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Datasheet: THAT2180C (THAT Corp.)

Pre-Trimmed IC Voltage Controlled Amplifiers

 

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THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
BIAS CURRENT
COMPENSATION
Vbe
MULTI-
PLIER
Output
Sym
Iset
V-
Vcc
Ec+
Ec-
Iadj
Input
Gnd
7
2
3
8
4
5
1
6
2k
25
Figure 1. 2180 Series Equivalent Circuit Diagram
Pin Name
SIP Pin
Input
1
Ec+
2
Ec
3
Sym
4
V
5
Gnd
6
V+
7
Output
8
Table 1. 2180 Series Pin Assignments
FEATURES
Wide Dynamic Range: >120 dB
Wide Gain Range: >130 dB
Exponential (dB) Gain Control
Low Distortion: < 0.01 % (2180A)
Wide Gain-Bandwidth: 20 MHz
Dual Gain-Control Ports (pos/neg)
Pin-Compatible with 2150-Series
APPLICATIONS
Faders
Panners
Compressors
Expanders
Equalizers
Filters
Oscillators
Automation System
Description
THAT 2180 Series integrated-circuit voltage con-
trolled amplifiers (VCAs) are very high-performance
current-in/current-out
devices
with
two
oppos-
ing-polarity, voltage-sensitive control ports. They offer
wide-range exponential control of gain and attenuation
with low signal distortion. The parts are trimmed at
wafer stage for low THD and control-voltage
feedthrough without further adjustment.
The VCA design takes advantage of a fully comple-
mentary dielectric isolation process which offers
closely matched NPN/PNP pairs, to deliver discrete
performance at IC prices. The parts are available in
three grades, selected for factory trimmed distortion,
alowing the user to optimize cost vs. performance. The
2180 Series is available in an 8-pin single-in-line (SIP)
package.
Max THD @1 V,
1 kHz, 0 dB
Plastic
SIP
0.01%
2180LA
0.02%
2180LB
0.05%
2180LC
Table 2. Ordering Information
T H A T
C o r p o r a t i o n
THAT 2180A, 2180B, 2180C
Pre-Trimmed IC
Voltage Controlled Amplifiers
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Page 2
THAT2180 Series IC VCAs
SPECIFICATIONS
1
Absolute-Maximum Ratings (T
A
= 25C)
Positive Supply Voltage (V
CC
)
+20 V
Negative Supply Voltage (V
EE
)
-20 V
Supply Current (I
CC
)
10 mA
Max DE
C
E
C+
- (E
C-
)
1V
Power Dissipation (P
D
) (T
A
= 75C)
330 mW
Operating Temperature Range (T
OP
)
0 to +70C
Storage Temperature Range (T
ST
)
-40 to +125C
Recommended Operating Conditions
2180A
2180B
2180C
Parameter
Symbol
Conditions
Min Typ
Max
Min Typ
Max
Min Typ
Max Units
Positive Supply Voltage V
CC
+4 +15 +18
+4 +15 +18
+4 +15 +18
V
Negative Supply Voltage V
EE
-4
-15 -18
-4
-15 -18
-4
-15 -18
V
Bias Current
I
SET
V
CC
- V
EE
= 30 V
1
2.4
5
1
2.4
5
1
2.4
5
mA
Signal Current
I
IN
+I
OUT
I
SET
= 2.4mA
-- 0.35 1.5
-- 0.35 1.5
-- 0.35 1.5
mA
rms
Electrical Characteristics
2180A
2180B
2180C
Parameter
Symbol
Conditions
Min Typ
Max
Min Typ
Max
Min Typ
Max Units
Supply Current
I
CC
No Signal
--
2.4
4
--
2.4
4
--
2.4
4
mA
Equiv. Input Bias Current I
B
No Signal
--
2
10
--
2
12
--
2
15
nA
Input Offset Voltage
V
OFF(IN)
No Signal
--
5
--
--
5
--
--
5
--
mV
Output Offset Voltage V
OFF(OUT)
R
out
