© 2004 Microchip Technology Inc. DS70064C-page 17-51
Section 17. 10-bit A/D Converter
10-bit A/D
Converter
17
17.25 Design Tips
Question 1: How can I optimize the system performance of the A/D converter?
Answer:
1. Make sure you are meeting all of the timing specifications. If you are turning the module
off and on, there is a minimum delay you must wait before taking a sample. If you are
changing input channels, there is a minimum delay you must wait for this as well and
finally, there is T
AD, which is the time selected for each bit conversion. This is selected in
ADCON3 and should be within a certain range as specified in the Electrical Characteris-
tics. If T
AD is too short, the result may not be fully converted before the conversion is
terminated, and if T
AD is made too long, the voltage on the sampling capacitor can decay
before the conversion is complete. These timing specifications are provided in the
“Electrical Specifications” section of the device data sheets.
2. Often the source impedance of the analog signal is high (greater than 10 kΩ), so the
current drawn from the source to charge the sample capacitor can affect accuracy. If the
input signal does not change too quickly, try putting a 0.1 µF capacitor on the analog input.
This capacitor will charge to the analog voltage being sampled and supply the
instantaneous current needed to charge the 4.4 pF internal holding capacitor.
3. Put the device into Sleep mode before the start of the A/D conversion. The RC clock
source selection is required for conversions in Sleep mode. This technique increases
accuracy because digital noise from the CPU and other peripherals is minimized.
Question 2: Do you know of a good reference on A/D’s?
Answer: A good reference for understanding A/D conversions is the “Analog-Digital Conversion
Handbook” third edition, published by Prentice Hall (ISBN 0-13-03-2848-0).
Question 3: My combination of channels/sample and samples/interrupt is greater than
the size of the buffer. What will happen to the buffer?
Answer: This configuration is not recommended. The buffer will contain unknown results.
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