14

LT1168

The LT1168 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 LT1168

is trimmed for critical DC parameters such as gain error

(0.04%, G = 10), input offset voltage (40

µV, RTI), CMRR

(90dB min, G = 1) and PSRR (103dB min, G = 1). These

trims allow the amplifier to achieve very high DC accuracy.

The LT1168 achieves low input bias current of just 250pA

(max) through the use of superbeta processing. 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 LT1168 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 voltage up to

±100V and improves the ESD immunity

to

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

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

resistors must be used, 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 resistors between 1k and 20k, see Figure 6.

The input resistors should be carbon and not metal film or

carbon film in order to withstand the fault conditions.

path. The first example is of a purely differential signal

source with a 10k

Ωinputcurrentpathtoground.Sincethe

impedance of the signal source is low, only one resistor is

needed. Two matching resistors are needed for higher

impedance signal sources as shown in the second

example. Balancing the input impedance improves both

common mode rejection and DC offset.

Figure 5. Providing an Input Common Mode Current Path

10k

1168 F05

THERMOCOUPLE

200k

MICROPHONE,

HYDROPHONE,

ETC

200k

CENTER-TAP PROVIDES

BIAS CURRENT RETURN

LT1168

LT1168

LT1168

Figure 6. Input Protection

APPLICATIO S I FOR ATIO

1168 F06

J2

2N4393

J1

2N4393

OUT

OPTIONAL FOR

LT1168

REF

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

THEORY OF OPERATIO