XE8802MI000 SEMTECH [Semtech Corporation], XE8802MI000 Datasheet - Page 126

• Manufacturer Part Number: XE8802MI000
• Description: Sensing Machine Data Acquisition MCU
• Manufacturer: SEMTECH [Semtech Corporation]

Using Table 17-22 or the graph plotted in Figure 17-6, resolution can be set between 6 and 16 bits. Notice that,
because of 16-bit register use for the ADC output, practical resolution is limited to 16 bits, i.e. n ≤ 16. Even if the
resolution is truncated to 16 bit by the output register size, it may make sense to set OSR and N
values in order to reduce the influence of the thermal noise in the PGA (see section 17.8.4).
17.7.6
As explained using Figure 17-5, conversion time is given by:
and throughput is then simply 1/T
conversions, and a sampling frequency of 500kHz (SET_OSR = "101", SET_NELC = "01", f
"00"). In this case, using Table 17-23, the conversion time is 515 sampling periods, or 1.03ms. This corresponds to
a throughput of 971Hz in continuous-time mode. The plot of Figure 17-7 illustrates the classic trade-off between
resolution and conversion time.
17.7.7
The ADC output code is a 16-bit word in two's complement format (see Table 17-24). For input voltages outside the
range, the output code is saturated to the closest full-scale value (i.e. 0x7FFF or 0x8000). For resolutions smaller
than 16 bits, the non-significant bits are forced to the values shown in Table 17-25. The output code, expressed in
LSBs, corresponds to:
© Semtech 2006
T
OUT
CONV
Conversion Time & Throughput
Output Code Format
ADC
=
(
Figure 17-7. Resolution vs. normalized conversion time for different SET_NELC[1:0]
N
=
ELCONV
2
16
Table 17-23. Normalized conversion time (T
⋅
V
⋅
V
IN
(
OSR
REF
,
ADC
SET_OSR
[2:0]
+
⋅
000
001
010
011
100
101
110
111
) 1
OSR
OSR
+
SET_NELC[1:0](normalized to sampling period 1/f
1
/ )
+
CONV
1
f
S
16.0
14.0
12.0
10.0
8.0
6.0
4.0
(s)
(LSB)
1026
. For example, consider an over-sampling ratio of 256, 2 elementary
130
258
514
10.0
00
10
18
34
66
XE8802 Sensing Machine Data Acquisition MCU
00
Normalized Conversion Time - T
01
SET_NELC[1:0]
1027
2051
10
131
259
515
01
19
35
67
100.0
11
17-14
SET_NELC
1029
2053
4101
133
261
517
10
37
69
with ZoomingADC™ and LCD driver
1000.0
CONV
1033
2057
4105
8201
137
265
521
CONV
11
73
(Eq. 16)
(Eq.17)
*f
⋅
f
S
S
) vs. SET_OSR[2:0] and
[-]
10000.0
S
)
RC
= 2MHz, and FIN =
ELCONV
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