xe8802 Semtech Corporation, xe8802 Datasheet - Page 136

• Manufacturer Part Number: xe8802
• Description: Data Acquisition Soc With 16+10 Bit Zoomingadc? Embedded Risc Controller And Lcd Driver
• Manufacturer: Semtech Corporation

As an example, consider the system where: GD
5V. In this case, the noise contribution V
V
Considering a 0.2V (rms) maximum signal amplitude, the signal-to-noise ratio is 90dB.
Implementing a software filter can also reduce noise. By taking an average on a number of subsequent
measurements, the apparent noise is reduced by the square root of the number of measurement used to make the
average.
17.8.5
Gain error is defined as the amount of deviation between the ideal transfer function (theoretical equation Eq. 18)
and the measured transfer function (with the offset error removed).
The actual gain of the different stages can vary relating to the fabrication tolerances of the different elements.
Although these tolerances are specified to a maximum of ±3%, most of the time they will be around ±0.5%.
Moreover, the tolerances between the different stages are not correlated and the probability of getting the maximal
error in the same direction in all stages is very low. Finally, the software can calibrate these gain errors at the same
time as the gain errors of the sensor for instance.
Figure 17-19 shows gain error drift vs. temperature for different PGA gains. The curves are expressed in % of Full-
Scale Range (FSR) normalized to 25°C.
© Semtech 2006
N,IN
= 6.4µV (rms) at the input of the acquisition chain, or, equivalently, 0.85 LSB at the output of the ADC.
Gain Error and Offset Error
Figure 17-17. ADC noise (PGA1, 2 & 3 bypassed, OSR=512,N
Figure 17-18. (a) Simple noise model for PGAs and ADC
V
PGA1
GD1
N,IN
PGA1
V
GD1
80
60
40
20
0
N1
and (b) total input referred noise
N1
-5
XE8802 Sensing Machine Data Acquisition MCU
of PGA1 is dominant over that of PGA2. Using equation Eq. 21, we get:
-4
Output Code Deviation From Mean Value [LSB]
PGA2
GD2
2
-3
= 10 (GD
PGA2
GD2
-2
V
N2
17-24
-1
(a)
(b)
1
0
= 1; PGA3 bypassed), OSR = 512, N
PGA3
with ZoomingADC™ and LCD driver
GD3
PGA3
GD3
1
V
2
N3
3
ADC
ADC
4
f
f
S
S
5
ELCONV
=2)
ELCONV
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= 2, V
REF
=