THAT1206 Datasheet, PDF (7/10 Pages) List of Unclassifed Manufacturers – InGenius High-CMRR Balanced Input Line Receiver

600033 Rev 0A
Preliminary Information
Page 7
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
( ^Ri1 and ^Rs2 ) will be equal to each other,
Ri1+ Rs1
Ri 2+ Rs2
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 kW. 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.
+Vin _Ri1
+
-
Ri2
-Vin
Vout
Figure 8. Basic differential amplifier
Rs1
+Vin Ri1
+
Rs1¹Rs2
-
Ri2
Rs2
-Vin
Vout
Figure 9. Basic differential amplifier showing
mismatched source impedances
Rs1
+Vin Ri1
+
Rs1¹Rs2
-
Vcm
Ri2
Rs2
-Vin
Vout
Figure 10. Basic differential amplifier driven
by common-mode input signal
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
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