AD8400 Analog Devices, AD8400 Datasheet - Page 17

• Manufacturer Part Number: AD8400
• Description: Single-channel Digital Potentiometer
• Manufacturer: Analog Devices

ACTIVE FILTER
One of the standard circuits used to generate a low-pass, high-
pass, or band-pass filter is the state variable active filter. The
digital potentiometer allows full programmability of the frequency,
gain and Q of the filter outputs. Figure 10 shows the filter circuit
using a 2.5 V virtual ground, which allows a ± 2.5 V
output swing. RDAC2 and 3 set the LP, HP, and BP cutoff and
center frequencies, respectively. These variable resistors should
be programmed with the same data (as with ganged potentiom-
eters) to maintain the best circuit Q. Figure 11 shows the measured
filter response at the band-pass output as a function of the RDAC2
and RDAC3 settings which produce a range of center frequencies
from 2 kHz to 20 kHz. The filter gain response at the band-pass
output is shown in Figure 12. At a center frequency of 2 kHz, the
gain is adjusted over a –20 dB to +20 dB range determined by
RDAC1. Circuit Q is adjusted by RDAC4. For more detailed
reading on the state variable active filter, see Analog Devices’
application note, AN-318.
V
IN
B
RDAC1
A1
RDAC4
B
10k
OP279
A2
RDAC2
2
10k
B
0.01 F
A3
RDAC3
B
P
0.01 F
A4
input and
LOW-
PASS
BAND-
PASS
HIGH-
PASS
–20
–40
–60
–80
–20
–40
–60
–80
40
20
40
20
0
0
20
20
100
100
AD8400/AD8402/AD8403
–19.01
–0.16
FREQUENCY – Hz
FREQUENCY – Hz
1k
1k
20.0000 k
2.00000 k
10k
10k
100k
100k
A
A
200k
200k