LTC1992 datasheet(27/42 Pages) AD

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LTC1992 Family

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APPLICATIONS INFORMATION

negative feedback and using an op amp’s differential input

to create the servo’s summing junction.

One servo controls the signal gain path. The differential

input of op amp A1 creates the summing junction of this

servo. Any voltage present at the input of A1 is amplified (by

the op amp’s large open-loop gain), sent to the summing

blocks and then onto the outputs. Taking note of the signs on

the summing blocks, op amp A1’s output moves +OUT and

–OUT in opposite directions. Applying a voltage step at

the INM node increases the +OUT voltage while the –OUT

voltage decreases. The RFB resistors connect the outputs

to the appropriate inputs establishing negative feedback and

closing the servo’s loop. Any servo loop always attempts

to drive its error voltage to zero. In this servo, the error

voltage is the voltage between the INM and INP nodes,

thus A1 will force the voltages on the INP and INM nodes

to be equal (within the part’s DC offset, open loop gain

and bandwidth limits). The “virtual short” between the

two inputs is conceptually the same as that for op amps

and is critical to understanding fully differential amplifier

applications.

The other servo controls the output common mode level.

The differential input of op amp A2 creates the summing

junction of this servo. Similar to the signal gain servo

above, any voltage present at the input of A2 is amplified,

sent to the summing blocks and then onto the outputs.

However, in this case, both outputs move in the same direc-

tion. The resistors RCMP and RCMM connect the +OUT and

–OUT outputs to A2’s inverting input establishing negative

feedback and closing the servo’s loop. The midpoint of

resistors RCMP and RCMM derives the output’s common

mode level (i.e., its average). This measure of the output’s

common mode level connects to A2’s inverting input while

A2’s noninverting input connects directly to the VOCM pin.

A2 forces the voltages on its inverting and noninverting

inputs to be equal. In other words, it forces the output

common mode voltage to be equal to the voltage on the

VOCM input pin.

For any fully differential amplifier application to function

properly both the signal gain servo and the common mode

level servo must be satisfied. When analyzing an applica-

tions circuit, the INP node voltage must equal the INM node

voltage and the output common mode voltage must equal

the VOCM voltage. If either of these servos is taken out of

the specified areas of operation (e.g., inputs taken beyond

the common mode range specifications, outputs hitting the

supply rails or input signals varying faster than the part

can track), the circuit will not function properly.

Fully Differential Amplifier Signal Conventions

Fully differential amplifiers have a multitude of signals and

signal ranges to consider. To maintain proper operation

with conventional op amps, the op amp’s inputs and its

output must not hit the supply rails and the input signal’s

common mode level must also be within the part’s speci-

fied limits. These considerations also apply to fully dif-

ferential amplifiers, but here there is an additional output

to consider and common mode level shifting complicates

matters. Figure 3 provides a list of the many signals and

specifications as well as the naming convention. The

phrase “common mode” appears in many places and often

leads to confusion. The fully differential amplifier’s ability

to uncouple input and output common mode levels yields

great design flexibility, but also complicates matters some.

For simplicity, the equations in Figure 3 also assume an

ideal amplifier and perfect resistor matching. For a detailed

analysis, consult the fully differential amplifier applications

circuit analysis section.

Basic Applications Circuits

Most fully differential amplifier applications circuits employ

symmetrical feedback networks and are familiar territory

for op amp users. Symmetrical feedback networks require

that the –VIN/+VOUT network is a mirror image duplicate of

the +VIN/–VOUT network. Each of these half circuits is basi-

cally just a standard inverting gain op amp circuit. Figure 4

shows three basic inverting gain op amp circuits and their

corresponding fully differential amplifier cousins. The vast

majority of fully differential amplifier circuits derive from

old tried and true inverting op amp circuits. To create a

fully differential amplifier circuit from an inverting op amp

circuit, first simply transfer the op amp’s VIN/VOUT network

to the fully differential amplifier’s –VIN/+VOUT nodes. Then,

take a mirror image duplicate of the network and apply it

to the fully differential amplifier’s +VIN/–VOUT nodes. Op

amp users can comfortably transfer any inverting op amp

circuit to a fully differential amplifier in this manner.

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