SA2159 Datasheet, PDF (8/11 Pages) Silan Microelectronics Joint-stock – VOLTAGE CONTROLLED AMPLIFIER FOR ELECTRONIC VOLUME CONTROL

SA2159
APPLICATIONS
Input
As mentioned above, because input and output signals are currents, not voltages, the current input/output
mode provides great flexibility in application.
Internal negative feedback loop provides a virtual ground to the input pin 1 (See Figure 1). Within the linear
range of the device, the input resistor (shown as 20kΩ in typical application circuit) should be scaled to convert
the available ac input voltage to a current. In order to obtain best distortion performance, peak input currents
should be kept under 1 mA. The input impedances must be less than 30kΩ to assure the circuit stability.
The feedback impedances around the internal op amp (essentially Q1/D1 and Q3/D3) are fixed. Low values for
the input resistor will require more closed-loop gain from the op amp.
Since the open-loop gain naturally falls off at high frequencies, this resistor should be kept to 10kΩ or above in
order to prevent high frequency distortion when the gain is too much. Distortion vs. frequency for a 1V signal at
0dB gain with a 20 kΩ input resistor is shown in Figure 9.
As mentioned above, any dc input currents will cause dc signals in the output. The dc signals will be modulated
by gain command, in turn causing audible thump. Therefore, in order to control quality audio applications,
capacitive coupling must be adopted in actual application, which can give acceptable low frequency performance.
Figure 9. Typical THD Vs. frequency, 0dB gain
Output
The output pin (pin 8) should be connected to a virtual ground node, so that current flowing in it may be
converted to a voltage. Choose the external op amp can improve audio performance.
A small feedback capacitor around the output op amp is necessary to cancel the output capacitance of the
VCA, so that most op amps can operate steadily. The capacitance at pin 8 is typically 30 pf.
Power supplies
The positive supply is connected directly to pin 7 without special bypassing circuit, but it is good practice to
include a small (~1µf) electrolytic close to the VCA IC on the PCB. There is no particularly dependence between
performance and supply voltage. The lowest supply voltage is determined by the sum of the input and output
currents plus ISET, which must be supplied through the resistor (~2kΩ) at the top of the core transistors while still
allowing enough voltage swing to bias the internal op amp and the core transistors themselves. Reducing signal
currents may help accommodate low supply voltages.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
Rev: 1.0 2005.04.22
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