LTC6362 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC6362
Linear Technology
LTC6362 Datasheet PDF : 22 Pages
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LTC6362
APPLICATIONS INFORMATION
RINM
RS
RI
RF
VS
R1
R1 CHOSEN SO THAT R1 || RINM = RS
R2 CHOSEN TO BALANCE R1 || RS
RI
R2 = RS || R1
–+
+–
RF
6405 F04
VINP +–
VCM +– VINM +–
RI2 V+IN
RF2
VVOCM
+
VOCM
–
RI1 V–IN
RF1
V–OUT
6362 F03 V+OUT
Figure 3. Real-World Application with
Feedback Resistor Pair Mismatch
Figure 2. Optimal Compensation for Signal Source Impedance
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:
VOUT(DIFF) = V+OUT – V–OUT
≈
VINDIFF
•
RF
RI
+
VCM
•
∆β
β AVG
–
VOCM
•
∆β
β AVG
where RF is the average of RF1 and RF2, and RI is the
average of RI1 and RI2.
βAVG is defined as the average feedback factor from the
outputs to their respective inputs:
βAVG
=
1
2
•
RI1
RI1 + RF1
+
RI2
RI2 +RF
2
∆β is defined as the difference in the feedback factors:
∆β = RI2 – RI1
RI2 + RF2 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
+
2
VINM
VINDIFF = VINP – VINM
12
When the feedback ratios mismatch (Δβ), 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 ≈ (VCM – VOCM) • ∆β/βAVG
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 VOCM 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:
eno =
( )
eni
•
1+
RF
RI
2
+
2
•
in •RF
2
+
2•
enRI
•
RF
RI
2
+
2 • enRF2
For example, if RF = RI = 1k, the output noise of the circuit
eno = 12nV/√Hz.
If the circuits surrounding the amplifier are well balanced,
common mode noise (envocm) does not appear in the dif-
ferential output noise equation given above.
6362fa
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