LTC1992 datasheet(27/40 Pages) LINER

27 / 40 page

LTC1992 Family

1992f

each employing 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 volt-

age decreases. The RFB resistors connect the outputs to the

appropriate inputs establishing negative feedback and clos-

ing 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 con-

ceptually the same as that for op amps and is critical to un-

derstanding 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

direction. 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 mid-

point 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 differ-

ential 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 de-

tailed analysis, consult the fully differential amplifier appli-

cations circuit analysis section..

Basic Applications Circuits

Most fully differential amplifier applications circuits em-

ploy symmetrical feedback networks and are familiar

territory for op amp users. Symmetrical feedback net-

works require that the –VIN/+VOUT network is a mirror

image duplicate of the +VIN/–VOUT network. Each of these

half circuits is basically 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 differ-

ential 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.

APPLICATIO S I FOR ATIO

LTC1992 Datasheet(PDF) 27 Page - Linear Technology

LTC1992 Datasheet(HTML) 27 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL