SX8724C Datasheet, PDF (49/67 Pages) Semtech Corporation – ZoomingADC for sensing data acquisition

SX8724C
ZoomingADC for sensing data acquisition
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
show short voltage spikes at the frequency fs, but these will not influence the measurement. Whereas if the bandwidth
of the node is lower, no spikes will arise, but a small offset can be generated by the integration of the charges
generated by the switched capacitors, this corresponds to the mean impedance effect.
Notes:
(1) One can increase the mean input impedance of the ZoomingADC by lowering the acquisition clock fs.
(2) One can increase the mean input impedance of the ZoomingADC by decreasing the gain of the first enabled amplifier.
(3) One can increase the effective input impedance of the ZoomingADC by having a source with a high electrical bandwidth (sensor electri-
cal bandwidth much higher than fs).
11.2 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 28,
page 50 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----^â‹---^f-^s--------------
OSR ⋅ N_ELCONV
For
i = 1, 2, …(N_ELCONV – 1)
Equation 27
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 27 and setting the appropriate values of OSR, fs and NELCONV.
Table 38. 50/60 Hz Line Rejection 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
Revision 1.01
© Semtech
January 2011
Page 49
www.semtech.com/products/