356 Chapter 12
megohm range of transformers. This overcomes the loss
of common-mode rejection when the impedances
feeding the line receiver are slightly unbalanced and
permits transformer like operation. The InGenius circuit
will be discussed in a following section.
THAT also offers the THAT1646 balanced line
driver, which has identical pinout to the SSM2142 and
DRV134/135. THAT’s 1606 balanced line driver is
unique among these parts in that it provides not only a
differential output, but also a differential input—
enabling a more direct connection to digital to analog
converters.
The THAT1646 and 1606 use a unique output
topology unlike conventional cross-coupled outputs
which THAT calls “OutSmarts” (another trademark).
OutSmarts is based on U.S. Patent 4,979,218 issued to
Chris Strahm, then of Audio Teknology Incorporated.^35
Conventional cross-coupled outputs lose common-mode
feedback when one output is shorted to ground to
accommodate a single-ended load. This allows large
signal currents to flow into ground, increasing crosstalk
and distortion. Strahm’s circuit avoids this by using an
additional feedback loop to provide current feedback.
Application circuits for the THAT1646 will be
described in the section “Balanced Line Outputs.”
12.3.6.4 Balanced Line Input Application Circuits
Conventional balanced line receivers from Analog
Devices, Texas Instruments, and THAT Corporation are
substantially equivalent to the THAT1240 circuit shown
in Fig. 12-61. Some variations exist in the values of
R 1 – R 4 from one manufacturer to the other that will
influence input impedance and noise. The ratio of R 1 /R 3
to R 2 /R 4 establishes the gain with R 1 =R 3 and R 2 =R 4.
Vout is normally connected to the sense input resistor
with the reference pin grounded.
Line receivers usually operate at either unity gain
(SSM2141, INA134, THAT1240, or THAT1250) or in
attenuation (SSM2143, INA137, THAT1243, or
THAT1246, etc.). When a perfectly balanced signal
(with each input line swinging ½ the differential
voltage) is converted from differential to single-ended
by a unity gain receiver, the output must swing twice
the voltage of either input line for a net voltage gain of
+6 dB. With only +21 dBu output voltage available
from a line receiver powered by bipolar 15 V supplies,
additional attenuation is often needed to provide head-
room to accommodate pro audio signal levels of
+24 dBu or more. The ratios R 1 /R 2 and R 3 /R 4 are 2:1 in
the SSM2143, INA137, and THAT1246 to provide 6 dB
attenuation. These parts accommodate up to +27 dBu
inputs without clipping their outputs when running from
bipolar 15 V supplies. The THAT1243, and THAT’s
other ±3 dB parts (the 1253, 1283, and 1293) are unique
with their 0.707 attenuation. This permits a line receiver
that accommodates +24 dBu inputs but avoids addi-
tional attenuation that increases noise. A 3 dB line
receiver is shown in Fig. 12-62.The ±6 dB parts from all three manufacturers (and
the ±3 dB parts from THAT) may be configured for gain
instead of attenuation. To accomplish this, the reference
and sense pins are used as inputs with the In– pin
connected to Vout and the In+ pin connected to ground.
A line receiver configured for 6 dB gain is shown in
Fig. 12-63.
Balanced line receivers may also be used to provide
sum-difference networks for mid-side (M/S or M-S)
encoding/decoding as well as general-purpose applica-
tions requiring precise difference amplifiers. Such
applications take advantage of the precise matching of
resistor ratios possible via monolithic, laser-trimmedFigure 12-61. THAT1240 with 0 dB gain. Courtesy THAT
Corporation.Figure 12-62. THAT1243 with 3 dB attenuation. Courtesy
THAT Corporation.63 12+In–In 9k 9k 59k 9kU1 1240Output74SenseVoutRefVCCVEEIn+In– R 1R 3R 2R 463 12+In- In 10.5k 7.5k 5
10.5k 7.5kU1 1243Output74SenseVoutRefVCCVEEIn+In–