that1570 THAT Corporation, that1570 Datasheet - Page 7

• Manufacturer Part Number: that1570
• Description: Low-noise, Differential Audio Preamplifier Ic
• Manufacturer: THAT Corporation

THAT1570 Low-Noise
Differential Audio Preamplifier
against ESD, they should not be relied upon to
protect against excessive input voltage, which can
result in significant current flow. This is a particular
problem when the preamplifier includes a source of
+48 V phantom power (see text below) but can be of
concern in any situation where the input may be
connected to high signal levels, in which the input
signal voltage could exceed the supply rails.
phantom power protection networks shown in
Figures 7 and 8 are worth considering even if
phantom power is not included in the design.
DC Offsets and C
Figure 2 and 3, the differential dc level at the output
of the 1570 will vary with gain. In most such applica-
tions, the output should be ac-coupled to the next
stage, in order to eliminate this varying offset.
recommends that R
Figures 5 and 6. (Figure 5 corresponds to Figure 2,
while Figure 6 corresponds to Figure 3.) By adding
C
constrains the differential output dc offset to just
over ±5 mV, and prevents it from varying with gain.
low-frequency response at the smallest values of R
With the values shown in Figures 5 and 6, the -3 dB
G
in series with R
However, while the internal diodes are effective
Because R
For applications where gain is variable, THAT
C
G
must be large enough not to interfere with
IN+
IN-
G
Figure 6. 1570 Typical Application Circuit With Dual-Gang Pot for Gain Control, AC-Coupled R
is dc coupled in the circuits of
G
G
PROTECTION
, dc gain is fixed at unity. This
G
be ac-coupled as shown in
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THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
RFI
Copyright © 2009, THAT Corporation; All rights reserved.
R1
1k0
R2
1k0
3300u
R
8.66
C
GF
G
The
G
R
R
Page 7 of 12
.
GV1
GV2
5k
5k
CW
CW
corner is about 5 Hz.
3,300 μF to maintain this low-frequency corner.
very small. Since polarized electrolytic capacitors
normally can withstand some small reverse bias, C
can usually be a polarized capacitor.
Phantom Power
microphones. THAT recommends the circuits of
Figure 7 and 8 when phantom power is included. R
R
flows through the 1570 inputs when overloaded.
These also protect the 1570 when the circuit inputs
(IN+ and IN-) are shorted to ground while phantom
power is turned on. This causes C4 and/or C5 to dis-
charge through other circuit components (including
the 1570 inputs), often generating transient currents
of several amps. R
limit destructive currents. (Higher values further
limit current flow, but introduce additional source
impedance and noise.) Take care to ensure that the
resistors used can handle the short-term inrush
current; many small surface-mount types cannot. D
through D
exceeding the supply rails.
the supply rails.
glass-passivated types (sometimes called “GP”) to
ensure low leakage. (Leakage manifests itself as noise
4
, and D
Note also that the voltage appearing across C
Phantom power is required for many condenser
D
THAT
1570
1
~D
IN1
R
R
IN2
4
G
G
1
4
prevent the input voltage from exceeding
1
2
prevent the IC’s inputs from significantly
~ D
2k5
2k5
R
+15V
R
-15V
Vcc
Vee
10k
R3
A
B
10k
R4
6
100n
100n
C10
OUT1
OUT2
are used to limit the current that
C9
3
For best results, they should be
and R
Both circuits require a C
4
should be at least 10 Ω to
Document 600132 Rev 01
G
OUT+
OUT-
G
G
of
is
3
G
1
,