SA2159 Datasheet, PDF (7/11 Pages) Silan Microelectronics Joint-stock – VOLTAGE CONTROLLED AMPLIFIER FOR ELECTRONIC VOLUME CONTROL

SA2159
Current programming
An external Iset can set the size of the current source at the bottom of the core. Iset is normally determined by
a resistor between V- and pin 5 (Vpin 5 = -2.7V, typically). Iset should be 200µA larger than the total of the peak
input and output signal currents because Iset divides into two portions: approximately 200µA is used for internal
biasing, and the rest is available for the current source at the bottom of the core.
Note that since the sum of the input and output currents plus ISET must be supplied through the output
impedance of the internal op amp, this impedance is approximately 2kΩ, the voltage available to drive the core
will lower.
Headroom
The logarithmic characteristics of the core transistors also limit maximum signal currents. The devices used in
the SA2159 are specially designed to conform to an ideal log-linear curve over a wide range of currents, but their
limit value is approximately 1mA. Distortion will increase with increasing current levels because of falling log
conformance. Figure 6 through 8 show typical distortion versus 1 kHz signal level for the SA2159 at -15dB, 0 dB,
and +15dB gain. Thus, the maximum signal level for a particular design will be determined by the acceptable
distortion.
Figure 6. 1KHz THD +Noise Vs. input,-15dB gain
Figure 7. 1kHz THD+Noise Vs. level, 0dB gain
Figure 8. 1kHz THD+Noise Vs. Input, +15dB gain
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
Rev: 1.0 2005.04.22
Page 7 of 11