EM78P5840/5841/5842
8-bit Micro-controller
EM785840’s 10 bit ADC characteristic
EM785840 build in 10 bit resolution, multi channel ADC function. In ideal, if ADC’s reference voltage is 5V, the
ADC’s LSB will be 5V/1024. But in practical, for some physics or circuit’s character, some un-ideal will effect the converter
result. As the next figure, offset voltage will reduce AD’s converter range. If AD’s input voltage less than VOFL, ADC will
output 0; in opposition, if input voltage is larger than (VDD-VOFH), ADC will output 1023. That is to say the physics AD
converter range will replace by (VDD-VOFH+LSB-VOFL+LSB). If we defined that VRB = VOFL – LSB and VRT = VDD-
VOFH+LSB, the physics LSB is:
LSB = (VRT - VRB) / 1024
= (VDD – (VOFH+VOFL) ) / 1022
For real operating, please think about the effect of AD’s offset voltage. If converter the range of (VRT - VRB), the AD
converter’s opposite result will be précised.
10-bit ADC
VDD
VRT
VOFH
Min. input for ADC output = 1023
(For 10-bit ADC, internally it takes this range to
average 1024 steps)
Min. input for ADC output = 1
VOFL
VRB
0V
Fig.19 The relative between ADC and offset voltage
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* This specification is subject to change without notice.
41
2004/11/10 V2.6
ELAN [ ELAN MICROELECTRONICS CORP ]
