
212
2570N–AVR–05/11
ATmega325/3250/645/6450
Figure 23-14. Differential Measurement Range
ADMUX = 0xFB (ADC3 - ADC2, 1.1V reference, left adjusted result)
Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.
ADCR = 512 * (300 - 500) / 1100 = -93 = 0x3A3
.
ADCL will thus read 0xC0, and ADCH will read 0xD8. Writing zero to ADLAR right adjusts the
result: ADCL = 0xA3, ADCH = 0x03.
Table 23-2.
Correlation Between Input Voltage and Output Codes
VADCn
Read Code
Corresponding Decimal Value
VADCm + VREF
0x1FF
511
V
ADCm +
511/
512 VREF
0x1FF
511
VADCm +
510/
512 VREF
0x1FE
510
...
V
ADCm +
1/
512 VREF
0x001
1
VADCm
0x000
0
VADCm -
1/
512 VREF
0x3FF
-1
...
VADCm -
511/
512 VREF
0x201
-511
V
ADCm - VREF
0x200
-512
0
Output Code
0x1FF
0x000
V
REF
Differential Input
Voltage (Volts)
0x3FF
0x200
- V
REF