isppac30 Lattice Semiconductor Corp., isppac30 Datasheet - Page 19

• Manufacturer Part Number: isppac30
• Description: In-system Programmable Analog Circuit
• Manufacturer: Lattice Semiconductor Corp.

Lattice Semiconductor
Precision Filter Configuration
Figure 5. Using the ispPAC30 as a Variable Lowpass Filter with Extended Frequency Range
Other filter frequencies are possible, in addition to the simple first order filters available by selecting the seven avail-
able capacitors of each ispPAC30 output amplifier. The ispPAC30 can be used to implement 1st-order tunable low-
pass filters over a range of 5kHz to over 600kHz.
Figure 5 shows the circuit for doing so. This circuit operates by using MDAC2 to emulate a programmable feedback
resistor around output amplifier OA1. In this technique, the effective feedback resistance is inversely proportional to
MDAC gain. Because negative feedback is essential to maintaining a stable loop, MDAC2’s gain must be set to
only negative values.
In addition to decreasing the closed-loop bandwidth of OA1, fractional feedback gain also increases the closed
loop DC gain. This increase must be compensated for if the filter is to maintain unity gain from input to output.
Because there are two MDACs in an ispPAC30, one way to do this is to attenuate the input signal through MDAC1
by the same amount the feedback signal is attenuated by MDAC2. To maintain signal polarity, however, MDAC1
should be set to a positive gain. Deliberately mismatching the values of MDAC1 and MDAC2 also allows one to
alter the gain dynamically, providing a variable gain control feature. The following expressions can be used to esti-
mate the resulting corner frequency (F
Note that MDAC2 (n%) must be negative, and that MDAC1 (n%) should normally be positive for the single-ended
system shown in Figure 5.
Although this technique can be used to control the corner frequency over a range of 128:1, the attenuation caused
by a very low MDAC1 setting can reduce the filter's overall signal-to-noise ratio and increase effective DC offset
and gain errors to unacceptable levels. Table 3 shows the ranges of corner frequencies that can be realized with
this technique when limiting MDAC2 settings between -10.16% and -100%.
V
IN
IN1
ispPAC30
C
Gain = |MDAC1(n%) / MDAC2(n%)|
) and gain, where F
MDAC1 n%
MDAC2 n%
F
C
MDAC1
MDAC2
= |F
CAP
MDAC2(n%)|
19
CAP
is the frequency associated with the feedback.
‘Integrator’ Mode
OA1
C
F
ispPAC30 Data Sheet
OUT1
(1)
(2)