Datasheet
–15–
REV.
3
AD7564
APPLICATIONS
Programmable State Variable Filter
The AD7564 with its multiplying capability and fast settling
time is ideal for many types of signal conditioning applications.
The circuit of Figure 25 shows its use in a state variable filter
design. This type of filter has three outputs: low pass, high pass
and bandpass. The particular version shown in Figure 25 uses
the AD7564 to control the critical parameters f
O
, Q and A
O
. In-
stead of several fixed resistors, the circuit uses the DAC equiva-
lent resistances as circuit elements.
Thus, R1 in Figure 25 is controlled by the 12-bit digital word
loaded to DAC A of the AD7564. This is also the case with R2,
R3 and R4. The fixed resistor R5 is the feedback resistor, R
FB
B.
DAC Equivalent Resistance, R
EQ
= (R
LADDER
×
4096)/N
where: R
LADDER
is the DAC ladder resistance
N is the DAC Digital Code in Decimal (0 < N < 4096)
In the circuit of Figure 25:
C1 = C2, R7 = R8, R3 = R4 (i.e., the same code is loaded to
each DAC).
Resonant Frequency, f
O
= 1/(2 π R3C1)
Quality Factor, Q = (R6/R8)
×
(R2/R5)
Bandpass Gain, A
O
= –R2/R1
Using the values shown in Figure 25, the Q range is 0.3 to 5 and
the f
O
range is 0 to 12 kHz.
R8
30kΩ
HIGH
PASS
OUTPUT
C1 1000pF C2 1000pF
LOW
PASS
OUTPUT
A2 A3 A4
A1
C3 10pF
BAND
PASS
OUTPUT
R6
10kΩ
V
IN
V
REF
A
I
OUT1
AI
OUT1
BR
FB
BV
REF
CI
OUT1
CV
REF
DI
OUT1
DV
REF
B
AD7564
I
OUT2
AI
OUT2
BI
OUT2
CI
OUT2
D
DAC B
(R2)
DAC C
(R3)
DAC D
(R4)
R7
30kΩ
R5
DAC A
(R1)
AGND
NOTES
1. A1, A2, A3, A4, : 1/4 X AD713.
2. DIGITAL INPUT CONNECTIONS ARE OMITTED.
3. C3 IS A COMPENSATION CAPACITOR TO ELIMINATE Q AND GAIN VARIATIONS
CAUSED BY AMPLIFIER GAIN AND BANDWIDTH LIMITATIONS.
Figure 25. Programmable 2nd Order State Variable Filter
B