SA2159 Datasheet, PDF (9/11 Pages) Silan Microelectronics Joint-stock – VOLTAGE CONTROLLED AMPLIFIER FOR ELECTRONIC VOLUME CONTROL

SA2159
The process characteristics and internal power consumption determine the highest supply voltage. +15V is the
nominal limit.
A resistive current source determining the current available for the core is connected to the negative supply
terminal. As mentioned before, this source must supply 200µA current over the sum of the required signal
currents including input signal current, output signal current and the bias to run the rest of the IC. 2.4mA is
recommended for most pro audio applications where +15V supplies are common and headroom is important.
Bypassing at pin 5 is not necessary because pin 5 is a current supply, not a voltage supply.
Pin 6 is used as a ground reference for the VCA which connect the non-inverting input of the internal op amp,
as a portion of the internal bias network. It may not be used as an additional input pin.
Voltage control
Pin 3 is the primary voltage control pin. This point controls gain is inversely proportional to applied voltage:
positive voltage causes loss, negative voltage causes gain. The current gain of the VCA is unity when pin 3 is at
0V and varies with voltage at approximately -5.9m/dB, at room temperature.
As implied by the equation for Av at the foot of page 3, the gain is sensitive to temperature. The constant of
proportionality is 0.33% of the decibel gain commanded, per degree Celsius, referenced to 27°C (300K). The
formula is:
Gain = (EC+ - EC-) / (0.0059 * 1.0033 * ǻT).
Where ∆T is the difference between the actual temperature and room temperature (27°C)
For most audio applications, this change with temperature is of little consequence. However, if necessary, it
may be compensated by a resistor which varies its value by 0.33%/°C.
When pin 3 is used for voltage control, Pin 2 is connected to ground and pin 4 is used to apply a small
symmetry voltage (~±4 mV) to correct for VBE mismatches within the VCA IC. Therefore, in order to obtain
optimum performance, pin 4 connects with an external impedance of approximately 50Ω. A trim pot is used to
adjust the voltage between pin 4 and pin 2 as shown in typical application circuit. Voltage adjustment range is ±4
mV.
Pin 2 and pin4 can be used together as an opposite sense voltage control port (See Figure 4). Pin 3 may be
grounded and pin 2 driven against the symmetry-adjustment voltage. The change of voltage at pin 4 does have a
small effect on the symmetry voltage, but this is of little practical consequence in most applications.
The chip can combine all control ports together with differential drive (See Figure 10). While the driving circuitry
is more complex, this configuration offers better performance at high attenuation levels (<-90dB) where the single
control port circuits begin to saturate Q1 (for EC- drive) or Q3 (for EC+ drive). When either of these transistors
saturates, the internal op amp will accommodate the change in current demand by responding with a small
change in its input offset voltage. This leads to an accumulation of charge on the input capacitor, which in turn
can cause thump when the high attenuation is suddenly removed(e.g., when a muted channel is opened).
Differential control drive avoids the large dc levels otherwise required to command high attenuation (+600mV or -
100dB gain at pin 3 alone, vs. ±300mV when using both pin 3 and pins 2 and 4).
Control port drive impedance
In order to reduce distortion, it is necessary to use low source impedance at the control port. Thus, this often
suggests an op amp is used to drive the control port directly (see below under noise considerations). However,
due to falling loop gain at high frequencies, the closed-loop output impedance of an op amp typically rises. The
output impedance is therefore inductive at high frequencies. Excessive inductance can cause the VCA to
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
Rev: 1.0 2005.04.22
Page 9 of 11