= 20 kW
0 dB gain
--
0.5
1
--
1
2
--
1.5
3
mV
+15 dB gain
--
1
3
--
1.5
4
--
3
10
mV
+30 dB gain
--
3
12
--
5
15
--
9
30
mV
Gain Cell Idling Current I
IDLE
--
20
--
--
20
--
--
20
--
mA
Gain-Control Constant
T
A
=25C (T
CHIP
@35C)
-60 dB < gain < +40 dB
E
C+
/Gain (dB)
Pin 2 (Fig. 14)
6.0
6.1
6.2
6.0
6.1
6.2
6.0
6.1
6.2
mV/dB
E
C-
/Gain (dB)
Pin 3
-6.2 -6.1 -6.0
-6.2 -6.1 -6.0
-6.2 -6.1 -6.0 mV/dB
Gain-Control TempCo DE
C
/ DT
CHIP
Ref T
CHIP
= 27C
-- +0.33 --
-- +0.33 --
-- +0.33 --
%/C
Gain-Control Linearity
-60 to +40 dB gain
--
0.5
2
--
0.5
2
--
0.5
2
%
1 kHz Off Isolation
E
C+
= -360 mV, E
C-
= +360 mV 110 115
--
110 115
110 115
--
dB
Output Noise
e
n(OUT)
20 Hz ~ 20 kHz
R
out
= 20kW
0 dB gain
--
-98 -97
--
-98 -96
--
-98 -95
dBV
+15 dB gain
--
-88 -86
--
-88 -85
--
-88 -84
dBV
Voltage at V-
V
V-
No Signal
-3.1 -2.85 -2.6
-3.1 -2.85 -2.5
-3.1 -2.85 -2.6
V
1. All specifications subject to change without notice.
2. Unless otherwise noted, T
A
=25C, V
CC
= +15V, V
EE
= 15V. Test circuit is as shown in Figure 2.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
600029 Rev 01
Page 3
Electrical Characteristics (Cont'd.)
2180A
2180B
2180C
Parameter
Symbol
Conditions
Min Typ
Max
Min Typ
Max
Min Typ
Max Units
Total Harmonic Distortion
THD
1 kHz, No External Trim
V
IN
= 0 dBV, 0 dB gain
-- 0.005 0.010
-- 0.010 0.020
-- 0.030 0.050
%
V
IN
= +10 dBV, -15 dB gain -- 0.020 0.030
-- 0.030 0.040
-- 0.040 0.070
%
V
IN
= -5 dBV, +15 dB gain
-- 0.020 0.030
-- 0.030 0.040
-- 0.040 0.070
%
Slew Rate
R
in
= R
out
= 20 kW
--
12
--
--
12
--
--
12
--
V/ms
Gain at 0 V Control Voltage
E
C-
= 0 mV
-0.1 0.0
+0.1 -0.15 0.0 +0.15 -0.2 0.0
+0.2
dB
Figure 3. 2180 Series Frequency Response Vs. Gain
Figure 4. 2180 Series Noise (20kHz NBW) Vs. Gain
Vcc
Ec-
IN
10u
20k
5.1k
Vee
OUT
22p
20k
OUT
OP275
7
3
8
4
2
6
5
1
V+
-IN
Ec-
Ec+
SYM
GND
V-
2180
Series
VCA
Power Supplies
Vcc = +15 V
Vee = -15 V
-
+
NC
Figure 2. Typical Application Circuit
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Theory of Operation
3
The THAT 2180 Series VCAs are designed for high
performance in audio-frequency applications requiring
exponential gain control, low distortion, wide dynamic
range and low control-voltage feedthrough. These parts
control gain by converting an input current signal to a
bipolar logged voltage, adding a dc control voltage, and
re-converting the summed voltage back to a current
through a bipolar antilog circuit.
Figure 5 presents a considerably simplified internal
circuit diagram of the IC. The ac input signal current
flows in pin1, the input pin. An internal operational
transconductance amplifier (OTA) works to maintain
pin 1 at a virtual ground potential by driving the emitters
of Q1 and (through the Voltage Bias Generator) Q3.
Q3/D3 and Q1/D1 act to log the input current, producing
a voltage, V3, which represents the bipolar logarithm of
the input current. (The voltage at the junction of D1 and
D2 is the same as V3, but shifted by four forward V
be
drops.)
