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LTC1992 Family

1992fb

APPLICATIONS INFORMATION

quantifies the undesired effect of signal level shifting

discussed earlier in the Signal Level Shifting section.

Asymmetrical Feedback Application Circuits

The basic signal equation in Figure 6 also gives insight

to another piece of intuition. The feedback factors may

be deliberately set to different values. One interesting

class of these application circuits sets one or both of the

feedback factors to the extreme values of either zero or

one. Figure 7 shows three such circuits.

At first these application circuits may look to be unstable

or open loop. It is the common mode feedback loop that

enables these circuits to function. While they are useful

circuits, they have some shortcomings that must be con-

sidered. First, due to the severe feedback factor asymmetry,

the VOCM level influences the differential output voltage

with about the same strength as the input signal. With

this much gain in the VOCM path, differential output offset

and noise increase. The large VOCM to VOUTDIFF gain also

necessitates that these circuits are largely limited to dual,

split supply voltage applications with a ground referenced

input signal and a grounded VOCM pin.

The top application circuit in Figure 7 yields a high input

impedance, precision gain of 2 block without any external

resistors. The on-chip common mode feedback servo

resistors determine the gain precision (better than 0.1

percent). By using the –VOUT output alone, this circuit is

also useful to get a precision, single-ended output, high

input impedance inverter. To intuitively understand this

circuit, consider it as a standard op amp voltage follower

(delivered through the signal gain servo) with a comple-

mentary output (delivered through the common mode level

servo). As usual, the amplifier’s input common mode range

must not be exceeded. As with a standard op amp voltage

follower, the common mode signal seen at the amplifier’s

input is the input signal itself. This condition limits the

input signal swing, as well as the output signal swing, to

be the input signal common mode range specification.

The middle circuit is largely the same as the first except

that the noninverting amplifier path has gain. Note that

Figure 6. Basic Equations for Mismatched or Asymmetrical Feedback Applications Circuits

–

RIN2

RIN1

2[+VIN • (1 – 1) – (–VIN) • (1 – 2)] + 2VOSDIFF + 2VOUTCM ( 1 – 2)

1 + 2

RFB1

VOCM

VOUTDIFF =

WHERE:

• FOR GROUND REFERENCED, SINGLE-ENDED INPUT SIGNAL, LET +VIN = VINSIG AND –VIN = 0V

RFB2

–VIN

VINDIFF

+VIN – –VIN

+VIN

–VOUT

+VOUT

1992 F06

LTC1992

VOUTDIFF

+VOUT – –VOUT

2 • VINSIG • (1 – 1) + 2VOSDIFF + 2VOUTCM ( 1 – 2)

1 + 2

VOUTDIFF =

• COMMON MODE REJECTION: SET +VIN = –VIN = VINCM, VOSDIFF = 0V, VOUTCM = 0V

ΔVINCM

ΔVOUTDIFF

CMRR =

= 2

; OUTPUT REFERRED

1 + 2

2 – 1

1 + 2

• OUTPUT DC OFFSET VOLTAGE: SET +VIN = –VIN = VINCM

VOSDIFFOUT = VOSDIFF

+ (VOUTCM – VINCM) 2

1 + 2

RIN1

RIN1 + RFB1

1 =

; 2 =

; VOSDIFF = AMPLIFIER INPUT REFERRED OFFSET VOLTAGE

VOUTCM = KCM • VOCM + VOSCM

0.999 < KCM < 1.001

RIN2

RIN2 + RFB2

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