Datasheet

LTC6362
12
6362fa
APPLICATIONS INFORMATION
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
OUT(DIFF)
= 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
.
β
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






∆β 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 (Δβ), 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
CM
– V
OCM
) • ∆β/β
AVG
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. A low impedance ground plane
should be used as a reference for both the input signal
source and the V
OCM
pin.
Noise
The LTC6362’s differential input referred voltage and current
noise densities are 3.9nV/√Hz and 0.8pA/√Hz, respectively.
In addition to the noise generated by the amplifier, the
surrounding feedback resistors also contribute noise. A
simplified noise model is shown in Figure 4. The output
noise generated by both the amplifier and the feedback
components is given by the equation:
e
no
=
e
ni
• 1+
R
F
R
I














2
+2• i
n
• R
F
( )
2
+ 2• e
nRI
•
R
F
R
I






2
+2• e
nRF
2
For example, if R
F
= R
I
= 1k, the output noise of the circuit
e
no
= 12nV/√Hz.
If the circuits surrounding the amplifier are well balanced,
common mode noise (e
nvocm
) does not appear in the dif-
ferential output noise equation given above.
Figure 2. Optimal Compensation for Signal Source Impedance
V
S
+
–
–
+
R
F
R
F
R
I
R
INM
R
S
R
I
R2 = R
S
|| R1
R1 CHOSEN SO THAT R1 || R
INM
= R
S
R2 CHOSEN TO BALANCE R1 || R
S
R1
6405 F04
–
+
R
F2
V
–OUT
V
+OUT
V
VOCM
V
INP
V
INM
V
OCM
6362 F03
R
F1
R
I2
R
I1
V
–IN
V
+IN
+
–
+
–
V
CM
+
–
Figure 3. Real-World Application with
Feedback Resistor Pair Mismatch