SA2159

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

Rev: 1.0

2005.04.22

Http: www.silan.com.cn

Page 9 of 11

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 200

µA 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 *

T).

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 50

Ω. 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