SX8722I070LF SEMTECH [Semtech Corporation], SX8722I070LF Datasheet - Page 73

• Manufacturer Part Number: SX8722I070LF
• Description: High gain acquisition for sensor interface
• Manufacturer: SEMTECH [Semtech Corporation]

As an example, consider the system where: GD2 = 10 (GD1 = 1; PGA3 bypassed), OSR = 512, N
this case, the noise contribution V
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.
Noise can also be reduced by the additional average filters implemented in the Measurement Engine. These filters are
described in section 11.4 Filtering. By making an average on a number of subsequent measurements, the apparent noise is
reduced the square root of the number of measurement used to make the average.
12.16. Gain Error and Offset Error
Gain error is defined as the amount of deviation between the ideal transfer function (theoretical Equation 18) and the
measured transfer function (with the offset error removed).
The actual gain of the different stages can vary depending on the fabrication tolerances of the different elements. Although
these tolerances are specified to a maximum of +/-3%, they will be most of the time around 0.5%. Moreover, the tolerances
between the different stages are not correlated and the probability to get the maximal error in the same direction in all stages
is very low. Finally, these gain errors can be calibrated by the software at the same time with the gain errors of the sensor for
instance.
Figure 40 shows gain error drift vs. temperature for different PGA gains. The curves are expressed in % of Full-Scale Range
(FSR) normalized to 25°C.
ACS - Revision 4.2
©2008 Semtech Corp.
ADVANCED COMMUNICATIONS & SENSING
V
N,IN
GD1
PGA1
GD1
October 2008
PGA1
Figure 38. Simple Noise Model for PGAs and ADC
N1
of PGA1 is dominant over that of PGA2. Using Equation 24, we get: V
V
N1
Figure 39. Total Input Referred Noise
V
N1
GD2
PGA2
GD2
PGA2
Page 73
V
N2
V
N2
GD3
PGA3
High gain acquisition for sensor interface
GD3
PGA3
V
N3
V
N3
ADC
f
s
ADC
f
s
ELCONV
DATASHEET
www.semtech.com
SX8722
= 2, V
N,IN
= 6.4 µV (rms)
REF
= 5 V. In