1646 Просмотр технического описания (PDF) - THAT Corporation
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1646 Datasheet PDF : 12 Pages
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Document 600078 Rev 05
Page 6 of 12
THAT1606/1646 Balanced Line Driver ICs
out of control. This causes the OutSmarts design to
more closely mimic the behavior of a true floating
balanced source (such as a transformer), compared
to the behavior of a conventional CCOS design.
Applications
1. Circuit implementations using the 1606 and
1646 are relatively straightforward. A quiet, solid
ground reference, stiff voltage supplies, and adequate
supply bypassing are all that are required to achieve
excellent performance out of both ICs.
2. Both devices must be driven from a
low-impedance source, preferably directly from
opamp outputs, to maintain the specified perfor-
mance.
3. Please refer to Figure 8 for a complete applica-
tions circuit.
Stability and Load Capacitance
The devices are stable into any capacitive load,
and the maximum capacitance is limited only by slew
rate and frequency response considerations.
For the purposes of the frequency response calcu-
lation, the line driver's 25W sense resistors can be
lumped into a single 50W resistor. The correct cable
capacitance to use for the balanced-signal case is the
sum of the inter-conductor capacitance and the two
conductor-to-shield capacitances in series. Some
manufacturers only specify the inter-conductor ca-
pacitance and the capacitance of one conductor to
the other while connected to the shield, and some ex-
traction may be required.
As an example, Belden 8451 is specified as hav-
ing with 34 pF/ft of inter-conductor capacitance and
67 pF/ft of conductor to "other conductor + shield
capacitance". Thus, we can assume a single conduc-
tor-to-shield capacitance of 33 pF/ft (the difference
between 67 and 34) for each conductor. For balanced
signals, the load capacitance across the 1646 outputs
will be 34 pF/ft + 16.5 pF/ft = 50.5 pF/ft. The corner
frequency of the THAT 1646 driving 500 ft of this ca-
ble (25.25 nF) will be 126 kHz.
fC
=
2×
p
×
50W ×
1
500 (34 pF
ft+16.5 pF
ft)
= 126kHz
One must also consider the slew rate limitations
posed by excessive cable and other capacitances. We
know that
i
=
C
dV
dt
and that
dV
dt
=
VPeak
2×
p×
f
Dennis Bohn of Rane Corporation has published
work specifying some of the requirements for a bal-
anced line driver, including a) stability into reactive
loads, b) differential output voltage swing of at least
±11 volts peak (+20dBu), and c) reliability5. This
Vcc
C4
100n
In
C5
100n
6
7
4
Vcc
In Sns+
8
Out+
3
Gnd Sns-
Vee
Out-
1
2 U1
5
THAT1646
Vee
Out+
Out-
Figure 4. THAT 1646 without common-mode offset reduction
5. Dennis A. Bohn, “Practical Line-Driving Current Requirements“(Rane Note 126), Rane Corporation, 1991, revised 5/1996. Available at www.rane.com/note126.html.
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