9

LT1189

Instrumentation Amplifier Rejects High Voltage

Instrumentation amplifiers are often used to process

slowly varying outputs from transducers. With the LT1189

it is easy to make an instrumentation amplifier that can

respond to rapidly varying signals. Attenuation resistors

in front of the LT1189 allow very large common-mode

signals to be rejected while maintaining good frequency

response. The input common-mode and differential-mode

signals are reduced by 100:1, while the closed-loop gain

is set to be 100, thereby maintaining unity-gain input to

output. The unique topology allows for frequency re-

sponse boost by adding 150pF to pin 8 as shown.

S

APPLICATI

I FOR ATIO

LT1189 • AI03

–

+

100*

100*

10k*

10k*

100

Ω

150pF

REF

FB

4

7

5V

–5V

LT1189

6

2

1

3

8

* 0.1% RESISTORS

WORST CASE CMRR = 48dB

10k

Output of Instrumentation Amplifier with 1MHz Square Wave

LT1189 • AI04

High Voltage Instrumentation Amplifier Response

Operating with Low Closed-Loop Gain

The LT1189 has been optimized for closed-loop gains of

10 or greater. The amplifier can be operated at much lower

the feedback resistor, (feedback zero). This capacitor

lowers the closed-loop 3dB bandwidth. The bandwidth

high frequency and the amplifier will appear configured

unity-gain. As an approximate guideline, make BW

= 200MHz. This expression expands to:

A

RC

MHz

VCL

FB

FB

2

200

π

()( )

=

or:

C

A

MHz

R

FB

VCL

FB

200

2

π

The effect of the feedback zero on the transient and

FREQUENCY (Hz)

20

LT1189 • AI05

100M

10M

1M

100k

0

–20

–60

–40

COMMON-MODE RESPONSE

DIFFERENTIAL-MODE RESPONSE