AD8421 datasheet(22/28 Pages) AD

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LT6372-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Input Protection

Additional input protection can be achieved by adding

external resistors in series with each input. If low value

resistors are needed, a clamp diode from the positive

supply to each input will help improve robustness. A

2N4394 drain/source to gate is a good low leakage diode

which can be used as shown in Figure 8. Robust input

resistors should be chosen, such as carbon composite

or bulk metal foil. Metal film and carbon film should not

be used because of their poor performance.

Figure 8.

VEE

6372-1 F08

VCC

2N4393

OUT

OPTIONAL FOR HIGHEST

ESD PROTECTION

RIN

LT6372-1

REF1,2

Input Protection

Maintaining AC CMRR

To achieve optimum AC CMRR, it is important to balance

the capacitance on the RG gain setting pins. Furthermore,

if the source resistance on each input is not equal, add-

ing an additional resistance to one input to improve input

source resistance matching will improve AC CMRR.

RFI Reduction/Internal RFI Filter

In many industrial and data acquisition applications, the

LT6372-1 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 LT6372-1.

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 LT6372-1 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 LT6372-1 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 9, where three capacitors have been added to

the inputs of the LT6372-1.

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

LT6372-1 CMRR. To avoid inadvertently reducing CMRR-

bandwidth performance, make sure that CC is at least one

magnitude smaller than CD.The effect of mismatched CCs

is reduced with a larger CD:CC ratio.

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