Datasheet THAT2181B, THAT2181C, THAT2181A, THAT2180C, THAT2180B Datasheet (THAT Corporation)
...Page 1
THAT Corporation
Trimmable IC
Voltage Controlled Amplifiers
THAT 2181A, 2181B, 2181C
FEATURES
Wide Dynamic Range: >120 dB
·
Wide Gain Range: >130 dB
·
Exponential (dB) Gain Control
·
Low Distortion:
·
~0.0025% (typical 2181A)
~0.005% (typical 2181C)
Wide Gain-Bandwidth: 20 MHz
·
Dual Gain-Control Ports (pos/neg)
·
Pin-Compatible with 2150-Series
·
Description
THAT 2181 Series integrated-circuit voltage controlled amplifiers (VCAs) are very high-performance
current-in/current-out devices with two opposing-polarity, voltage-sensitive control ports. They offer
wide-range exponential control of gain and attenuation
with low signal distortion. The parts are selected after
packaging based primarily on after-trim THD and control-voltage feedthrough performance.
APPLICATIONS
Faders
·
Panners
·
Compressors
·
Expanders
·
Equalizers
·
Filters
·
Oscillators
·
Automation Systems
·
The VCA design takes advantage of a fully complementary dielectric isolation process which offers
closely matched NPN/PNP pairs. This delivers performance unobtainable through any conventional process, integrated or discrete. The parts are available in
three grades, allowing the user to optimize cost vs.
performance. Both 8-pin single-in-line (SIP) and surface mount (SO) packages are available.
BIAS CURRENT
COMPENSATION
Input
1
6
Gnd
Pin Name SIP Pin SO Pin
7
Iset
25
Vcc
2
Ec+
Ec-
3
8
Output
4
Sym
5
Table 1. 2181 Series Pin Assignments
V-
Max Trimmed THD
@1V,1kHz,0dB
2k
Vbe
MULTI-
PLIER
Iadj
Figure 1. 2181 Series Equivalent Circuit Diagram
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Input 1 1
Ec+ 2 2
Ec– 3 3
Sym 4 4
V– 5 5
Gnd 6 6
V+ 7 7
Output 8 8
Plastic
SIP
Plastic
SO
0.005% 2181LA 2181SA
0.008% 2181LB 2181SB
0.02% 2181LC 2181SC
Table 2. Ordering Information
Page 2
Page 2 THAT2181 Series IC VCAs
SPECIFICATIONS
1
Absolute-Maximum Ratings (TA=25° C)
Positive Supply Voltage (VCC) +20 V
Negative Supply Voltage (V
Supply Current (I
Max DEE
)10mA
CC
-(EC-) ±1V
C+
) -20 V
EE
Power Dissipation (P
)(TA=75°C) 330 mW
D
Operating Temperature Range (T
Storage Temperature Range (T
) 0 to +70°C
OP
) -40 to +125°C
ST
Recommended Operating Conditions
2181A 2181B 2181C
Parameter Symbol Conditions Min Typ Max Min Typ Max Min Typ Max Units
Positive Supply Voltage V
Negative Supply Voltage V
Bias Current I
Signal Current I
IN+IOUT
SET
CC
EE
VCC-VEE= 30 V 1 2.4 3.5 1 2.4 3.5 1 2.4 3.5 mA
I
= 2.4mA — 0.35 2.5 — 0.35 2.5 — 0.35 2.5 mA
SET
+4 +15 +18 +4 +15 +18 +4 +15 +18 V
-4 -15 -18 -4 -15 -18 -4 -15 -18 V
Electrical Characteristics²
2181A 2181B 2181C
Parameter Symbol Conditions Min Typ Max Min Typ Max Min Typ Max Units
Supply Current I
Equiv. Input Bias Current I
Input Offset Voltage V
Output Offset Voltage V
CC
B
OFF(IN)
OFF(OUT)
No Signal — 2.4 4 — 2.4 4 — 2.4 4 mA
No Signal — 2 10 — 2 12 — 2 15 nA
No Signal — ±5— —±5— —±5— mV
R
=20kW
out
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
Gain-Control Constant T
=25°C(T
A
CHIP
@35°C)
—20— —20— —20— mA
-60 dB < gain < +40 dB
E
/Gain (dB) Pin 2 (Fig. 15) 6.0 6.1 6.2 6.0 6.1 6.2 6.0 6.1 6.2 mV/dB
C+
EC-/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 DEC/ DT
CHIP
Ref T
=27°C — +0.33 — — +0.33 — — +0.33 — %/°C
CHIP
Gain-Control Linearity -60 to +40 dB gain — 0.5 2 — 0.5 2 — 0.5 2 %
1 kHz Off Isolation EC+= -360 mV, EC-= +360 mV 110 115 — 110 11 5 110 11 5 — dB
Output Noise e
n(OUT)
