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參數(shù)資料
| 型號： | THAT1203 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | InGenius High-CMRR Balanced Input Line Receiver |
| 中文描述： | InGenius高共模抑制比平衡輸入線路接收器 |
| 文件頁數(shù)： | 7/10頁 |
| 文件大小： | 157K |
| 代理商： | THAT1203 |
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.
perfectly balanced source impedances (Rs1 = Rs2),
and
perfectly
balanced
(Ri1 = Ri2), the voltage dividers formed at each node
Ri
1
1
Ri
Rs
2
so the conventional input stage will maintain high
CMRR.
For
input
impedances
(
Ri
Rs
1and
Rs
2
2) will be equal to each other,
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.
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.
ventional balanced line receivers have common-mode
A common
Typically, con-
input impedances of approximately 10 k . In such
cases, a source impedance imbalance of only 10
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.
A 10
mis-
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
impractical for quality audio devices.
approach
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
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
600033 Rev 0A
Preliminary Information
Page 7
+
-
Rs1
Rs2
Rs1 Rs2
Vout
+Vin
-Vin
Ri1
Ri2
Vcm
Figure 10. Basic differential amplifier driven
by common-mode input signal
Rs1
Rs2
Rs1
s2
R
+
-
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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