Datasheet

LTC6409
11
6409fa
applicaTions inForMaTion
a termination resistor R
T
should be chosen (see Figure
2) such that:
R
T
=
R
INM
• R
S
R
INM
–R
S
According to Figure 2, the input impedance looking into
the differential amp (R
INM
) reflects the single-ended source
case, given above. Also, R2 is chosen as:
R2=R
T
||R
S
=
R
T
• R
S
R
T
+R
S
Figure 2. Optimal Compensation for Signal Source Impedance
Δb is defined as the difference in the feedback factors:
∆β=
R
I2
R
I2
+R
F2
–
R
I1
R
I1
+R
F1
Here, V
CM
and V
INDIFF
are defined as the average and
the difference of the two input voltages V
INP
and V
INM
,
respectively:
V
CM
=
V
INP
+ V
INM
2
V
INDIFF
= V
INP
– V
INM
When the feedback ratios mismatch (Δb), common mode
to differential conversion occurs. Setting the differential
input to zero (V
INDIFF
= 0), the degree of common mode
to differential conversion is given by the equation:
V
OUTDIFF
= V
+OUT
– V
–OUT
≈(V
CM
– V
OCM
)•
∆β
β
AVG
(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 V
OCM
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,
V
S
+
–
–
+
R
F
R
F
R
I
R
INM
R
S
R
I
R2 = R
S
|| R
T
R
T
CHOSEN SO THAT R
T
|| R
INM
= R
S
R2 CHOSEN TO BALANCE R
T
|| R
S
R
T
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:
V
OUTDIFF
= V
+OUT
– V
–OUT
≈ V
INDIFF
•
R
F
R
I
+
V
CM
•
∆β
β
AVG
– V
OCM
•
∆β
β
AVG
where R
F
is the average of R
F1
, and R
F2
, and R
I
is the
average of R
I1
, and R
I2
.
b
AVG
is defined as the average feedback factor from the
outputs to their respective inputs:
β
AVG
=
1
2
•
R
I1
R
I1
+R
F1
+
R
I2
R
I2
+R
F2






Figure 3. Real-World Application with Feedback
Resistor Pair Mismatch
–
+
R
F2
V
–OUT
V
+OUT
V
VOCM
V
OCM
6409 F03
R
F1
R
I2
R
I1
+
–
V
INP
–
+
V
INM
V
–IN
V
+IN