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LT1168

BLOCK DIAGRA

OUTPUT

REF

1168 F01

–

24.7k

400

Ω

400

Ω

RG 8

30k

DIFFERENCE AMPLIFIER STAGE

PREAMP STAGE

+IN

–IN

–

24.7k

+VS

–VS

+VS

–VS

+VS

4 –VS

Figure 1. Block Diagram

The LT1168 is a modified version of the three op amp

instrumentation amplifier. Laser trimming and monolithic

construction allow tight matching and tracking of circuit

parameters over the specified temperature range. Refer to

the block diagram (Figure 1) to understand the following

circuit description. The collector currents in Q1 and Q2 are

trimmed to minimize offset voltage drift, thus assuring a

high level of performance. R1 and R2 are trimmed to an

absolute value of 24.7k to assure that the gain can be set

accurately (0.6% at G = 100) with only one external

resistor RG. The value of RG in parallel with R1 (R2)

determines the transconductance of the preamp stage. As

RG is reduced for larger programmed gains, the transcon-

ductance of the input preamp stage increases to that of the

input transistors Q1 and Q2. This increases the open-loop

gain when the programmed gain is increased, reducing

the input referred gain related errors and noise. The input

voltage noise at gains greater than 50 is determined only

by Q1 and Q2. At lower gains the noise of the difference

amplifier and preamp gain setting resistors increase the

noise. The gain bandwidth product is determined by C1,

C2 and the preamp transconductance which increases

with programmed gain. Therefore, the bandwidth does not

drop proportionally with gain.

The input transistors Q1 and Q2 offer excellent matching,

which is inherent in NPN bipolar transistors, as well as

picoampere input bias current due to superbeta process-

ing. The collector currents in Q1 and Q2 are held constant

due to the feedback through the Q1-A1-R1 loop and

Q2-A2-R2 loop which in turn impresses the differential

input voltage across the external gain set resistor RG.

Since the current that flows through RG also flows through

R1 and R2, the ratios provide a gained-up differential

THEORY OF OPERATIO

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