XE8802MI000 SEMTECH [Semtech Corporation], XE8802MI000 Datasheet - Page 135

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

17.8.4
Ideally, a constant input voltage V
output code may vary for a fixed input voltage. Thus, a statistical analysis on the output code of 1200 conversions
for a constant input voltage was performed to derive the equivalent noise levels of PGA1, PGA2, and PGA3. The
extracted rms output noise of PGA1, 2, and 3 are given in Table 17-27: standard output deviation and output rms
noise voltage. Figure 17-17 shows the distribution for the ADC alone (PGA1, 2, and 3 bypassed). Quantitative
noise is dominant in this case, and, thus, the ADC thermal noise is below 16 bits.
The simple noise model of Figure 17-18(a) is used to estimate the equivalent input referred rms noise V
acquisition chain in the model of Figure 17-18(b). This is given by the relationship:
where V
stages 1 to 3 respectively. As shown in this equation, noise can be reduced by increasing OSR and N
(increases the ADC averaging effect, but reduces noise).
© Semtech 2006
Table 17-27. PGA noise measurements (n = 16 bits, OSR = 512, N
V
N1
N
, V
,
Noise
IN
2
N2
=
, and V
(
V
N
1
/
GD
N3
Note: see noise model of Figure 17-18 and equation Eq. 21.
are the output rms noise figures of Table 17-27, GD
Output rms noise (µV)
1
Standard deviation at
)
2
ADC output (LSB)
Figure 17-16. Differential non-linearity of the ADC converter.
+
(
Parameter
V
N
2
IN
/(
should result in a constant output code. However, because of circuit noise, the
GD
(
OSR
1
⋅
GD
XE8802 Sensing Machine Data Acquisition MCU
⋅
N
2
1
))
ELCONV
2
+
(
V
205 (V
)
N
PGA1
0.85
3
17-23
/(
GD
N1
)
1
with ZoomingADC™ and LCD driver
⋅
GD
340 (V
PGA2
2
1.4
⋅
GD
N2
)
3
))
2
365 (V
PGA3
ELCONV
1.5
(V
1
, GD
2
N3
rms)
)
2
= 2, V
, and GD
(Eq. 21)
REF
= 5V)
3
are the PGA gains of
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N,IN
ELCONV
of the