RFM22
8.3.1. ADC Differential Input Mode—Bridge Sensor Example
The differential input mode of ADC8 is designed to directly interface any bridge-type sensor, which is demonstrated
in the figure below. As seen in the figure the use of the ADC in this configuration will utilize two GPIO pins. The
supply source of the bridge and module should be the same to eliminate the measuring error caused by battery
discharging. For proper operation one of the VDD dependent references (VDD/2 or VDD/3) should be selected for
the reference voltage of ADC8. VDD/2 reference should be selected for VDD lower than 2.7 V, VDD/3 reference
should be selected for VDD higher than 2.7 V. The differential input mode supports programmable gain to match
the input range of ADC8 to the characteristic of the sensor and VDD proportional programmable offset adjustment
to compensate the offset of the sensor.
Figure 25. ADC Differential Input Example—Bridge Sensor
The adcgain[1:0] bits in "Register 0Eh. I/O Port Configuration" determine the gain of the differential/single ended
amplifier. This is used to fit the input range of the ADC8 to bridge sensors having different sensitivity:
Differential Gain
adcgain[1]
adcgain[0]
Input Range (% of VDD)
adcref[0] = 0
22/13
adcref[0] = 1
33/13
0
0
1
1
0
1
0
1
16.7
8.4
44/13
66/13
66/13
99/13
5.6
88/13
132/13
4.2
Note: The input range is the differential voltage measured between the selected GPIO pins corresponding to the full ADC
range (255).
The gain is different for different VDD dependent references so the reference change has no influence on input range
and digital measured values.
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