W79E825A Datasheet, PDF (102/127 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E825A/824A/823B/822B Data Sheet
DAC
VDAC
Vin
Comparator
-
+
MSB
Successive
Approximation
Register
LSB
Successive
Approximation
Control Logic
Start
Ready
(Stop)
Figure 24-1: Successive Approximation ADC
24.1 ADC Resolution and Analog Supply:
The ADC circuit has its own supply pins (AVDD and AVSS) and one pins (Vref+) connected to each
end of the DAC’s resistance-ladder that the AVDD and Vref+ are connected to VDD and AVSS is
connected to VSS. The ladder has 1023 equally spaced taps, separated by a resistance of “R”. The
first tap is located 0.5×R above AVSS, 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 VSS 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. Vref+ and AVSS may be between AVDD + 0.2V and VSS –
0.2 V. Vref+ should be positive with respect to VSS, and the input voltage (Vin) should be between
Vref+ and VSS.
The result can always be calculated from the following formula:
Vin
Vin
Result = 1024 ×
or Result = 1024 ×
Vref +
VDD
- 102 -