20Hz~20kHz
R
= 20kW
out
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 -2.6 -3.2 -2.85 -2.6 V
1. All specifications subject to change without notice.
2. Unless otherwise noted, T
=25°C, VC= +15V, VEE= –15V. Test circuit is as shown in Figure 2. SYM ADJ is ad
A
justed for minimum THD at 1 V, 1 kHz, Ec– = –Ec+=0V
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 3
600030 Rev 01 Page 3
Electrical Characteristics (Cont'd.)
2181A 2181B 2181C
Parameter Symbol Conditions Min Typ Max Min Typ Max Min Typ Max Units
Total Harmonic Distortion THD 1 kHz
= 0 dBV, 0 dB gain — 0.0025 0.005 — 0.004 0.008 — 0.005 0.02 %
V
IN
V
= +10 dBV, -15 dB gain — 0.018 0.025 — 0.025 0.035 — 0.035 0.07 %
IN
V
= -5 dBV, +15 dB gain — 0.018 0.025 — 0.025 0.035 — 0.035 0.07 %
IN
VIN= +10 dBV, 0 dB gain — 0.004 0.008 — 0.006 0.010 — .0015 — %
Slew Rate Rin=R
Symmetry Control Voltage V
Gain at 0 V Control Voltage E
SYMAV
= 0 dB, Minimum THD -0.5 — +0.5 -1.5 — +1.5 -2.5 — +2.5 mV
C-
2181
Series
VCA
IN
10u
Power Supplies
Vcc = +15 V
Vee=-15V
20k
=20kW —12 — —12 — —12 —V/ms
out
= 0 mV -0.1 0.0 +0.1 -0.15 0.0 +0.15 -0.2 0.0 +0.2 dB
Vcc
Ec-
7
3
V+
-IN
V-
GND
5
Ec-
Ec+
6
1
SYM
2
OUT
8
4
22p
-
OP275
+
20k
OUT
Vcc
5.1k
Rsym
50k
SYM
ADJ
680k (2181A)
220k (2181B)
130k (2181C)
Vee
Vee
Figure 2. Typical Application Circuit
Figure 3. 2181 Series Frequency Response Vs. Gain
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Figure 4. 2181 Series Noise (20kHz NBW) Vs. Gain
Page 4
Page 4 THAT2181 Series IC VCAs
Theory of Operation
The THAT 2181 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
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 replica of the input current. If pin 2 (Ec+) and pin 3 (Ec-)
are held at ground (with pin 4 - SYM - connected to a
high impedance current source), the output current will
equal the input current. For pin 2 positive or pin 3 negative, the output 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
3
Figure 6. Gain vs. Control Voltage (EC+, Pin 2) at 25°C
be
-
Figure 7. Gain vs. Control Voltage (Ec-, Pin 3) at 25°C
-
-
+
D1
2
25
Q1
D3
Vol tage
Bias
Generator
Ec+
1
IN
I
IN
D2
Q2
Q4Q3
D4
Figure 8. Gain vs. Control Voltage (Ec-) with Temp (°C)
3
Ec-
8
OUT
4
SYM
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 2181 versus the voltage at Ec+, while Fig
ure 7 shows gain versus the Ec-.
Temperature Effects
V
3
V-
Icell
5
Figure 5. Simplified Internal Circuit Diagram
3. For more details about the internal workings of the 2181 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.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Iadj
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
and Ic). As is well known, this relation
be
-
-
-
-
-
Page 5
600030 Rev 01 Page 5
ship is temperature dependent. Therefore, the gain of
any log-antilog VCA depends on its temperature.
