SX8723CWLTDT SEMTECH [Semtech Corporation], SX8723CWLTDT Datasheet - Page 49

• Manufacturer Part Number: SX8723CWLTDT
• Description: ZoomingADC for sensing data acquisition
• Manufacturer: SEMTECH [Semtech Corporation]

Revision 1.01
© Semtech
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)
(2)
(3)
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 V
Notice that the frequency axes are normalized to one elementary conversion period OSR / fs. The plots of
page 50
performed. In particular, notches appear for N
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
ADVANCED COMMUNICATIONS & SENSING
One can increase the mean input impedance of the ZoomingADC by lowering the acquisition clock fs.
One can increase the mean input impedance of the ZoomingADC by decreasing the gain of the first enabled amplifier.
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).
also show that the frequency response changes with the number of elementary conversions N
and setting the appropriate values of OSR, fs and N
Rejection [Hz]
60
50
f
NOTCH
January 2011
=
------------------------------------ -
OSR N
f
61
61
61
53
46
46
NOTCH
Table 38. 50/60 Hz Line Rejection Examples
⋅
i f
⋅
ELCONV
s
[Hz]
ELCONV
fs [kHz]
125
250
500
62.5
62.5
125
>= 2 These notches occur at:
For
Equation 27
Page 49
ELCONV
ZoomingADC for sensing data acquisition
i
OSR [-]
1024
1024
1024
1024
1024
1024
.
=
1 2 … N
, ,
(
ELCONV
N
2
4
8
8
4
8
www.semtech.com/products/
ELCONV
–
[-]
1
SX8723C
)
DATASHEET
ELCONV
Figure 28,
IN
.