10

LT1920

THEORY OF OPERATIO

Figure 3. Providing an Input Common Mode Current Path

10k

1920 F03

THERMOCOUPLE

200k

MICROPHONE,

HYDROPHONE,

ETC

200k

CENTER-TAP PROVIDES

BIAS CURRENT RETURN

LT1920

LT1920

LT1920

The LT1920 is a low power precision instrumentation

amplifier that requires only one external resistor to accu-

rately set the gain anywhere from 1 to 1000. The output

can handle capacitive loads up to 1000pF in any gain

configuration and the inputs are protected against ESD

strikes up to 13kV (human body).

Input Protection

The LT1920 can safely handle up to

±20mA of input

current in an overload condition. Adding an external 5k

input resistor in series with each input allows DC input

fault voltages up to

±100V and improves the ESD immu-

nity to 8kV (contact) and 15kV (air discharge), which is the

IEC 1000-4-2 level 4 specification. If lower value input

resistors are needed, a clamp diode from the positive

supply to each input will maintain the IEC 1000-4-2

specification to level 4 for both air and contact discharge.

A 2N4393 drain/source to gate is a good low leakage diode

for use with 1k resistors, see Figure 4. The input resistors

should be carbon and not metal film or carbon film.

RFI Reduction

In many industrial and data acquisition applications,

instrumentation amplifiers are used to accurately amplify

small signals 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 sensors that can be a significant distance

from the signal conditioning circuit. Although these sen-

Figure 4. Input Protection

1920 F04

J2

2N4393

J1

2N4393

OUT

OPTIONAL FOR HIGHEST

ESD PROTECTION

LT1920

REF

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 LT1920.

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.