Datasheet

LTC1992 Family
36
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 V
OCM
level influences the differential output voltage
with about the same strength as the input signal. With
this much gain in the V
OCM
path, differential output offset
and noise increase. The large V
OCM
to V
OUTDIFF
gain also
necessitates that these circuits are largely limited to dual,
split supply voltage applications with a ground referenced
input signal and a grounded V
OCM
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 –V
OUT
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
–
–
+
+
R
IN2
R
IN1
2[+V
IN
• (1 – B1) – (–V
IN
) • (1 – B2)] + 2V
OSDIFF
+ 2V
OUTCM
(B1 – B2)
B1 + B2
R
FB1
V
OCM
V
OCM
V
OUTDIFF
=
WHERE:
• FOR GROUND REFERENCED, SINGLE-ENDED INPUT SIGNAL, LET +V
IN
= V
INSIG
AND –V
IN
= 0V
R
FB2
–V
IN
V
INDIFF
+V
IN
– –V
IN
+V
IN
–V
OUT
+V
OUT
1992 F06
LTC1992
V
OUTDIFF
+V
OUT
– –V
OUT
2 • V
INSIG
• (1 – B1) + 2V
OSDIFF
+ 2V
OUTCM
(B1 – B2)
B1 + B2
V
OUTDIFF
=
• COMMON MODE REJECTION: SET +V
IN
= –V
IN
= V
INCM
, V
OSDIFF
= 0V, V
OUTCM
= 0V
ΔV
INCM
ΔV
OUTDIFF
CMRR = = 2 ; OUTPUT REFERRED
B1 + B2
B2 – B1
B2 – B1
B1 + B2
• OUTPUT DC OFFSET VOLTAGE: SET +V
IN
= –V
IN
= V
INCM
V
OSDIFFOUT
= V
OSDIFF
+ (V
OUTCM
– V
INCM
) 2
2
B1 + B2
R
IN1
R
IN1
+ R
FB1
B1 = ;B2 = ; V
OSDIFF
= AMPLIFIER INPUT REFERRED OFFSET VOLTAGE
V
OUTCM
= K
CM
• V
OCM
+ V
OSCM
0.999 < K
CM
< 1.001
R
IN2
R
IN2
+ R
FB2