LTC1992 Family

28

1992fb

Single-Ended to Differential Conversion

One of the most important applications of fully differential

amplifiers is single-ended signaling to differential signaling

conversion. Many systems have a single-ended signal that

must connect to an ADC with a differential input. The ADC

could be run in a single-ended manner, but performance

usually degrades. Fortunately, all of basic applications

circuits shown in Figure 4, as well as all of the fixed gain

LTC1992-X parts, are equally suitable for both differential

and single-ended input signals. For single-ended input

signals, connect one of the inputs to a reference voltage

(e.g., ground or mid-supply) and connect the other to

the signal path. There are no tradeoffs here as the part’s

performance is the same with single-ended or differential

input signals. Which input is used for the signal path only

affects the polarity of the differential output signal.

Signal Level Shifting

Another important application of fully differential ampli-

fier is signal level shifting. Single-ended to differential

conversion accompanied by a signal level shift is very

commonplace when driving ADCs. As noted in the theory of

operation section, fully differential amplifiers have a com-

mon mode level servo that determines the output common

mode level independent of the input common mode level.

To set the output common mode level, simply apply the

desired voltage to the VOCM input pin. The voltage range

on the VOCM pin is from (–VS + 0.5V) to (+VS – 1.3V).

Figure 3. Fully Differential Amplifier Signal Conventions (Ideal Amplifier and Perfect Resistor Matching is Assumed)

–

–

+

+

1992 F03

RIN

RIN

RFB

VOCM

VOCM

RFB

B

B

–B

–B

–VIN

–A

–A

VINCM

VOUTCM

VINDIFF

4AVP-PDIFF

A

A

+VIN

2AVP-P

2AVP-P

= VINDIFF = +VIN – –VIN

2BVP-P

2BVP-P

DIFFERENTIAL

INPUT VOLTAGE

= VINCM =

INPUT COMMON

MODE VOLTAGE

+VOUT =

•

•

+ VOCM ; VOSCM = 0V

+VIN – –VIN

+VIN + –VIN

2

= VOUTDIFF = +VOUT – –VOUT

DIFFERENTIAL

OUTPUT VOLTAGE

–VOUT

+VOUT

LTC1992

VOUTDIFF

4BVP-PDIFF

1

2

RFB

RIN

= VOUTCM =

OUTPUT COMMON

MODE VOLTAGE

+VOUT + –VOUT

2

()

–VOUT =

•

•

+ VOCM ; VOSCM = 0V

–VIN – +VIN

1

2

RFB

RIN

VOUTDIFF = VINDIFF •

RFB

RIN

rN ≈ (0.13nV/√Hz)

VAMPCM =

VINP + VINM

2

CMRR =

; +VIN = –VIN

ΔVAMPCM

ΔVAMPDIFF

OUTPUT BALANCE =

ΔVOUTCM

ΔVOUTDIFF

eNOUT =

WHERE: eNOUT = OUTPUT REFERRED NOISE VOLTAGE DENSITY

eNIN = INPUT REFERRED NOISE VOLTAGE DENSITY

(RESISTIVE NOISE IS ALREADY INCLUDED IN THE

SPECIFICATIONS FOR THE FIXED GAIN LTC1992-X PARTS)

+ 1

RFB

RIN

VOUTCM = VOCM

VAMPDIFF = VINP – VINM

VOSCM = VOUTCM – VOCM

()

()

VOSDIFFOUT = VOSDIFFIN •

+ 1

RFB

RIN

()

INM

INP

RIN • RFB

RIN + RFB

()

•

√eNIN2 + rN2

APPLICATIONS INFORMATION