8408 Datasheet, PDF (16/20 Pages) Maxwell Technologies – Quad 8-Bit Multiplying CMOS D/A Converter with Memory

Quad 8-Bit Multiplying CMOS
D/A Converter with Memory
8408
A DIGITALLY PROGRAMMABLE ACTIVE FILTER
A powerful D/A converter application is a programmable active filter design as shown in Figure 11. 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 frequency 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 programmability of
the filter extremely reliable and predictable. Note that low-pass, high-pass, and bandpass outputs are available. This
sophisticated function is achieved in only two IC packages.
The network analyzer photo shown in Figure 12 superimposes five actual bandpass responses ranging from the low-
est frequency 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 fC = 1/2πRC where R is the ladder resistance (RIN) of the 8408,
and C is 1000 pF. Note that from device to device, the resistance RIN varies. Thus some tuning may be necessary.
FIGURE 12. PROGRAMMABLE ACTIVE FILTER BAND-PASS FREQUENCY RESPONSE
All components used are available off-the-shelf. Using low drift thin-film resistors, the 8408 exhibits very stable perfor-
mance over temperature. The wide bandwidth of the OP-470 produces excellent high frequency and high Q response.
In addition, the OP470’s low input offset voltage assures an unusually low dc offset at the filter output.
08.20.02 REV 1
All data sheets are subject to change without notice 16
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