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ADVANCED COMMUNICATIONS & SENSING
USER GUIDE
SX8724SEVK
Evaluation Kit for SX8724S Data Acquisition System
ACS - Revision 1.0 May 2011
© Semtech Corp.
Now we can apply the following formula to get the gain:
As the total gain is more than 100, all PGAs have to be enabled to obtain a total amplification of 825. The
gains must be set as follow:
•
PGA1 = 10
•
PGA2 = 10
•
PGA3 = 8.25
17.6.3 Offset Cancellation
The offset computation uses the middle sensor output value at the ZoomingADC™ input as shown in the
formula below:
Note: Mw stands for measuring window.
Offset removing PGA after PGAs:
PGA1: Amplified sensor output = 6.5mV x 10 = 65mV
PGA2: Amplified sensor output = 65mV x 10 = 650mV
Offset cancellation: min +/- 0.2 x V
REF
(3.3V x 0.2 = 660mV).
-> The minimum value is too big we cannot use the offset cancellation of PGA2.
PGA3: Amplified sensor output = 650mV x 8.25 = 5362mV
Offset cancellation: min +/- 1/12 x V
REF
(1/12 x 3.3V = 275mV)
-> The minimum value fits up to 19 times in 5362mV 19/12 x 3.3 = 5225mV.
Note: 19/12 = 1.583
17.6.4 Final Gains Configuration
As computed, the PGA gains can be raised to 825.
The offset is 19/12 (see
“Offset Cancellation” on page 43
), as the input signal and the reference are non-
inverted, the offset has to be positive.
Equation 2
Equation 3
825
4
3300
=
=
=
mV
mV
SensorOut
ADCOut
Gain
Max
Max
mV
mV
mV
Mw
Mw
Offset
Min
Max
5
.
6
2
5
.
4
5
.
8
2
=
−
=
−
=
