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CHAPTER 11 A/D CONVERTER
11.7 Notes on Using the A/D Converter
This section contains notes on using the A/D converter.
I
Notes on Using the A/D Converter
H
Input impedance of the analog input pins
The A/D converter contains the sample hold circuit shown in Figure 11.7-1 "Equivalent Circuit
for Analog Input" and adds the voltage of the analog input pin to the sample hold capacitor in
about 16 instruction cycles after A/D conversion starts. Therefore, if the output impedance of
the external circuit for analog input is high, the analog input voltage may not be stabilized within
the analog input sampling period. To avoid this problem, keep the output impedance of the
external circuit low enough to stabilize the voltage. If the output impedance of the external
circuit cannot be kept low, it is recommended that an external capacitor of about 0.1
μ
F be
added to the analog input pin.
Figure 11.7-1 Equivalent Circuit for Analog Input
It shows the R and C values of sample and hold circuit.
H
Notes on setting with a program
In A/D conversion function operations, the ADDH and ADDL registers hold the previous
values until A/D conversion is started. As soon as A/D conversion is started, the contents of
the ADDH and ADDL registers become undefined.
During operation of the A/D conversion function, do not reselect an analog input channel
(ADC1: ANS3 to ANS0).
Resetting, stopping, or starting the watch mode stops the A/D converter.
If the interrupt request flag bit (ADC1: ADI) is set to 1 and the interrupt request is enabled
(ADC2: ADIE = 1), it is not possible to return from interrupt processing.
Be sure to clear the ADI bit.
Comparator
controller
C
R
AN0 to AN11
Sample & hold circuit
Analog channel
selector
Close for 16 instruction cycles after activating A/D conversion
Table 11.7-1 R and C values of sample and hold circuit
MB89P579A
MB89577
MB89PV570
R: Analog input equivalent
resistance
7.1 k
2.2 k
C: Analog input equivalent
capacitance
48.3 pF
45 pF