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=0Vis0dB,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 2181's gain characteristic is
Gain
=
(0.0061)(1 0.0033 T)
+-
+
D
,
Eq. 1
EE
-
CC
where DT is the difference between room temperature
(25°C) and the actual temperature, and Gain is the
gain in decibels. At room temperature, this reduces to
EE
-
CC
Gain
+-
=
0.0061
,
Eq. 2
If only the positive control port is used, this becomes
Gain
C=+
0.0061
,
Eq. 3
E
If only the negative control port is used, this becomes
E
-
C
Gain
=
0.0061
-
,
Eq. 4
larger value resistor to form a voltage divider connected
to the wiper of a trim pot across the supply rails.
-
-
This trim should be adjusted for minimum harmonic
distortion. This is usually done by applying a mid
dle-level, middle-frequency signal (e.g. 1 kHz at 1 V) to
the audio input, setting the VCA to 0 dB gain, and adjust
-
ing the SYM trim while observing THD at the output. In
the 2181, this adjustment coincides closely with the set
ting which produces minimum control-voltage
feedthrough, though the two settings are not always iden
tical.
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
DC Bias Currents
The 2181 current consumption is determined by the
resistor between pin 5 (V-) and the negative supply voltage
(V
). Typically, with 15V supplies, the resistor is 5.1 kW,
EE
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.
Trimming
The 2181-Series VCAs are intended to be adjusted for
minimum distortion by applying a small variable offset
voltage to pin 4, the SYM pin. Note that there is a 25 W re
sistor internal to the 2181 between pin 4 and pin 2. As
shown in Figure 2, Page 3, the usual method of applying
this offset is to use the internal 25 W resistor along with a
Figure 10. 1 kHz THD+Noise Vs. Input Level, 0 dB Gain
-
Figure 11. 1 kHz THD+Noise Vs. Input Level, +15 dB
Gain
Figure 9. Representative DC Offset Vs. Gain
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Figure 12. 1 kHz THD+Noise Vs. Input Level, -15 dB
Gain
Page 6
Page 6 THAT2181 Series IC VCAs
Audio Performance
The 2181-Series VCA design, fabrication and testing
ensure extremely good audio performance when used as
recommended. The 2181 maintains low distortion over a
wide range of gain, cut and signal levels. Figures 10
through 12 show typical distortion performance for rep
Figure 13. 2181A THD+N vs. Frequency, 0 dB Gain, 1 V
Applications
Input
As mentioned above, input and output signals are
currents, not voltages. While this often causes some
conceptual difficulty for designers first exposed to
this convention, the current input/output mode provides great flexibility in application.
The Input pin (pin 1) is a virtual ground with negative 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 avail
able ac input voltage to a current within the linear
range of the device. Generally, peak input currents
should be kept under 1 mA for best distortion perfor
mance.
Refer to Figures 10 through 12 to see how distor
tion varies with signal level for the three parts in the
2181 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 distor
tion will usually determine the maximum signal cur
rent level which may be used. Note that, with 20 kW
current-to-voltage converting resistors, distortion re
mains 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.
resentative samples of each grade of the part. At or near
unity gain, the 2181 behaves much like a good opamp,
with low distortion over the entire audio band. Figure
13 shows typical THD for a 2181A over frequency at
0 dB gain, with a 1 V input signal, while Figure 14 de
-
tails the harmonic content of the distortion in a typical
A–grade part.
-
Figure 14. FFT of THD, Typical 2181A,
0dB Gain, 1V, 1kHz
creased, 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 —a6dBreduction in noise because
10 kW is 1/2 of (6 dB lower than) 20 kW.
Conversely, if THD is more important than noise
performance, increasing these resistors to 40 kW will
increase 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 accommodate the actual voltages prevalent in the
circuit. Since the 2181 handles signals as currents,
these ICs can even operate with signal levels far ex
ceeding the 2181's supply rails, provided appropri
ately large resistors are used.
-
-
High-Frequency Distortion
The choice of input resistor has an additional,
-
-
-
subtle effect on distortion. Since the feedback imped
ances 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 fre
quencies, asking for too much gain will lead to in
creased high-frequency distortion. For best results,
this resistor should be kept to 10 kW or above.
-
-
-
Distortion vs. Noise
A designer may trade off noise for distortion by
decreasing the 20 kW current-to-voltage converting re
sistors used at the input and output in Figure 2,
Page 3. For every dB these resistor values are de
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Stability
An additional consideration is stability: the inter
-
-
nal op amp is intended for operation with source im
pedances of less than 60 kW at high frequencies. For
most audio applications, this will present no problem
-
-
Page 7
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 desired current-to-voltage conversion constant. Since the
input resistor determines the voltage-to-current conversion at the input, the familiar ratio of R
for an invert-
f/Ri
ing opamp will 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.
