SX8724C Datasheet, PDF (55/67 Pages) Semtech Corporation – ZoomingADC for sensing data acquisition

ADVANCED COMMUNICATIONS & SENSING
SX8724C
ZoomingADC for sensing data acquisition
DATASHEET
Analog
Inputs
VSS
VREF
AC2
AC3
AC4
AC5
AC6
AC7
V_IN
V_N1
V_N2
V_N3
±Vin
±Vin
±Vin
S
PGA1
±Voff _PGA2
±Voff _PGA3
V_IN,ADC
±Vin
±Vref _ADC
Reference
Inputs
VBATT
VSS
VREF
VSS
V_REFN,WB
V_REF,ADC
gains: GD₁
offsets:
GD₂
GD_OFF2
GD₃
GD_OFF3
Figure 46. Simple Noise Model for PGAs and ADC
VN1, VN2, and VN3 are the output rms noise figures of Table 39, GD1, GD2, and GD3 are the PGA gains of stages 1 to 3
respectively. VREFN,WB is the wide band noise on the reference voltage.
The simple noise model of Figure 46 is used to estimate the equivalent input referred rms noise VN,IN of the acquisition
chain in the model of Figure 48, page 56. This is given by the relationship:
( ) _{[ ] V 2}
( ) ^{N ,IN}
=
⎜⎜⎝⎛
V_N1
GD₁
⎟⎟⎠⎞²
+
⎜⎜⎝⎛
V_{N 2}
GD₁ ⋅ GD₂
⎟⎟⎠⎞²
+
⎜⎜⎝⎛
V_{N 3}
GD_TOT
⎟⎟⎠⎞²
+ _{⎜⎜⎝⎛} ^{VREFN ,WB}
GD₂ ⋅ GD_{OFF 2}
GD_TOT
OSR ⋅ N _ELCONV
+ GD_{OFF 3}
⎟⎟⎠⎞²
+
⎜⎜⎝⎛
1
2
⋅
V_{REFN ,WB}
GD_TOT
⎟⎟⎠⎞²
V ²rms
Equation 28
On the numerator of Equation 28 :
1 the first parenthesis is the PGA1 gain amplifier contribution to noise
2 the second parenthesis is the PGA2 gain amplifier contribution to noise
3 the third parenthesis is the PGA3 gain amplifier contribution to noise
4 the fourth parenthesis is PGA2 and PGA3 offset amplifiers contributions to noise
5 the last parenthesis is the contribution of the noise on the references of the ADC
Revision 1.01
© Semtech
January 2011
Page 55
www.semtech.com/products/