sx8725 Semtech Corporation, sx8725 Datasheet - Page 44

• Manufacturer Part Number: sx8725
• Description: Sx8725 Zoomingadc? For Pressure And Temperature Sensing
• Manufacturer: Semtech Corporation

ADVANCED COMMUNICATIONS & SENSING
Noise
Ideally, a constant input voltage VIN should result in a constant output code. However, because of circuit noise,
the 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 33: standard output deviation
and output rms noise voltage. Figure 23 shows the distribution for the ADC alone (PGA1, 2, and 3 bypassed).
Quantization noise is dominant in this case, and, thus, the ADC thermal noise is below 16 bits.
The simple noise model of Figure 24 is used to estimate the equivalent input referred rms noise V
acquisition chain in the model of Figure 25. 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).
Standard
output (LSB)
Output rms noise (µV)
V1.5 © 2007 Semtech Corp.
Parameter
N1
deviation
, V
Table 33 - PGA Noise Measurements (n = 16 bits, OSR = 512, N
N2
, and V
at
V
N
N3
ADC
,
IN
are the output rms noise figures of Table 33, GD
2

 

GD
V
N
1
1

 

2
205 (V
PGA1
0.85

 

ZoomingADC™ for Pressure and Temperature Sensing
GD
N1
)
V
1
OSR
N
GD
2
Equation 25
2
N

 

44
2
ELCONV

 

GD
1
GD
V
N
340 (V
PGA2
3
2
1.4
GD
N2
1
)
, GD
3

 

2
ELCONV
2
, and GD
(V
= 2, V
2
rms)
3
are the PGA gains of
REF
www.semtech.com
= 5 V)
365 (V
PGA3
1.5
SX8725
N3
N,IN
)
ELCONV
of the