ispPAC10-01PI Lattice Semiconductor, ispPAC10-01PI Datasheet - Page 12

• Manufacturer Part Number: ispPAC10-01PI
• Description: In-System Programmable Analog Circuit
• Manufacturer: Lattice Semiconductor

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
Figure 7a. Biquad Bandpass Filter Block Diagram
and the schematic from PAC-Designer is shown in Figure
7b. 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.
(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)
12
It can also be seen that the transfer function V
implements a lowpass filter. This application is discussed
further in a separate application note.
Figure 7b. Biquad Bandpass Filter Schematic
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
The result is a circuit whose transfer function is:
The gains k
this circuit can either amplify or attenuate an input signal.
The one in the denominator is due to R
F
), as shown in Figure 8.
OUT1
OUT2
V
IN1
IN2
IN1
Specifications ispPAC10
1
IA1
IA2
and k
k
k
1
2
V
V
OUT
IN
2
are independently set by the user;
=
IA1
IA2
IA3
IA4
V
-
-1
-1
1
1
< 1
k
2
PACblock 1
PACblock 2
–
k
1
sC
2
g
OA1
m
F
2.5V
F
C
R
; if R
F
F
F
OA1
OA2
16.05pF
FB
8.19pF
is disabled,
2.5V
2.5V
(s)/V
V
OUT
IN
(8)
(s)