SA2159 Datasheet, PDF (10/11 Pages) Silan Microelectronics Joint-stock – VOLTAGE CONTROLLED AMPLIFIER FOR ELECTRONIC VOLUME CONTROL

SA2159
oscillate internally. In such cases, a 100Ω resistor in series with a 1.5nf capacitor between the control port and
ground will usually suffice to prevent the instability.
Input
10­
1k
+15V
47p
7
SA2159
3
VCA
20k
V+
EC-
OUT
20k
IN
EC+(sym)
EC+
8
LF351
GND
4
V-
2
6
5
+15V
5.1K
51
Rsym
150K
50K
SYM
ADJ
1k
-15V EC+
-15V
Output
Figure 10. Using both control ports (differential drive)
Noise considerations
For good audio designers, it is important to consider the effects of noisy devices on the signal path. As is well
known, this includes not only active devices, but also impedance. High value resistors have inherent thermal
noise. And the wrong choice of impedance levels will easily bring on the noise performance of an otherwise quiet
circuit being spoiled.
However, the effect of noisy circuitry and high impedance levels in the control path of voltage-control circuitry is
easily neglected. When no signal is present at the signal input, The SA2159 VCA noise at the control input is
rejected like double-balanced multipliers. Most everyone measures noise in the absence of signal, so noise of
control circuitry is often neglected. However, noise at the control port of these parts will cause noise modulation
of the signal. If driving the control ports is not quiet signals, this problem will become very grave.
Quiet electronics throughout be control-voltage circuitry can be used to avoid excessive noise. One useful
technique is to process control voltages at a multiple of the eventual control constant (e.g., 59mV/dB –ten times
higher than the VCA requires). And then attenuate the control signal just before the final drive amplifier. It is
necessary to pay attention to Impedance levels and noisy op amps.
Figure 11. Typical frequency response vs. gain
Figure 12. Typical noise (20kHz NBW) vs. gain
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
Rev: 1.0 2005.04.22
Page 10 of 11