SX8723 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SX8723' PDF : 53 Pages View PDF
SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Frequency Response
The incremental ADC is an over-sampled converter with two main blocks: an analog modulator and a low-pass
digital filter. The main function of the digital filter is to remove the quantization noise introduced by the
modulator. This filter determines the frequency response of the transfer function between the output of the ADC
and the analog input VIN. Notice that the frequency axes are normalized to one elementary conversion period
OSR / fs. The plots of Figure 21 also show that the frequency response changes with the number of elementary
conversions NELCONV performed. In particular, notches appear for NELCONV ≥ 2. These notches occur at:
f NOTCH
(i)
=
i⋅ fs
OSR ⋅ N ELCONV
(Hz) for i = 1,2,..., (NELCONV −1)
Equation 24
and are repeated every fs / OSR.
Information on the location of these notches is particularly useful when specific frequencies must be filtered out
by the acquisition system. This chip has no dedicated 50/60 Hz rejection filtering but some rejection can be
achieved by using Equation 24 and setting the appropriate values of OSR, fs and NELCONV.
Examples:
Rejection [Hz]
60
50
fNOTCH [Hz]
61
61
61
53
46
46
fs [kHz]
125
250
500
62.5
62.5
125
OSR [-]
1024
1024
1024
1024
1024
1024
NELCONV [-]
2
4
8
8
4
8
Table 32 - 60/50 Hz Line Rejection Examples
1.2
1
0.8
0.6
0.4
0.2
0
0
NELCONV = 1
1
2
3
4
Normalized Frequency - f *(OSR/fS) [-]
1.2
1
0.8
0.6
0.4
0.2
0
0
NELCONV = 2
1
2
3
4
Normalized Frequency - f *(OSR/fS) [-]
1.2
1
0.8
0.6
0.4
0.2
0
0
NELCONV = 4
1
2
3
4
Normalized Frequency - f *(OSR/fS) [-]
1.2
1
0.8
0.6
0.4
0.2
0
0
NELCONV = 8
1
2
3
4
Normalized Frequency - f *(OSR/fS) [-]
Figure 21 - Frequency Response: Normalized Magnitude vs. Frequency for Different NELCONV
V1.5 © 2007 Semtech Corp.
38
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