dsPIC30F Family Reference Manual
DS70065C-page 18-32 © 2004 Microchip Technology Inc.
18.24 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 by leakage, and 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 18 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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