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LTC1992 Family

1992fb

BLOCK DIAGRAMS (1992-X)

–

+VS

–IN

VMID

+IN

–VS

+VS

–VS

+OUT

–OUT

VOCM

200k

RIN

RFB

RIN

RFB

1992-X BD

PART

LTC1992-1

LTC1992-2

LTC1992-5

LTC1992-10

RIN

30k

15k

RFB

30k

60k

150k

APPLICATIONS INFORMATION

Theory of Operation

The LTC1992 family consists of five fully differential, low

power amplifiers. The LTC1992 is an unconstrained fully

differential amplifier. The LTC1992-1, LTC1992-2, LTC1992-

5 and LTC1992-10 are fixed gain blocks (with gains of

1, 2, 5 and 10 respectively) featuring precision on-chip

resistors for accurate and ultra stable gain.

In many ways, a fully differential amplifier functions much

like the familiar, ubiquitous op amp. However, there are

several key areas where the two differ. Referring to Figure 1,

an op amp has a differential input, a high open-loop gain

and utilizes negative feedback (through resistors) to set

the closed-loop gain and thus control the amplifier’s gain

with great precision. A fully differential amplifier has all of

these features plus an additional input and a complemen-

tary output. The complementary output reacts to the input

signal in the same manner as the other output, but in the

opposite direction. Two outputs changing in an equal but

opposite manner require a common reference point (i.e.,

opposite relative to what?). The additional input, the VOCM

pin, sets this reference point. The voltage on the VOCM input

directly sets the output signal’s common mode voltage and

allows the output signal’s common mode voltage to be

set completely independent of the input signal’s common

mode voltage. Uncoupling the input and output common

mode voltages makes signal level shifting easy.

For a better understanding of the operation of a fully dif-

ferential amplifier, refer to Figure 2. Here, the LTC1992

functional block diagram adds external resistors to real-

ize a basic gain block. Note that the LTC1992 functional

block diagram is not an exact replica of the LTC1992

circuitry. However, the Block Diagram is correct and is

a very good tool for understanding the operation of fully

differential amplifier circuits. Basic op amp fundamentals

together with this block diagram provide all of the tools

needed for understanding fully differential amplifier circuit

applications.

The LTC1992 Block Diagram has two op amps, two sum-

ming blocks (pay close attention the signs) and four resis-

tors. Two resistors, RMID1 and RMID2, connect directly to

the VMID pin and simply provide a convenient mid-supply

reference. Its use is optional and it is not involved in the

operation of the LTC1992’s amplifier. The LTC1992 functions

through the use of two servo networks each employing

LTC1992 Datasheet(PDF) 25 Page - Analog Devices

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