LTC1992 Family

35

1992fb

APPLICATIONS INFORMATION

Fully Differential Amplifier Applications

Circuit Analysis

All of the previous applications circuit discussions have as-

sumed perfectly matched symmetrical feedback networks.

To consider the effects of mismatched or asymmetrical

feedback networks, the equations get a bit messier.

Figure 6 lists the basic gain equation for the differential

output voltage in terms of +VIN, –VIN, VOSDIFF, VOUTCM

and the feedback factors

β1 and β2. The feedback factors

are simply the portion of the output that is fed back to the

input summing junction by the RFB-RIN resistive voltage

divider.

β1 and β2 have the range of zero to one. The

VOUTCM term also includes its offset voltage, VOSCM, and

its gain mismatch term, KCM. The KCM term is determined

by the matching of the on-chip RCMP and RCMM resistors

in the common mode level servo (see Figure 2).

While mathematically correct, the basic signal equation

does not immediately yield any intuitive feel for fully

differential amplifier application operation. However, by

nulling out specific terms, some basic observations and

sensitivities come forth. Setting

β1 equal to β2, VOSDIFF

to zero and VOUTCM to VOCM gives the old gain equation

from Figure 3. The ground referenced, single-ended input

signal equation yields the interesting result that the driven

side feedback factor (

β1) has a very different sensitivity

than the grounded side (

β2). The CMRR is twice the

feedback factor difference divided by the feedback fac-

tor sum. The differential output offset voltage has two

terms. The first term is determined by the input offset

term, VOSDIFF, and the application’s gain. Note that this

term equates to the formula in Figure 3 when

β1 equals

β2. The amount of signal level shifting and the feedback

factor mismatch determines the second term. This term

Table 1. Input Signal Limitations for Some Common Applications

Single Supply Ground Referenced Single-Ended Input Signal, VOCM at Mid-Supply. (The VINSIG Min and Max values listed account for

both the input common mode limits and the output clipping)

Single Supply Ground Referenced Single-Ended Input Signal, VOCM at Typical ADC Reference Levels. (The VINSIG Min and Max values

listed account for both the input common mode limits and the output clipping)

+VS

(V)

–VS

(V)

GAIN

(V/V)

VOCM

(V)

VINREF

(V)

VINSIG(MAX)

(V)

VINSIG(MIN)

(V)

VINSIGP-P(MAX)

(VP-P AROUND VINREF)

VOUTDIFF(MAX)

(VP-PDIFF)

2.7

0

1

1.35

0

2.700

–2.700

5.40

5.40

2.7

0

2

1.35

0

1.350

–1.350

2.70

5.40

2.7

0

5

1.35

0

0.540

–0.540

1.08

5.40

2.7

0

10

1.35

0

0.270

–0.270

0.54

5.40

5

0

1

2.5

0

5.000

–5.000

10.00

10.00

5

0

2

2.5

0

2.500

–2.500

5.00

10.00

5

0

5

2.5

0

1.000

–1.000

2.00

10.00

5

0

10

2.5

0

0.500

–0.500

1.00

10.00

+VS

(V)

–VS

(V)

GAIN

(V/V)

VOCM

(V)

VINREF

(V)

VINSIG(MAX)

(V)

VINSIG(MIN)

(V)

VINSIGP-P(MAX)

(VP-P AROUND VINREF)

VOUTDIFF(MAX)

(VP-PDIFF)

2.7

0

1

1

0

2.000

–2.000

4.00

4.00

2.7

0

2

1

0

1.000

–1.000

2.00

4.00

2.7

0

5

1

0

0.400

–0.400

0.80

4.00

2.7

0

10

1

0

0.200

–0.200

0.40

4.00

5

0

1

2

0

4.000

–4.000

8.00

8.00

5

0

2

2

0

2.000

–2.000

4.00

8.00

5

0

5

2

0

0.800

–0.800

1.60

8.00

5

0

10

2

0

0.400

–0.400

0.80

8.00