PAC80 Datasheet, PDF (9/19 Pages) Lattice Semiconductor – In-System Programmable Analog Circuit

Specifications ispPAC80
Theory of Operation
Reference Configuration
In the specifications table, specific filter configurations
are referred to, such as the CC051042. This is simply a
shorthand notation for the classical filter type that in-
cludes all the characteristic parameters of the particular
configuration. In this case, the CC refers to complete
Chebyshev, or a Chebyshev ripple response in both the
pass and stopbands of the filter. Another common name
for this type of filter is the Elliptic family of filters. The next
set of numbers, “05” refers to the order of the filter. In the
case of ispPAC80 this will always be fifth order. The next
two digits signify the reflection coefficient (rf), in this case
10%, and has a direct mathematical relationship to the
passband ripple magnitude of the filter, where the pass-
band ripple expressed in dB = 10*Log(1-rf²). The final two
digits are the passband to stopband attenuation ratio
expressed as an angle, in this case 42 degrees or 1.49
(1/sin((π/180)*42)). This configuration corresponds to
the elliptical filter with the ID# 3083 (Fp=50.00kHz;
Fc=56.23kHz) in the filter configuration database utility of
PAC-Designer. Because of the almost limitless number
of configurations realizable with ispPAC80, standard test
configurations had to be chosen. The Elliptical family was
chosen since it has many parameters that can be easily
and directly measured to insure that all the internal
circuits of ispPAC80 are operating correctly.
SPI Power-On Condition
The SPI shift register is always reset to all zeroes when
an ispPAC80 powers on. That means that if the ENSPI
pin is high at power on, the initial configuration will be set
to a gain of 1X (0dB) and configuration “A” is selected as
the “wake-up” configuration. The only way to prevent this
behavior would be to hold the ENSPI pin low while
applying power to the device. Because this is usually
impractical, it is advised that if the ispPAC80 is used in
SPI mode that it be reloaded to the desired first configu-
ration every time power is cycled to the device and/or that
Figure 1. Simplified ispPAC80 Filter Core Schematic
Rin
100K
Vin
R21
100K
R32
-100K
R11 100K
C1
R12
-100K
A1
V1
‚
C231
C213
L2
R23
100K
A2
V2
ƒ
R43
100K
C3
R34
-100K
A3
V3
„
R54
-200K
C453
C435
L4
R45
100K
A4
V4
…
R55
200K
C5
A5
Vout
VOUT ₌
VIN
-2G ^{}(L2 L4 C4 C2)s4 + ((L2 G2 C2) + (G ² L4 C4)) s2 + G4^{}
_{-(L2 C42 L4 C1) + 2( L2 C1 C3 L4 C5) - 2(C22 L2 L4 C5)}_s5
+-(L2 G C42 L4) + (L2 C1 C3 L4 G) - (C22 L2 L4 G) + 2(L2 G C3 L4 C5)^{}s4
+(L2 C1 C3 G2 ) + (L2 G2 C3 L4) - (G2 L4 C42 ) + 2(G2 L4 C5 C3) - 4(G2 L4 C5 C2)
-2(L2 C1 C4 G2 ) + 2(G2 C1 L4 C5) + 2(L2 C5 G2 C1)-(C22 L2 G2 )^{}s3
+2(L2 C5 G3 ) - 2(G3 L4 C2) + (G3 L4 C3) + (L2 G3 C1) + 2(G3 L4 C5) - 2( L2 C4 G3 ) + (G3 C1 L4) + (L2 C3 G3 )^{} s2
+2(G4
C5)
+
(G4
C3)
-
2(G4
C4)
+
(G4
L4)
+
(G4
C1)
+
(L2
G4 )
-
2(G4
C2)^{}
s
+
₂
G5


9