Datasheet

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 V
OCM
input pin. The voltage range
on the V
OCM
pin is from (–V
S
+ 0.5V) to (+V
S
– 1.3V).
Figure 3. Fully Differential Amplifier Signal Conventions (Ideal Amplifier and Perfect Resistor Matching is Assumed)
–
–
+
+
1992 F03
R
IN
R
IN
R
FB
V
OCM
V
OCM
R
FB
B
B
–B
–B
–V
IN
–A
–A
V
INCM
V
OUTCM
V
INDIFF
4AV
P-PDIFF
A
A
+V
IN
2AV
P-P
2AV
P-P
= V
INDIFF
= +V
IN
– –V
IN
2BV
P-P
2BV
P-P
DIFFERENTIAL
INPUT VOLTAGE
= V
INCM
=
INPUT COMMON
MODE VOLTAGE
+V
OUT
= • • + V
OCM
; V
OSCM
= 0V +V
IN
– –V
IN
+V
IN
+ –V
IN
2
= V
OUTDIFF
= +V
OUT
– –V
OUT
DIFFERENTIAL
OUTPUT VOLTAGE
–V
OUT
+V
OUT
LTC1992
V
OUTDIFF
4BV
P-PDIFF
1
2
R
FB
R
IN
= V
OUTCM
=
OUTPUT COMMON
MODE VOLTAGE
+V
OUT
+ –V
OUT
2
()
–V
OUT
= • • + V
OCM
; V
OSCM
= 0V –V
IN
– +V
IN
1
2
R
FB
R
IN
V
OUTDIFF
= V
INDIFF
•
R
FB
R
IN
r
N
≈ (0.13nV/√Hz)
V
AMPCM
=
V
INP
+ V
INM
2
CMRR = ; +V
IN
= –V
IN
ΔV
AMPCM
ΔV
AMPDIFF
OUTPUT BALANCE =
ΔV
OUTCM
ΔV
OUTDIFF
e
NOUT
= WHERE: e
NOUT
= OUTPUT REFERRED NOISE VOLTAGE DENSITY
e
NIN
= INPUT REFERRED NOISE VOLTAGE DENSITY
(RESISTIVE NOISE IS ALREADY INCLUDED IN THE
SPECIFICATIONS FOR THE FIXED GAIN LTC1992-X PARTS)
+ 1
R
FB
R
IN
V
OUTCM
= V
OCM
V
AMPDIFF
= V
INP
– V
INM
V
OSCM
= V
OUTCM
– V
OCM
()
()
V
OSDIFFOUT
= V
OSDIFFIN
• + 1
R
FB
R
IN
()
INM
INP
R
IN
• R
FB
R
IN
+ R
FB
()
• √e
NIN
2
+ r
N
2
APPLICATIONS INFORMATION