W79E217A Datasheet, PDF (170/207 Pages) Winbond – 8-bit Microcontroller

Preliminary W79E217A Data Sheet
Figure 19-2: ADC Block Diagram
19.2 ADC Resolution and Analog Supply
The ADC circuit has its own supply pins (AV_DD and AV_SS) and one pins (Vref+) connected to each end
of the DAC’s resistance-ladder that the AV_DD and Vref+ are connected to V_DD and AV_SS is connected to
V_SS. The ladder has 1023 equally spaced taps, separated by a resistance of “R”. The first tap is
located 0.5×R above AV_SS, and the last tap is located 0.5×R below Vref+. This gives a total ladder
resistance of 1024×R. This structure ensures that the DAC is monotonic and results in a symmetrical
quantization error.
For input voltages between AV_SS and [(Vref+) + ½ LSB], the 10-bit result of an A/D conversion will be
0000000000B = 000H. For input voltages between [(Vref+) – 3/2 LSB] and Vref+, the result of a
conversion will be 1111111111B = 3FFH. AVref+ and AV_SS may be between AV_DD + 0.2V and AV_SS –
0.2 V. Avref+ should be positive with respect to AV_SS, and the input voltage (Vin) should be between
AVref+ and AV_SS.
The result can always be calculated from the following formula:
Vin
Result = 1024 ×
or Result = 1024 × ^VDD
AVref +
V _SS
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Publication Release Date: December 14, 2007
Revision A3.0