SA2159

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

Rev: 1.0

2005.04.22

Http: www.silan.com.cn

Page 7 of 11

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 200

µA larger than the total of the peak

input and output signal currents because Iset divides into two portions: approximately 200

µA 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 2k

Ω, 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