LT6370 datasheet(21/28 Pages) AD

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LT6370

Rev. 0

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APPLICATIONS INFORMATION

RFI Reduction/Internal RFI Filter

In many industrial and data acquisition applications, the

LT6370 will be used to amplify small signals accurately

in the presence of large common mode voltages or high

levels of noise. Typically, the sources of these very small

signals (on the order of microvolts or millivolts) are sen-

sors that can be a significant distance from the signal

conditioning circuit. Although these sensors may be con-

nected to signal conditioning circuitry using shielded or

unshielded twisted-pair cabling, the cabling may act as

an antenna, conveying very high frequency interference

directly into the input stage of the LT6370.

The amplitude and frequency of the interference can have

an adverse effect on an instrumentation amplifier’s input

stage by causing any 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 enve-

lope 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 appears in

series with the instrumentation amplifier’s inputs.

To help minimize this effect, the LT6370 has 50MHz on-

chip RFI filters to help attenuate high frequencies before

they can interact with its input transistors. These on-chip

filters are well matched due to their monolithic construc-

tion, which helps minimize any degradation in AC CMRR.

To reduce the effect of these out-of-band signals on the

input offset voltage of the LT6370 further, an additional

external low-pass filter can be used at the inputs. The

filter should be located very close to the input pins of

the circuit. An effective filter configuration is illustrated

in Figure 10, where three capacitors have been added to

the inputs of the LT6370.

The filter limits the input signal according to the following

relationship:

FilterFreqDIFF =

2πR(2CD +CC)

FilterFreqCM =

2πRCC

where CD ≥10CC.

CD affects the difference signal. CC affects the common-

mode signal. Any mismatch in R × CC degrades the LT6370

CMRR. To avoid inadvertently reducing CMRR-bandwidth

performance, make sure that CC is at least one order of

magnitude smaller than CD.The effect of mismatched CCs

is reduced with a larger CD:CC ratio.

Figure 10.

V–

V+

IN+

IN–

6370 F06

VOUT

10n

100n

1.54k

EXTERNAL RFI

FILTER

LT6370

f–3dB ≈ 500Hz

Adding a Simple External RC Filter at the Inputs to

an Instrumentation Amplifier Is Effective in Further Reducing

Rectification of High Frequency Out-Of-Band Signals.

To avoid any possibility of common mode to differential

mode signal conversion, match the common mode low-

pass filter on each input to 1% or better. Here are the steps

to help determine appropriate values for the filter:

1. Pick R and CD to have a low pass pole at least 10x

higher than the highest signal of interest (e.g. 500Hz for

a 50Hz signal) using:

FilterFreqDIFF =

2πR(2CD +CC)

2πR(2CD +0.1CD)

4.2πRCD

2. Select CC = CD/10.

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