Gain Control
Since pin 8, the output, is usually connected to a vir-
tual ground, Q2/D2 and Q4/D4 take the bipolar antilog
of V3, creating an output current which is a precise rep-
lica of the input current. If pin 2 (Ec+) and pin 3 (Ec-)
are held at ground, the output current will equal the in-
put current. For pin 2 positive or pin 3 negative, the out-
put current will be scaled larger than the input current.
For pin 2 negative or pin 3 positive, the output current is
scaled smaller than the input.
The scale factor between the output and input cur-
rents is the gain of the VCA. Either pin 2 (Ec+) or pin 3
(Ec-), or both, may be used to control gain. Gain is expo-
nentially proportional to the voltage at pin 2, and expo-
nentially proportional to the negative of the voltage at
pin 3. Therefore, pin 2 (Ec+) is the positive control port,
while pin 3 (Ec-) is the negative control port. Because of
the exponential characteristic, the control voltage sets
gain linearly in decibels. Figure 6 shows the decibel cur-
rent gain of a 2180 versus the voltage at Ec+, while Fig-
ure 7 shows gain versus the Ec-.
Temperature Effects
The logging and antilogging in the VCA depends on
the logarithmic relationship between voltage and current
in a semiconductor junction (in particular, between a
transistor's V
be
and I
c
). As is well known, this relation-
ship is temperature dependent. Therefore, the gain of
any log-antilog VCA depends on its temperature.
Page 4
THAT2180 Series IC VCAs
3. For more details about the internal workings of the 2180 Series of VCAs, see An Improved Monolithic Volt-
age-Controlled Amplifier, by Gary K. Hebert (Vice-President, Engineering, for THAT Corporation), presented at the 99th
convention of the Audio Engineering Society, New York, Preprint number 4055.
Figure 6. Gain vs. Control Voltage (E
C+
, Pin 2) at 25C
Figure 7. Gain vs. Control Voltage (Ec-, Pin 3) at 25C
Figure 8. Gain vs. Control Voltage (Ec-) with Temp (C)
D1
IN
OUT
SYM
Ec-
D4
D3
Ec+
25
V-
+
Voltage
Bias
Generator
V
3
I
IN
Q1
Q4
Q3
Q2
Icell
Iadj
5
4
8
3
1
2
D2
Figure 5. Simplified Internal Circuit Diagram
Figure 8 shows the effect of temperature on the nega-
tive control port. (The positive control port behaves in the
same manner.) Note that the gain at Ec = 0 V is 0 dB, re-
gardless of temperature. Changing temperature changes
the scale factor of the gain by 0.33%/C, which pivots the
curve about the 0 dB point.
Mathematically, the 2180's gain characteristic is
Gain
E
E
(0.0061)(1 0.0033DT)
C
C
=
-
+
+
-
,
Eq. 1
where DT is the difference between room temperature
(25C) and the actual temperature, and Gain is the
gain in decibels. At room temperature, this reduces to
Gain
E
E
0.0061
C
C
=
-
+
-
,
Eq. 2
If only the positive control port is used, this becomes
Gain
E
0.0061
C
=
+
,
Eq. 3
If only the negative control port is used, this becomes
Gain
E
0.0061
C
=
-
-
,
Eq. 4
DC Bias Currents
The 2180 current consumption is determined by the
resistor between pin 5 (V-) and the negative supply voltage
(V
EE
). Typically, with 15V supplies, the resistor is 5.1 kW,
which provides approximately 2.4 mA. This current is
split into two paths: 570 mA is used for biasing the IC,
and the remainder becomes Icell as shown in Figure 5.
Icell is further split in two parts: about 20 mA biases the
core transistors (Q1 through Q4), the rest is available for
input and output signal current
DC Feedthrough
Normally, a small dc error term flows in pin 8 (the
output). When the gain is changed, the dc term changes.
