Datasheet
AD5233
Rev. B | Page 25 of 32
PROGRAMMABLE STATE-VARIABLE 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
AD5233 digital potentiometer allows full programmability
of the frequency, the gain, and the Q of the filter outputs.
Figure 52 shows a filter circuit using a 2.5 V virtual ground,
which allows a ±2.5 V peak input and output swing. RDAC2
and RDAC3 set the low-pass, high-pass, and band-pass cutoff
and center frequencies, respectively. RDAC2 and RDAC3
should be programmed with the same data (as with ganged
potentiometers) to maintain the best Circuit Q.
A3
A2
A1
0.01µF
0.01µF
RDAC3
LOW-PASS
BAND-PASS
HIGH-PASS
RDAC2
B
RDAC4
B
V
IN
B
RDAC1
2.5V
OP279 × 2
R2
10kΩ
R1
10kΩ
A3
B
0
2794-053
Figure 52. Programmable Stable-Variable Filter
The transfer function of the band-pass filter is
2
2
O
O
O
O
i
BP
S
Q
S
S
Q
A
V
V
ω+
ω
+
ω
= (9)
where A
O
is the gain.
For R
WB2(D2)
= R
WB3(D3)
, R1 = R2, and C1 = C2:
C1R
WB2
O
1
=ω (10)
WA1
WB1
O
R
R
A =
(11)
R1
R
R
R
Q
WB1
WB4
WA4
×= (12)
Figure 53 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 54. At a center frequency of 2 kHz, the gain is adjusted
over the −20 dB to +20 dB range, determined by RDAC1.
Circuit Q is adjusted by RDAC4 and RDAC1. Suitable op amps
for this application are OP4177, AD8604, OP279, and AD824.
40
20 100 1k 10k 100k 200k
FREQUENCY (Hz)
AMPLITUDE (dB)
0
–20
–40
–60
–80
20
20k–16
*
02794-054
Figure 53. Programmed Center Frequency Band-Pass Response
40
20 100 1k 10k 100k 200k
FREQUENCY (Hz)
AMPLITUDE (dB)
0
–20
–40
–60
–80
20
2.0k–19.01
02794-055
Figure 54. Programmed Amplitude Band-Pass Response