1646 データシートの表示（PDF） - THAT Corporation
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1646 Datasheet PDF : 12 Pages
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THAT1606/1646 Balanced Line Driver ICs
Page 7 of 12
Document 600078 Rev 05
work suggests a reasonable rule by which to calculate
the output current requirements at 20kHz. The au-
thor concludes that the actual worst case peak level
for various types of music and speech will be flat out
to 5kHz, and roll off at 6dB/octave above this fre-
quency. Thus the peak levels at 20kHz will be 12dB
below those at 5kHz.
Using these, we can calculate the required slew
rate and current drive. For the +26 dBu output lev-
els that the 1646 is capable of, VPeak is 22V (below
5kHz), and at 20kHz, VPeak is 5.5V. Therefore,
dV
dt
=
2p ×
5.5V ×
20kHz
=
0.69
V
ms
As a consequence,
i = 500 ft ×
(34
pF
ft
+ 16.5
pfFt ) ×
0.69
V
ms
= 17.5mA
.
Thus, driving this 25.25 nF cable requires 17.5
mAPeak, which is well within the capability of the
1606 and 1646.
Gain structure
The 1606 and 1646 both provide +6 dB gain (fac-
tor of 2) between their inputs and differential out-
puts. This is appropriate, since with a balanced
output, twice the voltage between the power supply
rails is available at the output of the stage. The sin-
gle-ended input of the 1646 can accept signals that
swing to nearly the power supply rails without distor-
tion, when driving into a differential (floating) load.
The balanced input of the 1606 can accept signals at
each input that swing to nearly one-half the power
supply rails without distortion, when driving into dif-
ferential loads.
Both devices, when driving single-ended loads,
will clip at about half the output voltage as compared
to a differential load. This is because only one of the
two output signals will be available. Despite the out-
put clipping, the input to the devices does not need to
be constrained - they will work without undue prob-
lems being overdriven at their inputs when the out-
puts are clipping into single-ended loads.
1646 circuits
Figure 4 shows the most basic connection for a
1646. The only external components needed are the
local 100nF bypass capacitors. These should be
within 1 inch of the 1646 pins.
Output DC offset
Because the 1646's outputs are connected directly
to their respective sense inputs, this circuit may pro-
duce up to 250mV of common-mode dc offset at its
outputs. As shown, the outputs are DC coupled to
the output connector, so this dc will appear directly
at the output of the system.
The output common-mode offset of a 1646 may
be reduced by adding capacitors in the feedback
loop, as shown in Figure 5. Capacitors C1 and C2
ac-couple the common-mode feedback loop. This
changes the loop operation from servoing the com-
mon-mode output current at audio frequencies to
servoing the common-mode output voltage to 0 at
DC. This results in much lower common-mode out-
C4
100n
In
C5
100n
Vcc
C1
6
7
10u
NP
4
Vcc
In Sns+
8
Out+
3
Gnd Sns-
Vee
Out-
1
2 U1
5
THAT1646
C2
Vee
10u
NP
Out+
Out-
Figure 5. THAT 1646 with common-mode offset reduction
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