Theory of Operation
Conventional high-CMRR balanced input stages
cancel common-mode interference using a differential
amplifier with matched (trimmed) resistance ele-
ments (Figure 8). When driven from a true voltage
source, these conventional stages offer extremely high
CMRR (>80dB).
However, when driven from real-
world sources, the CMRR of these stages degrades
rapidly for even small source impedance imbalances.
The reason why this occurs is easily shown. Fig-
ure 9 shows that a voltage divider is formed between
the impedance of the external signal source and the
input impedance of the differential amplifier.
For
perfectly balanced source impedances (Rs1 = Rs2),
and
perfectly
balanced
input
impedances
(Ri1 = Ri2), the voltage dividers formed at each node
(
Ri
Ri
Rs
1
11
+
and
Rs
Ri
Rs
2
22
+
) will be equal to each other,
so the conventional input stage will maintain high
CMRR.
However, if the source impedances are not pre-
cisely equal, the voltage divider action will result in
unequal signals at the plus and minus inputs of the
input stage. In this case, no amount of CMRR is suf-
ficient to reject the differential voltage that is gener-
ated by the impedance mismatch.
To illustrate, consider Figure 10.
A common
mode input signal is shown as Vcm. It couples to the
positive and negative input of the balanced line re-
ceiver via Rs1 and Rs2, repectively.
Typically, con-
ventional balanced line receivers have common-mode
input impedances of approximately 10 k
W. In such
cases, a source impedance imbalance of only 10
W
can degrade CMRR to about 65 dB.
A 10
W mis-
match may be easily caused by tolerances in coupling
capacitors or output resistors, and variations in con-
tact and wire resistance. The situation becomes even
worse when a conventional balanced line receiver is
driven from an unbalanced source.
The best solution to this problem is to increase
the line receiver’s common-mode input impedance
enough to minimize the imbalanced voltage divider
effect, preferably on the order of several megohms.
However, with a conventional differential amplifier,
this requires the use of high resistances in the cir-
cuit. High resistance carries with it a high noise pen-
alty,
making
this
straightforward
approach
impractical for quality audio devices.
An alternative approach is to use the classic in-
strumentation amplifier configuration shown in Fig-
ure 11.
In this circuit, the common-mode input
impedance is the parallel combination of Ri1 and
Ri2.
Unfortunately for this approach, to achieve
multi-megohm input impedances, the input devices
used in the input amplifiers must have extremely low
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
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600033 Rev 0A
Page 7
Preliminary Information
+
-
Rs1
Rs2
Rs1 Rs2
¹
Vout
+Vin
-Vin
Ri1
Ri2
Vcm
Figure 10. Basic differential amplifier driven
by common-mode input signal
Rs1
Rs2
Rs1
s2
¹R
+
-
Vout
+Vin
-Vin
Ri1
Ri2
Figure 9. Basic differential amplifier showing
mismatched source impedances
+
-
Vout
+Vin
-Vin
Ri1
Ri2
Figure 8. Basic differential amplifier
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