THAT2180A Datasheet, PDF (7/8 Pages) List of Unclassifed Manufacturers – Pre-Trimmed IC Voltage Controlled Amplifiers

600029 Rev 01
Page 7
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 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
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
Vcc
2180
Series
VCA
7
V+ ³
IN
10u
20k
1
Ec-
-IN
SYM
VG-₅^ND₆^Ec+2_NC⁴
OUT
8
Power Supplies 5.1k
Vcc = +15 V
Vee = -15 V
Vee Ec+
22p
20k
-
OP275
+
OUT
Figure 14. Positive Control Port Using Pin 2 (Ec+)
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 14. 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. In order to maintain the wafer level adjustment
which minimizes THD, leave pin 4 open.
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.
There is no reason why both control ports cannot be
used simultaneously.
Control Port Drive Impedance
The control ports (pins 2 through 4) are connected
directly to the bases of the logging and/or antilogging
transistors. The accuracy of the logging and antilogging
is dependent on the E_C+ and E_C- voltages being exactly
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.
The 2180 Series VCAs are designed to be operated
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.
This often suggests driving the control port directly
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.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com