SX8725C View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SX8725C' PDF : 68 Pages View PDF
SX8725C
ZoomingADC for sensing data acquisition
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
As shown in Equation 28, noise can be reduced by increasing OSR and NELCONV (increases the ADC averaging effect,
but reduces noise).
Table 39. PGA Noise Measurement (n = 16bits, OSR = 512, NELCONV = 2, VREF = 5V)
Parameter
Output RMS noise(uV)
PGA1
VN1 = 205
PGA2
VN2 = 340
PGA3
VN3 = 365
Figure 47 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.
80
60
40
20
0
-5 -4 -3 -2 -1 0 1 2 3 4 5
Output Code Deviation From Mean Value [LSB]
Figure 47. ADC Noise (PGA1, 2 & 3 Bypassed, OSR = 512, NELCONV = 2)
Analog
Inputs
VSS
VREF
AC2
AC3
N.C.
N.C.
N.C.
N.C.
VN,IN
VIN
±Vin
S
PGA1
±Vin
±Voff PGA2
±Vin
±Voff PGA3
VIN,ADC
±Vin
±Vref ADC
Reference
Inputs
VBATT
VSS
VREF
VSS
VREF,ADC
Figure 48. Total Input Referred Noise
As an example, consider the system where: GD2 = 10 (GD1 = 1; PGA3 bypassed), OSR = 512, NELCONV = 2, VREF = 5 V. In
this case, the noise contribution VN1 of PGA1 is dominant over that of PGA2. Using Equation 28, page 55, we get: VN,IN
= 6.4 μV (rms) at the input of the acquisition chain, or, equivalently, 0.85 LSB at the output of the ADC. Considering 0.2
V (rms) maximum signal amplitude, the signal-to-noise ratio is 90dB.
Revision 1.01
© Semtech
January 2011
Page 56
www.semtech.com/products/
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