bias currents since their input bias flows through Ri1
and Ri2. Because of the difficulty of maintaining low
noise with low input bias currents, FET op amps
may be employed, but they impose their own limita-
tions, as described further on.
The THAT 1200 series of balanced line receivers
overcomes this problem by way of an AC bootstrap
technique, shown in simplified form in Figure 12. By
driving the lower end of R2 to nearly the same AC
voltage as the upper end, AC current flow through R2
is greatly reduced, effectively increasing its value. At
DC, of course, the input impedance Z is simply R1 +
R2.
If gain G is unity, for frequencies within the
passband of the high-pass filter formed by Cb and
R1, the effective value of the input impedance is in-
creased to infinity at sufficiently high frequencies.
Input impedance Z, at frequency f, is described
the following equation:
ZR
R
G
i
f
fn
f
fD
=+
+
+-
()
()
() (
)
12
2
22
1
11
where
f
C
f
RC
N
RR
RR
D
==
´
+
1
2
1
2
12
12
1
p
p
()
,
For example, if R1 and R2 are 10 k
W each, Z
DC is
20 k
W. This resistance provides a DC path for am-
plifier bias current as well as leakage current that
might flow from a signal source. At higher frequen-
cies, the bootstrap greatly increases the input imped-
ance,
limited
ultimately
by
how
close
gain
G
approaches unity. With the THAT 1200 input stages,
common-mode input impedances of several meg-
ohms across much of the audio spectrum can be ex-
pected.
Figure 1 shows a complete equivalent circuit for
the THAT 1200-series ICs.
OA1 and OA2 are
high-impedance buffers feeding differential amplifier
OA3 in an instrumentation amplifier configuration.
The common mode signal is extracted at the junction
of Rc and Rd, buffered by OA4, and fed back to both
inputs via capacitor Cb and resistors Ra and Rb.
The junction of Ra, Rb and R5 is driven to the same
potential as the common-mode input voltage. Hence
no common-mode current flows in resistors Ra and
Rb. Since, ideally, no current flows, the input imped-
ance to common mode signals is infinite.
The effectiveness of this topology is limited by the
unity gain precision of OA4 and the input imped-
ances of OA1 and OA2, all of which are optimized in
THAT’s integrated circuit process.
Note that OA1
and OA2 isolate OA3 from external source imped-
ances. Therefore, the performance of the differential
amplifier OA3 and its associated components are not
affected by imbalances in the source impedances.
Alternatives
In the following section we will compare other so-
lutions for minimizing CMRR degradation in the
presence of source impedance mismatch, and con-
trast them with THAT’s InGenius topology.
Precision 4-resistor op amp stage
This stage (Figure 8) was discussed earlier.
To
summarize, this solution offers high common-mode
rejection only when the source impedances are per-
fectly balanced, or a tiny fraction of the common-
mode input impedance.
Because differential- and
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
Page 8
InGenius Balanced Line Receiver
Preliminary Information
Z
R2
R1
G=1
Cb
Figure 12. InGenius bootstrap topology
Out
+In
-In
+
-
+
-
+
-
OA1
OA2
OA3
Ri1
Ri2
Figure 11. Instrumentation amplifier
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