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
range. The frequency is determined by f
C
= 1/2πRC where R is
the ladder resistance (R
IN
) of the DAC8408, and C is 1000 pF.
Note that from device to device, the resistance R
IN
varies. Thus
some tuning may be necessary.
