15

LT1168

voltages or high levels of noise. Typically, the sources of

these very small signals (on the order of microvolts or

millivolts) are sensors that can be a significant distance

from the signal conditioning circuit. Although these sen-

sors may be connected to signal conditioning circuitry,

using shielded or unshielded twisted-pair cabling, the ca-

bling may act as antennae, conveying very high frequency

interference directly into the input stage of the LT1168.

The amplitude and frequency of the interference can have

an adverse effect on an instrumentation amplifier’s input

stage by causing an unwanted DC shift in the amplifier’s

input offset voltage. This well known effect is called RFI

rectification and is produced when out-of-band interfer-

ence is coupled (inductively, capacitively or via radiation)

and rectified by the instrumentation amplifier’s input tran-

sistors. These transistors act as high frequency signal

detectors, in the same way diodes were used as RF

envelope detectors in early radio designs. Regardless of

the type of interference or the method by which it is

coupled into the circuit, an out-of-band error signal ap-

pears in series with the instrumentation amplifier’s inputs.

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 7, 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.

To avoid any possibility of inadvertently affecting the

signal to be processed, set the common mode time

constant an order of magnitude (or more) smaller than the

differential mode time constant. Set the common mode

time constants such that they do not degrade the LT1168

inherent AC CMR. Then the differential mode time con-

stant can be set for the bandwidth required for the appli-

cation. Setting the differential mode time constant close to

the sensor’s BW also minimizes any noise pickup along

the leads. 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 and is in proximity to the

can be omitted.

Figure 7. Adding a Simple RC Filter at the Inputs to an

Instrumentation Amplifier is Effective in Reducing Rectification

of High Frequency Out-of-Band Signals

APPLICATIO S I FOR ATIO

1168 F07

0.001

µF

0.001

µF

0.1

µF

1.6k

1.6k

EXTERNAL RFI

FILTER

LT1168

Nerve Impulse Amplifier

The LT1168’s low current noise makes it ideal for EMG

monitors that have high source impedances. Demonstrat-

ing the LT1168’s ability to amplify low level signals, the

circuit in Figure 8 takes advantage of the amplifier’s high

gain and low noise operation. This circuit amplifies the low

level nerve impulse signals received from a patient at

set a gain of ten. The potential on LT1112’s Pin 1 creates