that2162 THAT Corporation, that2162 Datasheet - Page 7

• Manufacturer Part Number: that2162
• Description: Dual Pre-trimmed Blackmer Voltage Controlled Amplifier
• Manufacturer: THAT Corporation

Document 600087 Rev 00G
Input
2162’s VCA IN pins. These pins are virtual grounds
with negative feedback provided internally.
input resistor R
convert the available ac input voltage to a current
within the linear range of the device. Generally, peak
input currents should be kept under 750 μA for best
distortion performance.
distortion typically varies with signal level for 0 dB,
+15 dB and -15 dB gain. The circuit of Figure 2,
Page 3 was used to generate these curves.
tion will usually determine the maximum signal
current level which may be used. Note that, with
20 kΩ current-to-voltage converting resistors, distor-
tion remains low even at 10 V rms input at 0 dB or
-15 dB gain, and at 1.7 V rms input at +15 dB gain
(~10 V rms output).
AC Coupling
dc offset away from ground. It is important to
prevent this dc offset from becoming a dc current in
the input, since any dc input currents will be
modulated by gain changes, thereby becoming
audible as thumps. To prevent the dc input offset
voltage and the previous stage’s dc output offset from
causing dc input currents, the input pins are
normally ac-coupled (C
such offset currents and reduces dc offset variation
with gain. Choose a capacitor which will give accept-
able low frequency performance for the application.
The mean offset voltage is slightly negative, so if a
polarized capacitor is used, it should be oriented
with the negative side toward the VCA input.
Summing Multiple Input Signals
resistors, just as with an inverting opamp configura-
tion. In such a case, a single coupling capacitor may
be located next to pin 1 rather than multiple capaci-
tors at the driven ends of the summing resistors.
However, take care that the capacitor does not pick
up stray signals.
Stability
source impedance seen at the VCA IN terminal must
be less than 5 kΩ above approximately 250 kHz.
The R
this.
Output
current, inverted with respect to the input current. In
Input signals are currents in pin 7 [10] -- the
Refer to Figures 10 through 12 to see how
For a specific application, the acceptable distor-
Pin 7 [10], the VCA IN pin will also have a small
Multiple signals may be summed via multiple
In order to guarantee stability at low gains, the
The VCA output signal, at pin 2 (15), is also a
4
-C
4
and R
3
(R
8
-C
7
) in Figure 2 should be scaled to
8
networks in Figure 2 ensure
2
, C
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7
in Figure 2). This blocks
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Applications
The
normal operation, the output current is connected to
a virtual ground node, and converted to a voltage via
an external op-amp. The current-to-voltage conver-
sion ratio is determined by the feedback resistor, R
[R
output and its inverting input. The resulting signal
path through the VCA plus op-amp is non-inverting.
voltage-to-current conversion at the input, and R
(R
to-voltage conversion rate at the output. As a result,
the familiar ratio of Rf /Ri for an inverting opamp will
determine the overall voltage gain when the 2162 is
set for 0 dB current gain. Since the VCA performs
best at settings near unity gain, use the input and
feedback resistors to provide design-center gain or
loss, if necessary.
opamp is needed to cancel the output capacitance of
the VCA. Without it, this capacitance will destabilize
most opamps. The capacitance at pin 2 [15] is
typically 3 pF The 22 pF capacitor shown at C
ensures stability.
Voltage Control
between pin 6 [11] -- E
that any unused control ports should be connected to
ground (as E
is proportional to (E
tionality is 6.4 mV/dB for the voltage at E
to E
or E
ground.
Positive and Negative
lower offset change at very low gains than the E
port. For best performance with large attenuations
both control ports can be utilized simultaneously
with differential drives.
Symmetry
under “Triming”, Pin 3 [14] -- the SYM pin -- can be
used to improve the preprogrammed distortion
setting, allowing for finer resolution than available
on-chip, and for shifts that may occur during IC
packaging.
circuitry is shown in Figure 13. The wiper resistor
R1, shown as 510 kΩ, is recommended for the
supplies shown.
scale R1 directly proportional to the supply voltage.
with a modest level (e.g., ~1 Vrms), low-frequency
(e.g., ~1 kHz) sine wave input. Since the SYM pins
are connected to internal bias generators, if an exter-
nal symmetry adjustment is omitted, leave the SYM
pins open.
6
6
] in Figure 2 connected between the op-amp's
) -- the output resistor -- determines the current-
C-
R3 [R7] -- the input resistor -- determines the
A small feedback capacitor around the output
The VCA gain is controlled by the voltage applied
C-
Note for Figures 8 and 9 that the E
As described more fully in the Theory section
Adjust the Symmetry control for minimum THD
). See Figure 5 through 7. Note that neither E
should be driven more than ±0.6 V away from
C+
The
is in Figure 2). The gain (in decibels)
For other power supply voltages,
recommended
C+
- E
C+
C-
and pin 4 [13] -- E
). The constant of propor-
additional
C-
Page 7 of 12
port yields
C+
(relative
C-
. Note
1
±
trim
15V
(C
C+
C+
6
2
2
)