SX8801IRXXMLTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SX8801IRXXMLTRT' PDF : 135 Pages View PDF
XE8801A – SX8801R
16.7 ADC Characteristics
The main performance characteristics of the ADC (resolution, conversion time, etc.) are determined by three
programmable parameters. The setting of these parameters and the resulting performances are described later.
• sampling frequency fS,
• over-sampling ratio OSR, and
• number of elementary conversions NELCONV.
16.7.1 Conversion Sequence
A conversion is started each time the bit START or the bit DEF is set. As depicted in Figure 16-5, a complete analog-
to-digital conversion sequence is made of a set of NELCONV elementary incremental conversions and a final
quantization step. Each elementary conversion is made of (OSR+1) sampling periods TS=1/fS, i.e.:
TELCONV = (OSR + 1) / fS
(s)
(Eq. 12)
The result is the mean of the elementary conversion results. An important feature is that the elementary conversions
are alternatively performed with the offset of the internal amplifiers contributing in one direction and the other to the
output code. Thus, converter internal offset is eliminated if at least two elementary sequences are performed (i.e. if
NELCONV ≥ 2). A few additional clock cycles are also required to initiate and end the conversion properly.
TELCONV= (OSR+1)/fS
Init
Elementary
Conversion
Conversion index 1
Offset +
Elementary
Conversion
Elementary
Conversion
2
NELCONV- 1
-
TCONV
+
Elementary
Conversion
Conversion
End Result
NELCONV
-
Figure 16-5 Analog-to-digital conversion sequence
16.7.2 Sampling Frequency
The word FIN[1:0] is used to select the sampling frequency fS (Table 16-19). Three sub-multiples of the internal
RC-based frequency fRCEXT can be chosen. For FIN = "11", sampling frequency is about 8kHz. Additional information
on oscillators and their control can be found in the clock block documentation.
FIN[1:0]
00
01
10
11
Sampling Frequency fS (Hz)
XE8801A, SX8801
and XE8805A
XE8802
1/4⋅fRC
1/8⋅fRC
1/32⋅fRC
∼8kHz
1/8⋅fRCEXT
1/16⋅fRCEXT
1/64⋅fRCEXT
∼4kHz
Table 16-19 Sampling frequency settings (fRC or fRCEXT is the RC oscillator frequency)
© Semtech 2005
16-12
www.semtech.com
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