AD8400AN10 Analog Devices, AD8400AN10 Datasheet - Page 16

• Manufacturer Part Number: AD8400AN10
• Description: 1-/2-/4-Channel Digital Potentiometers
• Manufacturer: Analog Devices

AD8400/AD8402/AD8403
ACTIVE FILTER
One of the standard circuits used to generate a low-pass, high-
pass or bandpass filter is the state variable active filter. The digi-
tal potentiometer allows full programmability of the frequency,
gain and Q of the filter outputs. Figure 44 shows the filter cir-
cuit using a +2.5 V virtual ground, which allows a 2.5 V
and 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 po-
tentiometers) to maintain the best circuit Q. Figure 45 shows
the measured filter response at the bandpass output as a func-
tion of the RDAC2 and RDAC3 settings which produce a range
of center frequencies from 2 kHz to 20 kHz. The filter gain re-
sponse at the bandpass output is shown in Figure 46. At a cen-
ter 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
Figure 44. Programmable State Variable Active Filter
~
B
RDAC1
2.5V
A1
RDAC4
B
A2
10k
OP279
RDAC2
2
10k
B
0.01µF
A3
RDAC3
B
0.01µF
A4
P
input
BAND-
PASS
HIGH-
PASS
LOW-
PASS
–16–
Figure 46. Programmed Amplitude Bandpass Response
Figure 45. Programmed Center Frequency Bandpass
Response
–20
–40
–60
–80
–20
–40
–60
–80
40
20
40
20
0
0
20
20
100
100
–19.01
–0.16
FREQUENCY – Hz
FREQUENCY – Hz
1k
1k
2.00000 k
20.0000 k
10k
10k
100k
100k
200k
200k
REV. B