DAC8408

–13–

REV. A

Figure 9. A Digitally Programmable Universal Active Filter

Figure 10. Programmable Active Filter Band-Pass

Frequency Response

All components used are available off-the-shelf. Using low drift

thin-film resistors, the DAC8408 exhibits very stable perfor-

mance over temperature. The wide bandwidth of the OP-470

produces excellent high frequency and high Q response. In addi-

tion, the OP470’s low input offset voltage assures an unusually

low dc offset at the filter output.

A DIGITALLY PROGRAMMABLE ACTIVE FILTER

A powerful D/A converter application is a programmable active

filter design as shown in Figure 9. The design is based on the

state-variable filter topology which offers stable and repeatable

filter characteristics. DAC B and DAC D can be programmed in

tandem with a single digital byte load which sets the center fre-

quency of the filter. DAC A sets the Q of the filter. DAC C sets

the gain of the filter transfer function. The unique feature of this

design is that varying the gain of filter does not affect the Q of

the filter. Similarly, the reverse is also true. This makes the pro-

grammability of the filter extremely reliable and predictable.

Note that low-pass, high-pass, and bandpass outputs are avail-

able. This sophisticated function is achieved in only two IC

packages.

The network analyzer photo shown in Figure 10 superimposes

five actual bandpass responses ranging from the lowest fre-

quency of 75 Hz (1 LSB ON) to a full-scale frequency of 19.132

kHz (all bits ON), which is equivalent to a 256 to 1 dynamic

some tuning may be necessary.