Datasheet SSM2164 (Analog Devices) - 8

SSM2164
but higher noise, and the opposite is true for less current. The a low cutoff frequency. The main exception to this is in increased noise is due to higher current noise in the gain core dynamic processing applications, where faster attack or decay transistors as their operating current is increased. THD has the times may be needed. opposite relationship to collector current. The lower distortion is due to the decrease in the gain core transistors’ emitter
+5V
impedance as their operating current increases.
100pF 3 VC
This classical tradeoff between THD and noise in VCAs is
100k
usually expressed as the choice of using a VCA in either Class A
1µF 30k I I IN 2 4 IOUT VCA1
or Class AB mode. Class AB operation refers to running a VCA
30k 1/4 V OP482 VOUT1
with less current in the gain core, resulting in lower noise but
IN1 500
higher distortion. More current in the core corresponds to
560pF
Class A performance with its lower THD but higher noise.
+5V 100pF VC 6
Figures 11 and 12 show the THD and noise performance of the
100k
SSM2164 as the bias current is adjusted. Notice the two
1µF 30k I 7 IOUT 5
characteristics have an inverse characteristic.
IIN VCA2 30k 1/4 V OP482 VOUT2
The quiescent current in the core is set by adding a single
IN2 500
resistor from the positive supply to the MODE pin. As the
560pF
simplified schematic shows, the potential at the MODE pin is
+5V 100pF VC 11
one diode drop above the ground pin. Thus, the formula for the
100k
MODE current is:
1µF 30k I IIN 10 12 IOUT VCA3 30k 1/4 V OP482 VOUT3
I = (V +) − 0.6V
IN3
MODE R
500
B
+5V 560pF 100pF VC 14
With ± 15 V supplies, an R of 7.5k gives Class A biasing with a B current of 1.9 mA. Leaving the MODE pin open sets the
100k 1µF
SSM2164 in Class AB with 30 µA of current in the gain core.
30k I IIN 15 13 IOUT VCA4 30k 1/4 Basic VCA Configuration V V IN4 OP482 OUT4
Figure 24 shows the basic application circuit for the SSM2164.
500
Each of the four channels is configured identically. A 30 kΩ
560pF POWER SUPPLY
resistor converts the input voltage to an input current for the
AND BIASING CIRCUITRY
VCA. Additionally, a 500 Ω resistor in series with a 560 pF
9 8 16 1
capacitor must be added from each input to ground to ensure
V– GND V+ MODE
stable operation. The output current pin should be maintained
0.1µF 0.1µF RB (7.5k
Ω
CLASS A)
at a virtual ground using an external amplifier. In this case the
(OPEN CLASSAB)
OP482 quad JFET input amplifier is used. Its high slew rate,
–15V +15V
wide bandwidth, and low power make it an excellent choice for the current-to-voltage converter stage. A 30 kΩ feedback resistor is chosen to match the input resistor, giving unity gain Figure 24. Basic Quad VCA Configuration for a 0.0 V control voltage. The 100 pF capacitors ensure stability and reduce high frequency noise. They can be
Low Cost, Four-Channel Mixer
increased to reduce the low pass cutoff frequency for further The four VCAs in a single package can be configured to create a noise reduction. simple four-channel mixer as shown in Figure 25. The inputs and control ports are configured the same as for the basic VCA, For this example, the control voltage is developed using a
OBSOLETE
but the outputs are summed into a single output amplifier. The 100 kΩ potentiometer connected between +5 V and ground. OP176 is an excellent amplifier for audio applications because This configuration results in attenuation only. To produce both of its low noise and distortion and high output current drive. gain and attenuation, the potentiometer should be connected The amount of signal from each input to the common output between a positive and negative voltage. The control input has can be independently controlled using up to 20 dB of gain or as an impedance of 5 kΩ. Because of this, any resistance in series much as 100 dB of attenuation. Additional SSM2164s could be with V will attenuate the control signal. If precise control of C added to increase the number of mixer channels by simply the gain and attenuation is required, a buffered control voltage summing their outputs into the same output amplifier. Another should be used. possible configuration is to use a dual amplifier such as the Notice that a capacitor is connected from the control input to OP275 to create a stereo, two channel mixer with a single ground. Because the control port is connected directly to the SSM2164. gain core transistors, any noise on the V pin will increase the C output noise of the VCA. Filtering the control voltage ensures that a minimal amount of noise is introduced into the VCA, allowing its full performance to be realized. In general, the largest possible capacitor value should be used to set the filter at –8– REV. 0