Specifications

dsPIC33F Family Reference Manual

16.25 DESIGN TIPS

Question 1: How can I optimize the system performance of the ADC module?

Answer:

1. Make sure you are meeting all of the timing specifications. If you are turning the ADC mod-

ule 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. Finally,

there is T

AD, which is the time selected for each bit conversion. TAD is selected in ADCON3

and should be within a range as specified in the Electrical Characteristics. If T

AD is too

short, the result may not be fully converted before the conversion is terminated. If T

AD is

too long, the voltage on the sampling capacitor can decay before the conversion is com-

plete. 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 charges to the analog voltage being sampled and supplies 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 ADCs?

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.