THAT2181 Datasheet, PDF (7/10 Pages) List of Unclassifed Manufacturers – Trimmable IC Voltage Controlled Amplifiers

600030 Rev 01
Page 7
DC Coupling
The quiescent dc voltage level at the input (the input
offset voltage) is approximately 0 V, but, as in many gen-
eral-purpose opamps, this is not well controlled. Any dc
input currents will cause dc in the output which will be
modulated by gain; this may cause audible thumps. If
the input is dc coupled, dc input currents may be gener-
ated due to the input offset voltage of the 2181 itself, or
due to offsets in stages preceeding the 2181. Therefore,
capacitive coupling is almost mandatory for quality au-
dio applications. Choose a capacitor which will give ac-
ceptable low frequency performance for the application.
Summing Multiple Input Signals
Multiple signals may be summed via multiple resis-
tors, just as with an inverting opamp configuration. In
such a case, a single coupling capacitor may be located
next to pin 1 rather than multiple capacitors at the
driven ends of the summing resistors. However, take
care that the capacitor does not pick up stray signals.
Output
The Output pin (pin 8) is intended to be connected
to a virtual ground node, so that current flowing in it
may be converted to a voltage (see Figures 2 & 15).
Choose the external opamp for good audio performance.
The feedback resistor should be chosen based on the de-
sired current-to-voltage conversion constant. Since the
input resistor determines the voltage-to-current conver-
sion at the input, the familiar ratio of R_f /R_i for an invert-
ing opamp will determine the overall voltage gain when
the 2181 is set for 0 dB current gain. Since the VCA per-
forms best at settings near unity gain, use the input and
feedback resistors to provide design-center gain or loss,
if necessary.
A small feedback capacitor around the output
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
The positive supply is connected directly to V+
(pin 7). No special bypassing is necessary, but it is good
practice to include a small (~1 mf) electrolytic or
(~0.1 mf) ceramic capacitor close to the VCA IC on the
PCB. Performance is not particularly dependent on sup-
ply voltage. The lowest permissible supply voltage is de-
termined by the sum of the input and output currents
plus I_SET , which must be supplied through the output
of the internal transconductance amplifier and down
through the core and voltage bias generator. Reducing
signal currents may help accommodate low supply volt-
ages. THAT Corporation intends to publish an applica-
tion note covering operation on low supply voltages.
Please inquire for its availability.
The highest permissible supply voltage is fixed by the
process characteristics and internal power consumption.
+18 V is the nominal limit.
Negative
The negative supply terminal is V- (pin 5). Unlike
normal negative supply pins, this point is intended to be
connected to a current source I_set (usually simply a re-
sistor to V _EE), which determines the current available
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 mA over the sum of the re-
quired signal currents. Usually, I_set 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.
Mathematically, this can be expressed as
I_cell ³ Peak (I_in) + Peak (I_out) + 220 mA; and
I_cell = I_set - 350 mA. Therefore,
I_set ³ Peak (I_in) + Peak (I_out) + 570 mA.
The voltage at V- (pin 5) is four diode drops 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
The GND pin (pin 6) is used as a ground reference
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 addi-
tional input pin.
Voltage Control
Negative Sense
E_C- (pin 3) is the negative voltage control port. This
point controls gain inversely with applied voltage: posi-
tive 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 tem-
perature.
Positive Sense
As mentioned earlier, E_C+ (pin 2) is the posi-
tive-sense voltage control port. A typical circuit using
this approach is shown in Figure 15. E_C- (Pin 3) should
be grounded, and E_C+ (pin 2) driven from a
low-impedance voltage source. Using the opposite sense
of control can sometimes save an inverter in the control
path.
Positive and Negative
It is also possible (and sometimes advantageous) 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.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com