isppac20 Lattice Semiconductor Corp., isppac20 Datasheet - Page 15

• Manufacturer Part Number: isppac20
• Description: In-system Programmable Analog Circuit
• Manufacturer: Lattice Semiconductor Corp.

Integrator . Switching out R
the feedback element as shown in Figure 6. The
integrator’s transfer function can be derived from Equa-
tion 5b by setting g
Figure 6. PAC-Designer PACblock Integrator
(IAF Disabled; g
The integrator slope is proportional to 1/f and, for the case
of a single input, the transfer function magnitude equals
|k| when the frequency is (1/2 )(2g
should not be used as a stand-alone circuit element. It
needs to be used in configurations that provide DC
feedback to ensure the output does not saturate, as
illustrated by the biquad filter circuit below.
Application Examples
Biquad Filter . By simply combining the two structures,
the integrator providing feedback around the lossy inte-
grator, creates a useful circuit. The block diagram is
shown in Figure 7a and the schematic from PAC-De-
signer is shown in Figure 7b.
Figure 7a. Biquad Bandpass Filter Block Diagram
(IN1)
Theory of Operation (Continued)
V IN
V OUT2
(OUT2)
V
V
IN1
IN2
IA1
IA2
V
V FB
k
k
OUT
1
2
m3
Error
m3
= 0)
=
= 0 (open circuit IAF (R
k
1
V
IN1
1 +
F
sC
2
A
B
(turning off IAF) removes
s
+
g
m
p
k
F
s
1
2
V
OA1
IN2
m
/C). The integrator
2.5V
C
R
F
F
F
V
)).
OUT
V OUT1
(OUT1)
(7)
15
Figure 7b. Biquad Bandpass Filter Schematic
The transfer function OUT1(s)/IN1(s) is a band pass filter
with programmable gain, Q and center frequency. Note
the presence of DC feedback around the integrator. It can
also be seen that the transfer function V
implements a lowpass filter. This application is discussed
further in a separate application note.
To ease the design of Biquad Filters, PAC-Designer
contains a macro tool that allows a user to simply specify
filter corner, q factor and gain. This macro is accessed
under the Tools menu.
Attenuator . The PACblock architecture makes varia-
tions possible on these two basic building blocks just
described. An example uses summation to connect an
input amplifier (IA2) in parallel with the feedback element
(R
Figure 8. PACblock A
F
), as shown in Figure 8.
V
IN1
Specifications ispPAC20
IA1
IA2
k
k
1
2
V
< 1
OA1
2.5V
C
R
F
F
FB
(s)/V
V
OUT
IN
(s)