11

LT1920

To significantly reduce the effect of these out-of-band

signals on the input offset voltage of instrumentation

amplifiers, simple lowpass filters can be used at the

inputs. This filter should be located very close to the input

pins of the circuit. An effective filter configuration is

illustrated in Figure 5, where three capacitors have been

unwanted signal that would appear across the input traces.

common mode rejection is not degraded due to common

mode capacitive imbalance. The differential mode and

common mode time constants associated with the capaci-

tors are:

Setting the time constants requires a knowledge of the

frequency, or frequencies of the interference. Once this

frequency is known, the common mode time constants

can be set followed by the differential mode time constant.

Set the common mode time constants such that they do

not degrade the LT1920’s inherent AC CMR. Then the

differential mode time constant can be set for the band-

width required for the application. Setting the differential

mode time constant close to the sensor’s BW also mini-

mizes any noise pickup along the leads. To avoid any

possibility of inadvertently affecting the signal to be pro-

cessed, set the common mode time constant an order of

magnitude (or more) larger than the differential mode time

constant. To avoid any possibility of common mode to

differential mode signal conversion, match the common

mode time constants to 1% or better. If the sensor is an

RTD or a resistive strain gauge, then the series resistors

instrumentation amplifier.

N8 1197

0.100

± 0.010

(2.540

± 0.254)

0.065

(1.651)

TYP

0.045 – 0.065

(1.143 – 1.651)

0.130

± 0.005

(3.302

± 0.127)

0.020

(0.508)

MIN

0.018

± 0.003

(0.457

± 0.076)

0.125

(3.175)

MIN

12

3

4

87

6

5

0.255

± 0.015*

(6.477

± 0.381)

0.400*

(10.160)

MAX

0.009 – 0.015

(0.229 – 0.381)

0.300 – 0.325

(7.620 – 8.255)

0.325

+0.035

–0.015

+0.889

–0.381

8.255

()

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)

N8 Package

8-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

Dimensions in inches (millimeters) unless otherwise noted.

Figure 5. Adding a Simple RC Filter at the Inputs to an

Instrumentation Amplifier is Effective in Reducing Rectification

of High Frequency Out-of-Band Signals

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

1920 F05

0.001

µF

0.001

µF

0.1

µF

1.6k

1.6k

EXTERNAL RFI

FILTER

LT1920

f(–3dB)

≈ 500Hz