THAT1206 THAT Corporation, THAT1206 Datasheet - Page 7

• Manufacturer Part Number: THAT1206
• Description: (THAT1200 / THAT1203 / THAT1206) Ingenius High-cmrr Balanced Input Line Receiver
• Manufacturer: THAT Corporation

600033 Rev 0A
Preliminary Information
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).
world sources, the CMRR of these stages degrades
rapidly for even small source impedance imbalances.
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.
perfectly balanced source impedances (Rs1 = Rs2),
and
(Ri1 = Ri2), the voltage dividers formed at each node
(
so the conventional input stage will maintain high
CMRR.
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.
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.
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 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,
impractical for quality audio devices.
Ri
1
Conventional high-CMRR balanced input stages
The reason why this occurs is easily shown. Fig-
However, if the source impedances are not pre-
To illustrate, consider Figure 10.
The best solution to this problem is to increase
Ri
+
1
Rs
making
perfectly
1
and
Theory of Operation
Ri
2
Rs
this
+
However, when driven from real-
balanced
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
2
Rs
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2
) will be equal to each other,
straightforward
input
A 10 W mis-
impedances
A common
approach
For
strumentation amplifier configuration shown in Fig-
ure 11.
impedance is the parallel combination of Ri1 and
Ri2.
multi-megohm input impedances, the input devices
used in the input amplifiers must have extremely low
Vcm
+Vin
-Vin
An alternative approach is to use the classic in-
Unfortunately for this approach, to achieve
Figure 9. Basic differential amplifier showing
Figure 10. Basic differential amplifier driven
Rs1
In this circuit, the common-mode input
Rs2
Rs1
Figure 8. Basic differential amplifier
Ri1
Ri2
¹R
mismatched source impedances
Rs2
Rs1
Rs1 Rs2
by common-mode input signal
s2
¹
+Vin
-Vin
+Vin
-Vin
Ri2
Ri1
Ri2
Ri1
-
+
-
+
-
+
Vout
Page 7
Vout
Vout