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
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.
, which must be supplied through the output
SET
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
sistor to V
), which determines the current available
EE
(usually simply a re
set
-
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
should equal 2.4 mA
set
-
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
³ Peak (Iin) + Peak (I
cell
I
cell=Iset
I
set
- 350 mA. Therefore,
³ Peak (Iin) + Peak (I
) + 220 mA; and
out
) + 570 mA.
out
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 additional input pin.
Voltage Control
Negative Sense
EC-(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, EC+(pin 2) is the posi
tive-sense voltage control port. A typical circuit using
this approach is shown in Figure 15. E
-
-
be grounded, and E
(pin 2) driven from a
C+
low-impedance voltage source. Using the opposite sense
(Pin 3) should
C-
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: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Page 8
Page 8 THAT2181 Series IC VCAs
There is no reason why both control ports cannot be
used simultaneously.
Symmetry
The SYM pin (pin 4) is actually a sort of additional
positive-sense control port. It is provided to allow V
mismatches in the core transistors to be adjusted after
packaging and installation in the circuit board. It should
only be used for this purpose. Connect pin 4 only to a
high-impedance source as shown in Figures 2 and 15.
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
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 2181 Series VCAs are designed to be operated
with zero source impedance at pins 2 and 3, and a high
(³50 kW) source impedance at pin 4. To realize all the
performance designed into a 2181, keep the source im-
pedance 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 impedance of an opamp typically rises at high frequencies because open loop gain falls off as frequency increases. A
typical opamp's output impedance is therefore inductive
and EC-voltages being exactly
C+
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.
be
-
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 2181 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 2181 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 minimum 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—4dBofnoise 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—6dBofnoise modulation.
-
-
-
-
-
-
-
IN
Power Supplies
Vcc = +15 V
Vee = -15 V
Vcc
2181
22p
Series
Vee
7
3
V+
-IN
V-
GND
5
Ec-
Ec+
6
SYM
2
4
OUT
8
1
Rsym
20k
-
OP275
+
50k
680k (2181A)
220k (2181B)
Ec+
130k (2181C)
Vcc
VCA
10u
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
20k
5.1k
Figure 15. Positive Control Port Using Pin 4
OUT
Vee
SYM
ADJ
Page 9
600030 Rev 01 Page 9
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 2181 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 clue to the
presence of this problem is that the symmetry null for
minimum THD varies with frequency. It is often possible
to counteract a small amount of pure fundamental
picked up in the control path by "misadjusting" the symmetry setting. Since the amount of pickup usually varies
with frequency, the optimum trim setting will vary with
frequency and level. A useful technique to confirm this
problem 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 Equation 1 (Page 5), the gain of a 2181
VCA is sensitive to temperature in proportion to the
amount of gain or loss commanded. The constant of pro
portionality is 0.33% of the decibel gain commanded,
per degree Celsius, referenced to 27°>C (300°K). 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 10°C 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 [+1(603) 669-0054], and KOA/Speer Electron
ics, PO Box 547, Bradford, PA, 16701 USA
[+1(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.
A
H
M
1
B
D
C
N
ITEM
MILLIMETERS
A
19.5 +0.2/-0
B
1.25
C
0.65
D
0.85
E
2.54 ±0.2
F
0.9
G
1.2
H
5.8 +0.2/-0
I
2.8 +0.1/-0
J
10.5 ±0.5
K
1.3
L
0.3
M
3.5 ±0.5
N 17.78 ±0.3 0.700 ±0.012
G
F
E TYP.
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
J
I
K
L
Figure 16. -L (SIP) Version Package Outline Drawing
E
F
CB
H
D
G
A
ITEM MILLIMETERS
A
4.80/4.98
B
3.81/3.99
C
5.80/6.20
D
0.36/0.46
E
1.27
F
1.35/1.73
G
0.19/0.25
0.41/1.27H 0.016/0.05
INCHES
0.189/0.196
0.150/0.157
0.228/0.244
0.014/0.018
0.050
0.053/0.068
0.0075/0.0098
Figure 17. -S (SO) Version Package Outline Drawing
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 10
Page 10 THAT2181 Series IC VCAs
Notes
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
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