RFM22B/23B
8.3. General Purpose ADC
An 8-bit SAR ADC is integrated for general purpose use, as well as for digitizing the on-chip temperature sensor
reading. Registers 0Fh "ADC Configuration", 10h "Sensor Offset" and 4Fh "Amplifier Offset" can be used to
configure the ADC operation.
Every time an ADC conversion is desired, bit 7 "adcstart/adcbusy" in Register 1Fh “Clock Recovery Gearshift
Override” must be set to 1. This is a self clearing bit that will be reset to 0 at the end of the conversion cycle of the
ADC. The conversion time for the ADC is 350 µs. After this time or when the "adcstart/adcbusy" bit is cleared, then
the ADC value may be read out of “Register 11h. ADC Value."
The architecture of the ADC is shown in Figure 27. The signal and reference inputs of the ADC are selected by
adcsel[2:0] and adcref[1:0] in register 0Fh “ADC Configuration”, respectively. The default setting is to read out the
temperature sensor using the bandgap voltage (VBG) as reference. With the VBG reference the input range of the
ADC is from 0-1.02 V with an LSB resolution of 4 mV (1.02/255). Changing the ADC reference will change the LSB
resolution accordingly.
A differential multiplexer and amplifier are provided for interfacing external bridge sensors. The gain of the amplifier
is selectable by adcgain[1:0] in Register 0Fh. The majority of sensor bridges have supply voltage (VDD) dependent
gain and offset. The reference voltage of the ADC can be changed to either V /2 or V /3. A programmable V
DD
DD
DD
dependent offset voltage can be added using soffs[3:0] in register 10h.
.
Diff. MUX
Diff. Amp.
Input MUX
aoffs [4:0]
soffs [3:0]
adcsel [2:0]
adcgain [1:0]
GPIO0
GPIO1
GPIO2
8-bit ADC
Temperature Sensor
Vin
adc [7:0]
adcsel [2:0]
Vref
0 -1020mV / 0-255
Ref MUX
VDD / 3
DD / 2
V
VBG (1.2V)
adcref [1:0]
Figure 27. General Purpose ADC Architecture
Add R/W
Function/Description
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
0F
10
11
R/W
R/W
R
ADC Configuration
adcstart/adcbusy adcsel[2] adcsel[1] adcsel[0] adcref[1]
soffs[3]
adcref[0] adcgain[1] adcgain[0]
00h
Sensor Offset
ADC Value
soffs[2]
adc[2]
soffs[1]
adc[1]
soffs[0]
adc[0]
00h
—
adc[7]
adc[6]
adc[5]
adc[4]
adc[3]
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