settling

in response to changes in either the input voltage or the gain.

The bandwidth and the settling times are mostly determined

by the output stage and are therefore independent of gain,

except at the highest gain of the PGA202 where other factors

in the input stage begin to dominate.

APPLICATIONS

In addition to general purpose applications, the PGA202/203

are designed to handle two important and demanding classes

of applications: inputs with high source impedances, and

rapid scanning data acquisition systems requiring fast set-

tling time. Because the user has access to output sense and

output common pins, current sources can also be constructed

with a minimum of external components. Some basic appli-

cation circuits are shown in Figures 6 through 12.

PGA203

ISO

8

7

1

2

4

11

12

V

OUT

D

Digital

Opto-

Coupler

14

+

–

102

IN

R

L

+5

FIGURE 6. Isolated Programmable Gain Instrumentation

Amplifier.

FIGURE 7. Auto Gain Ranging.

PGA202

8

7

1

2

4

11

12

V

OUT

1M

Ω

14

+

–

1M

Ω

1µF

1µF

V

IN

Gain Control

FIGURE 8. AC-Coupled Differential Amplifier for

Frequencies Above 0.16Hz.

PGA202

8

7

1

2

4

11

12

V

OUT

Down

Dual

Comparator

2-Bit

Up/Down

Counter

Up

+

–

–

+

V

10V

1

R

10k

Ω

2

R

1k

Ω

REF

V

IN

14

–

+

OSC