This control-voltage feedthrough is more pronounced
with gain; the A version of the part produces the least
feedthrough, the C version the most. See Figure 9 for
typical curves for dc offset vs. gain
Audio Performance
The 2180-Series VCA design, fabrication and testing
ensure extremely good audio performance when used as
recommended. In particular, the 2180 maintains low
distortion over a wide range of gain, cut and signal lev-
els. Figures 10 through 12 show typical distortion per-
formance for representative samples of each grade of the
part. Figure 13 shows the harmonic content of the dis-
tortion in a typical B-grade part.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
600029 Rev 01
Page 5
Figure 11. 1 kHz THD+Noise Vs. Input Level,
+15 dB Gain
Figure 12. 1 kHz THD+Noise Vs. Input Level,
-15 dB Gain
Figure 9. Representative DC Offset Vs. Gain
Figure 13. FFT of THD, 0dB gain, 1kHz, 0dBV,
Typical 2180B
Figure 10. 1 kHz THD+Noise Vs. Input Level, 0 dB Gain
Applications
Input
As mentioned above, input and output signals are
currents, not voltages. While this often causes some con-
ceptual difficulty for designers first exposed to this con-
vention, the current input/output mode provides great
flexibility in application.
The Input pin (pin 1) is a virtual ground with nega-
tive feedback provided internally (see Figure 5, Page 4).
The input resistor (shown as 20 kW in Figure 2, Page 3)
should be scaled to convert the available ac input voltage
to a current within the linear range of the device. Gen-
erally, peak input currents should be kept under 1 mA
for best distortion performance.
Figures 10 through 12 show distortion vs. Signal
level for the three parts in the 2180 Series for 0 dB,
+15 dB and -15 dB gain. The circuit of Figure 2, Page 3
was used to generate these curves.
For a specific application, the acceptable distortion
will usually determine the maximum signal current level
which may be used. Note that, with 20 kW cur-
rent-to-voltage converting resistors, distortion 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).
This is especially true in the A and B grades of the
part.
Distortion vs. Noise
A designer may trade off noise for distortion by de-
creasing the 20 kW current-to-voltage converting resis-
tors used at the input and output in Figure 2, Page 3. For
every dB these resistor values are decreased, the voltage
noise at the output of the OP275 is reduced by one dB.
For example, with 10 kW resistors, the output noise floor
drops to 104 dBV (typical) at 0 dB gain -- a 6 dB re-
duction in noise because 10 kW is 1/2 of (6 dB lower
than) 20 kW.
Conversely, if THD is more important than noise per-
formance, increasing these resistors to 40 kW will in-
crease the noise level by 6 dB, while reducing distortion
at maximum voltage levels. Furthermore, if maximum
signal levels are higher (or lower) than the traditional
10 V rms, these resistors should be scaled to accommo-
date the actual voltages prevalent in the circuit. Since the
2180 handles signals as currents, these ICs can even op-
erate with signal levels far exceeding the 2180's supply
rails, provided appropriately large resistors are used.
High-Frequency Distortion
The choice of input resistor has an additional, subtle
effect on distortion. Since the feedback impedances
around the internal opamp (essentially Q1/D1 and
Q3/D3) are fixed, low values for the input resistor will
require more closed-loop gain from the opamp. Since
the open-loop gain naturally falls off at high frequencies,
asking for too much gain will lead to increased
high-frequency distortion. For best results, this resistor
should be kept to 10 kW or above.
Stability
An additional consideration is stability: the internal
op amp is intended for operation with source imped-
ances of less than 60 kW at high frequencies. For most
audio applications, this will present no problem
DC Coupling
The quiescent dc voltage level at the input (the input
offset voltage) is approximately +0 mV, but, as in many
general-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 2180 itself, or
due to offsets in stages preceeding the 2180. 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 & 14).
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 2180 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-
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Page 6
THAT2180 Series IC VCAs
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
Peak (I
in
) + Peak (I
out
) + 220 mA; and
I
cell
= I
set
- 350 mA. Therefore,
I
set
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
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: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
600029 Rev 01
Page 7
Vcc
Ec+
IN
10u
20k
5.1k
Vee
OUT
22p
20k
OUT
OP275
7
3
8
4
2
6
5
1
V+
-IN
Ec-
Ec+
SYM
GND
V-
2180
Series
VCA
Power Supplies
Vcc = +15 V
Vee = -15 V
-
+
NC
Figure 14. Positive Control Port Using Pin 2 (Ec+)
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Page 8
THAT2180 Series IC VCAs
Noise Considerations
It is second nature among good audio designers to con-
sider the effects of noisy devices on the signal path. As is
well known, this includes not only active devices such as
opamps and transistors, but extends to the choice of im-
pedance levels as well. High value resistors have higher in-
herent thermal noise, and the noise performance of an
otherwise quiet circuit can be easily spoiled by the wrong
choice of impedance levels.
