2181-DEMO THAT Corporation, 2181-DEMO Datasheet - Page 8

• Manufacturer Part Number: 2181-DEMO
• Description: Audio Modules & Development Tools Trimmable VCA Demo Board
• Manufacturer: THAT Corporation

Specifications of 2181-DEMO

• Description/function: Audio Amplifiers
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT2181
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Document 600030 Rev 02
the familiar ratio of R
determine the overall voltage gain when the 2181 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 8 is typically
15 pF.
Power Supplies
Positive
(pin 7). No special bypassing is necessary, but it is
good practice to include a small (~1 μf) electrolytic
or (~0.1 μf) ceramic capacitor close to the VCA IC on
the PCB. Performance is not particularly dependent
on supply voltage. The lowest permissible supply
voltage is determined by the sum of the input and
output currents plus I
through the output of the internal transconductance
amplifier and down through the core and voltage bias
generator. Reducing signal currents may help accom-
modate low supply voltages. THAT Corporation
intends to publish an application note covering
operation on low supply voltages. Please inquire for
its availability.
the process characteristics and internal power
consumption. +18 V is the nominal limit.
Negative
normal negative supply pins, this point is intended to
be connected to a current source I
a resistor to V
able for the device. As mentioned before, this source
must supply the sum of the input and output signal
currents, plus the bias to run the rest) of the IC. The
minimum value for this current is 570 μA over the
sum of the required signal currents. Usually, I
should equal 2.4 mA for most pro audio applications
with ±15 V supplies. Higher bias levels are of limited
value, largely because the core transistors become
ineffective at logging and antilogging at currents over
1 mA.
A small feedback capacitor around the output
The positive supply is connected directly to V+
The highest permissible supply voltage is fixed by
The negative supply terminal is V- (pin 5). Unlike
EE
), which determines the current avail-
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
f
/R
SET
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
i
for an inverting opamp will
, which must be supplied
SET
(usually simply
Copyright © 2008, THAT Corporation
SET
Page 8 of 12
below ground, which, for the 2181, is approximately
-2.85 V. Since this pin connects to a (high
impedance) current supply, not a voltage supply,
bypassing at pin 5 is not normally necessary.
Ground
ence for the VCA. The non-inverting input of the
internal opamp is connected here, as are various
portions of the internal bias network. It may not be
used as an additional input pin.
Voltage Control
Negative Sense
This point controls gain inversely with applied
voltage: positive voltage causes loss, negative voltage
causes gain. As described on Page 5, the current gain
of the VCA is unity when pin 3 is at 0 V with respect
to pin 2, and varies with voltage at approximately
-6.1 mV/dB, at room temperature.
Positive Sense
sense voltage control port. A typical circuit using this
approach is shown in Figure 15. E
grounded,
low-impedance voltage source. Using the opposite
sense of control can sometimes save an inverter in
the control path.
Positive and Negative
to drive both control ports, either with differential
drive (in which case, the control sensitivities of each
port are summed), or through two different control
signals. There is no reason why both control ports
cannot be used simultaneously.
Mathematically, this can be expressed as
I
I
I
The voltage at V- (pin 5) is four diode drops
The GND pin (pin 6) is used as a ground refer-
E
As mentioned earlier, E
It is also possible (and sometimes advantageous)
CELL
CELL
SET
C-
≥ Peak (I
(pin 3) is the negative voltage control port.
≥ Peak (I
= I
SET
and
- 350 μA. Therefore,
IN
IN
) + Peak (I
) + Peak (I
E
C+
Blackmer® Trimmable IC VCAs
(pin
C+
OUT
OUT
(pin 2) is the positive-
) + 570 μA.
2)
) + 220 μA; and
C-
driven
THAT 2181 Series
(Pin 3) should be
from
a