LTC6409
11
6409fa
applicaTions inForMaTion
a termination resistor RT should be chosen (see Figure
2) such that:
RT =
RINM RS
RINM –RS
According to Figure 2, the input impedance looking into
thedifferentialamp(RINM)reflectsthesingle-endedsource
case, given above. Also, R2 is chosen as:
R2
=RT ||RS =
RT RS
RT +RS
Figure 2. Optimal Compensation for Signal Source Impedance
Δ
b is defined as the difference in the feedback factors:
β =
RI2
RI2 +RF2
–
RI1
RI1+RF1
Here, VCM and VINDIFF are defined as the average and
the difference of the two input voltages VINP and VINM,
respectively:
VCM =
VINP + VINM
2
VINDIFF = VINP – VINM
When the feedback ratios mismatch (Δ
b), common mode
to differential conversion occurs. Setting the differential
input to zero (VINDIFF = 0), the degree of common mode
to differential conversion is given by the equation:
VOUTDIFF = V+OUT – V–OUT ≈(VCM – VOCM)
β
βAVG
(3)
In general, the degree of feedback pair mismatch is a
source of common mode to differential conversion of
both signals and noise. Using 0.1% resistors or better
will mitigate most problems and will provide about 54dB
worst case of common mode rejection. A low impedance
ground plane should be used as a reference for both the
input signal source and the VOCM pin.
There may be concern on how feedback factor mismatch
affects distortion. Feedback factor mismatch from using
1% resistors or better, has a negligible effect on distortion.
However, in single supply level shifting applications where
there is a voltage difference between the input common
mode voltage and the output common mode voltage,
VS
+
–
+
RF
RI
RINM
RS
RI
R2 = RS || RT
RT CHOSEN SO THAT RT || RINM = RS
R2 CHOSEN TO BALANCE RT || RS
RT
6409 F02
Effects of Resistor Pair Mismatch
Figure 3 shows a circuit diagram which takes into consid-
eration that real world resistors will not match perfectly.
Assuming infinite open loop gain, the differential output
relationship is given by the equation:
VOUTDIFF = V+OUT – V–OUT ≈ VINDIFF
RF
RI
+
VCM
β
βAVG
– VOCM
β
βAVG
where RF is the average of RF1, and RF2, and RI is the
average of RI1, and RI2.
bAVG is defined as the average feedback factor from the
outputs to their respective inputs:
βAVG =
1
2
RI1
RI1+RF1
+
RI2
RI2 +RF2
Figure 3. Real-World Application with Feedback
Resistor Pair Mismatch
–
+
RF2
V–OUT
V+OUT
VVOCM
VOCM
6409 F03
RF1
RI2
RI1
+
–
VINP
–
+
VINM
V–IN
V+IN