that2180a THAT Corporation, that2180a Datasheet - Page 7

• Manufacturer Part Number: that2180a
• Description: Pre-trimmed Ic Voltage Controlled Amplifiers
• Manufacturer: THAT Corporation

600029 Rev 01
ply voltage. The lowest permissible supply voltage is de-
termined by the sum of the input and output currents
plus I
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.
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
sistor to V
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
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.
ground, which, for the 2180, 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
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
IN
The highest permissible supply voltage is fixed by the
The negative supply terminal is V- (pin 5). Unlike
Mathematically, this can be expressed as
I
I
I
The voltage at V- (pin 5) is four diode drops below
The GND pin (pin 6) is used as a ground reference
Figure 14. Positive Control Port Using Pin 2 (Ec+)
cell
cell
set
Power Supplies
SET
Vcc = +15 V
Vee = -15 V
³ Peak (I
³ Peak (I
= I
10u
, which must be supplied through the output
set
EE
- 350 mA. Therefore,
), which determines the current available
in
Series
in
2180
20k
VCA
) + Peak (I
) + Peak (I
5.1k
Vee
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
1
Vcc
7
V+
-IN
V-
GND
5
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
6
Ec+
Ec-
Ec+
3
SYM
2
out
NC
out
4
) + 570 mA.
OUT
) + 220 mA; and
8
set
set
(usually simply a re-
should equal 2.4 mA
+
-
OP275
22p
20k
OUT
Negative Sense
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
tive-sense voltage control port. A typical circuit using
this approach is shown in Figure 14. E
be
low-impedance voltage source. Using the opposite sense
of control can sometimes save an inverter in the control
path. In order to maintain the wafer level adjustment
which minimizes THD, leave pin 4 open.
Positive and Negative
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.
Control Port Drive Impedance
directly to the bases of the logging and/or antilogging
transistors. The accuracy of the logging and antilogging
is dependent on the E
as desired to control gain. The base current in the core
transistors will follow the collector currents, of course.
Since the collector currents are signal-related, the base
currents are therefore also signal-related. Should the
source impedance of the control voltage(s) be large, the
signal-related base currents will cause signal-related
voltages to appear at the control ports, which will inter-
fere with precise logging and antilogging, in turn causing
distortion.
with zero source impedance at pins 2 and 3, and an
infinite source impedance at pin 4. (Pin 4 should be left
open.) To realize all the performance designed into a
2180, keep the source impedance of the control voltage
driver well under 50 W.
with an opamp. However, the closed-loop output imped-
ance of an opamp typically rises at high frequencies be-
cause open loop gain falls off as frequency increases. A
typical opamp's output impedance is therefore inductive
at high frequencies. Excessive inductance in the control
port source impedance can cause the VCA to oscillate in-
ternally. In such cases, a 100 W resistor in series with a
1.5 nf capacitor from the control port to ground will
usually suffice to prevent the instability.
E
As mentioned earlier, E
It is also possible (and sometimes advantageous) to
The control ports (pins 2 through 4) are connected
The 2180 Series VCAs are designed to be operated
This often suggests driving the control port directly
grounded,
C-
(pin 3) is the negative voltage control port. This
and
C+
E
C+
and E
(pin
C+
C-
(pin 2) is the posi-
voltages being exactly
2)
C-
driven
(Pin 3) should
from
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