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ADVANCED COMMUNICATIONS & SENSING
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
-> 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
-> 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
inverted, the offset has to be positive.
ACS - Revision 1.0
© Semtech Corp.
ADCOut
=
Gain
SensorOut
−
Mw
Mw
=
Max
Offset
2
(3.3V x 0.2 = 660mV).
REF
(1/12 x 3.3V = 275mV)
REF
"Offset Cancellation" on page
May 2011
Evaluation Kit for SX8724S Data Acquisition System
3300
mV
=
=
Max
825
4
mV
Max
Equation 2
−
8
5 .
mV
4
5 .
mV
=
Min
2
Equation 3
43), as the input signal and the reference are non-
Page 43
SX8724SEVK
USER GUIDE
=
6
5 .
mV
www.semtech.com
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