AD8253 datasheet(22/34 Pages) AD

22 / 34 page

LTC6373

Rev. 0

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APPLICATIONS INFORMATION

Figure 5. Providing an Input Common Mode Current Path

Figure 6. Adding a Simple External RC Filter at the Inputs of the

LTC6373 Is Effective in Suppressing RF Interference.

RF Interference

In many industrial and data acquisition applications, the

LTC6373 will be used to process 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 LTC6373.

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 radia-

tion) and rectified by the instrumentation amplifier’s input

transistors. 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, high frequency signals

can be filtered with a low pass RC network placed at

the input of the LTC6373, as illustrated in Figure 6. The

6373 F05

–

LTC6373

THERMOCOUPLE

THIS IS AN AC ONLY APPLICATION

THIS IS AN AC AND DC APPLICATION

10k

–

LTC6373

MICROPHONE

HYDROPHONE,

ETC.

200k

–

LTC6373

CENTER-TAP PROVIDES

BIAS CURRENT RETURN

6373 F06

–

LTC6373

V–

V+

–VS

–OUT

+OUT

+VS

IN–

IN+

filter limits the input signal bandwidth according to the

following formulas:

FilterFreqDIFF = 1/[2 • π • RS • (CC + 2CD)]

FilterFreqCM = 1/[2 • π • RS • CC]

Setting the filter frequencies requires knowledge of the

frequency (or frequencies) of the RF interference. Once

the interference frequency is known, the common mode

filter frequency can be set (low enough to filter out the

interference frequency) followed by the differential mode

filter frequency. To avoid any possibility of inadvertently

affecting the differential signal of interest, set the common

mode filter frequency an order of magnitude (or more)

higher than the differential mode filter frequency. Set

the common mode filter frequency such that it does not

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