VTI Instruments EX1629 - Appendix B Ex1629 Filtering; Introduction; Analog Anti-Aliasing Filter; Appendixb

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EX1629 Filtering 315
APPENDIX B
EX1629 FILTERING
INTRODUCTION
The diagram below details the filter chain present in the EX1629 channels, which contains both
analog and digital components. These components are discussed sequentially in the following
sections.
FPGA
CIC
(R=1-10000,
N=5, M=1)
6th order
modulator
ADC
ADS1271
f
so
50 kSa/s
Analog
Ant-Aliasing
Filter
(1-pole RC)
CIC
compensation
filter and FIR
decimation
filter
Butterworth/
Bessel IIR
filters with
variable fc
C6713
DSP
f
s
FIR
100dB stopband
attn.
ripple=0.005dB
where f
so
= input frequency to FIR decimator, f
s
= user sample rate
FIGURE B-1: EX1629 FILTER CHAIN
Analog Anti-Aliasing Filter
The analog anti-aliasing filter consists of a single-pole RC filter with R = 5.62 kΩ and C = 470 pF,
which has a -3 dB point of f
c
= 1/(2π*RC), or 60.254 kHz.
The rejection at 6.25 MHz (the ADC sigma delta modulator sampling rate) is:
2
1
1
c
f
f
f
= -40.52426 dB