VTI Instruments Corp.
318
EX1629 Filtering
FIR decimation filter
There are up to 128 tap filters that decimate up to a factor of 10. These filters can operate in the
linear range of the CIC response. Hence, the CIC filter is used to decimate frequencies up to
max_
f
c
*20 or more, then the FIR filter is used to further decimate the data such that a passband
close to the Nyquist frequency is achieved. The ripple of this filter for
f
s
≥ 3125 is 0.01 dB and for
f
s
< 3125 is 0.001 dB with an alias rejection of 100 dB.
FIR filtering is done in floating point.
IIR filters
The IIR filters are designed on-the-fly in the DSP. Given the following:
filter type (i.e. Butterworth or Bessel)
sampling frequency,
f
s
cutoff
f
c
– -3 dB point can be, at most, max_
f
c
to operate in the linear region of the decimating
filters
transform type (Bilinear/Matched Z)
IIR filter order – supported ranges are 1-10. For the Butterworth filter type, if the filter order
is set to 0, the DSP determines the order by assuming attenuation of 200 dB in stopband. The
filter order MUST be specified for Bessel filters.
the DSP designs a filter implemented as biquad sections. The maximum filter order is 10. The
DSP also determines the risetime, overshoot, and group delay of the filter.
IIR filtering operations are done in double-precision floating point.
Filtering Limitations
IIR filtering is done only at rates 10 kHz and below due to DSP processing constraints
Group Delay
FIR filters
The group delay (τ
g
) through the FIR filters is constant over frequency and are calculated as
follows:
τ
g FIR
= (
Ntaps
-1)/2, where
Ntaps
is the number of taps.
The settling time of a FIR filter is twice the group delay (2τ
g FIR
).
In the EX1629, the ADC and the CIC are FIR filters. Their group delays are:
ADC = 39 samples @ 50 kSa/s
2
)
1
(
RM
N
CIC
@ 50 kSa/s
Where
R
=decimation
N
=number of stages = 5
M
=differential delay = 1
FIR filter: (
fir_Ntaps
-1)/2 @ CIC output rate, where
fir_ntaps
is the number of taps of the FIR
filter.
Hence, the total delay through the FIRs at the ADC sample rate (50 kHz) is:
τ
g FIR
=
s
dec
cic
Ntaps
fir
dec
cic
20
_
1
_
2
1
_
5
39
.