Less well known, however, is the effect of noisy circuitry
and high impedance levels in the control path of volt-
age-control circuitry. The 2180 Series VCAs act like multi-
pliers: when no signal is present at the signal input, noise at
the control input is rejected. So, when measuring noise (in
the absence of signal as most everyone does), even very
noisy control circuitry often goes unnoticed. However, noise
at the control port of these parts will cause noise modula-
tion of the signal. This can become significant if care is not
taken to drive the control ports with quiet signals.
The 2180 Series VCAs have a small amount of inherent
noise modulation because of its class AB biasing scheme,
where the shot noise in the core transistors reaches a mini-
mum with no signal, and increases with the square root of
the instantaneous signal current. However, in an optimum
circuit, the noise floor rises only to -94 dBV with a
50 mA rms signal at unity gain -- 4 dB of noise modulation.
By contrast, if a unity-gain connected, non-inverting 5534
opamp is used to directly drive the control port, the noise
floor will rise to 92 dBV -- 6 dB of noise modulation.
To avoid excessive noise, one must take care to use
quiet electronics throughout the control-voltage circuitry.
One useful technique is to process control voltages at a
multiple of the eventual control constant (e.g., 61 mV/dB
-- ten times higher than the VCA requires), and then at-
tenuate the control signal just before the final drive am-
plifier. With careful attention to impedance levels,
relatively noisy opamps may be used for all but the final
stage.
Stray Signal Pickup
It is also common practice among audio designers to
design circuit boards to minimize the pickup of stray
signals within the signal path. As with noise in the con-
trol path, signal pickup in the control path can ad-
versely effect the performance of an otherwise good VCA.
Because it is a multiplier, the 2180 produces second
harmonic distortion if the audio signal itself is present at
the control port. Only a small voltage at the control port
is required: as little as 10 mV of signal can increase dis-
tortion to over 0.01%. This can frequently be seen at
high frequencies, where capacitive coupling between the
signal and control paths can cause stray signal pickup.
Because the signal levels involved are very small, this
problem can be difficult to diagnose. One useful tech-
nique is to temporarily bypass the control port to
ground via a modest-sized capacitor (e.g., 10 mF). If the
distortion diminishes, signal pickup in the control path
is the likely cause.
Temperature Sensitivity
As shown by the equation for A
V
(Page 5), the gain of
a 2180 VCA is sensitive to temperature in proportion to
the amount of gain or loss commanded. The constant of
proportionality is 0.33% of the decibel gain commanded,
per degree Celsius, referenced to 27C (300K). This
means that at 0 dB gain, there is no change in gain with
temperature. However, at -122 mV, the gain will be
+20 dB at room temperature, but will be 20.66 dB at a
temperature 10C lower.
For most audio applications, this change with tem-
perature is of little consequence. However, if necessary, it
may be compensated by a resistor embedded in the con-
trol voltage path whose value varies with temperature at
the same rate of 0.33%/C. Such parts are available from
RCD Components, Inc, 3301 Bedford St., Manchester,
NH, USA [(603) 669-0054], and KOA/Speer Electronics,
PO Box 547, Bradford, PA, 16701 USA [(814)362-5536].
Closing Thoughts
THAT Corporation welcomes comments, questions
and suggestions regarding these devices, their design
and application. Our engineering staff includes designers
who have decades of experience in applying our parts.
Please feel free to contact us to discuss your applications
in detail.
I
K
L
G
E TYP.
F
B
D
C
1
A
N
M
H
N
17.78 0.3
0.700 0.012
J
MILLIMETERS
19.5 +0.2/-0
1.25
0.65
0.85
2.54 0.2
0.9
1.2
5.8 +0.2/-0
2.8 +0.1/-0
10.5 0.5
1.3
0.3
3.5 0.5
INCHES
0.77 +0.008/-0
0.049
0.026
0.033
0.100 0.008
0.04
0.05
0.23 +0.008/-0
0.11 +0.004/-0
0.413 0.02
0.05
0.012
0.14 0.02
ITEM
A
B
C
D
E
F
G
H
I
J
K
L
M
Figure 16. -L (SIP) Version Package Outline Drawing
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