1203P08-U THAT Corporation, 1203P08-U Datasheet - Page 5

• Manufacturer Part Number: 1203P08-U
• Description: Difference Amplifiers High-CMRR Bal Input Line Rec. -3dB DIP-8
• Manufacturer: THAT Corporation

Specifications of 1203P08-U

• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: 0 C
• Mounting Style: Through Hole
• Operating Temperature Range: 0 C to + 85 C
• Output Current: +/- 25 mA
• Operating Supply Voltage: +/- 18 V
• Package / Case: DIP-8
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

600033 Rev 00
Source Impedance Mismatches Ruin Good CMRR
the source impedances R
voltage dividers to be unequal between the two input
legs. This means that V
longer equal to each other. Essentially, imbalances in
the two source impedances convert the common
mode signal to a differential signal, which will not be
rejected by the input stage no matter how high its
theoretical CMRR is.
case of a typical unity-gain conventional balanced line
receiver with common-mode input impedance of
10 kΩ. In such cases, a source impedance imbalance
of only 10 Ω can degrade CMRR to no better than
66 dB. A 10 Ω mismatch could be caused by toler-
ances in coupling capacitors or output build-out re-
sistors. The situation becomes much worse when a
conventional balanced line receiver is driven from an
unbalanced source, where it is common to use at
least 100 Ω in series with the output for protection.
(With a 100 Ω unbalanced output impedance, and a
10 kΩ common-mode input impedance, even a per-
fect simple input stage can provide no more than
46 dB CMRR!)
the line receiver's common-mode input impedance
enough to minimize the unbalancing effect of the volt-
age divider. Preferably, this means achieving input
impedances on the order of several megohms. How-
ever, in a conventional differential amplifier, this re-
quires high-value resistances in the circuit.
resistance carries with it a high noise penalty, making
this straightforward approach impractical for quality
audio devices.
Instrumentation Amplifiers
proach of an instrumentation amplifier, as shown in
Figure 6. In this circuit, it is possible to raise the in-
Vcm
Zcm
Z
Even if R
To see how this plays out in practice, consider the
The best solution to this problem is to increase
Some designers prefer the more elaborate ap-
CM
Figure 5. Basic differential amplifier driven
=
R
1
3
+
+
1
R
R
by common-mode input signal
R
1
3
perfectly matches R
4
Rs-
Rs+
Zcm+
Zcm-
.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Vin-
Vin+
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
in-
and V
S+
R1
R3
and R
in+
3
, any mismatch in
in Figure 5 are no
S-
+
-
will cause the
R4
R2
Vout
High
put impedance (both common-mode and differential)
of the stage because the load seen by the source is
decoupled by OA
(OA
Z
match in source impedance would require R
R
values of R
ance common mode signals as well, so these resis-
tors would ideally be trimmed just like the resistors
in the single opamp stage of Figure 3. Unfortunately
for this approach, it is difficult and expensive to
make precision trimmed resistors with such high val-
ues.
plifiers OA
their input currents must be extremely low if they are
not to cause significant offsets. Practically, this neces-
sitates using FET input stages for OA
While FETs may be a viable alternative, it is difficult
to achieve with them the low noise performance of
modern bipolar input stages.
Transformer Input Stages
impedance, as well as that of electrical isolation, a
transformer in front of the first active input stage is
really the best possible solution. Transformers are
the only approach of which we are aware that pro-
vides true electrical isolation with reasonable fidelity.
Furthermore, their common-mode input impedance
is easily extremely high (tens of Megohms), and al-
most completely decoupled from their differential in-
put impedance.
They do not offer dc coupling, and suffer from satu-
ration at low frequencies unless they are physically
large and carefully made.
carefully made (which usually equates to high cost),
they introduce phase shift at high audio-band fre-
quencies.
heavy and pick up external magnetic fields, some-
CM- = Ri1
i2
In+
In-
To retain 90 dB CMRR in the face of a 10 Ω mis-
Furthermore, since the input bias current for am-
From the point of view of common mode input
But, transformers have many other limitations.
to be > 317 kΩ. Of course, any difference in the
3
along with R
Ri2
Ri1
, and Z
i1
1
Figure 6. Instrumentation amplifier
Furthermore, they tend to be big and
and R
and OA
CM+
+
+
OA1
-
-
OA2
1
1
i2
and OA
, R
= R
2
themselves would further unbal-
2
flows through these resistors,
, R
i2
.
3
2
, and R
from the balanced stage
Again, unless they are
R1
R3
4
). In this circuit,
+
OA3
-
R2
R4
1
and OA
i1
Page 5
Out
and
2
.