PIC A/D Converter

I have used one of the PIC16C7X series devices and found that the analog-to-digital conversion results are not always accurate. How can I improve the accuracy?

1. Make sure all timing specifications are met. If you turn the ADC off and then on, you should wait at least a minimum delay before sampling; if you change the input channel, you should also wait a minimum delay; and finally, Tad, which is the time selected for each bit conversion. This time is selected in ADCON0 and should be between 2 and 6us. If Tad is too short, the result will not be fully converted at the end of the conversion, and if Tad is too long, the voltage on the sampling capacitor may have dropped before the conversion is completed. These timing specifications are provided in the data sheet in the form of tables or formulas and should be reviewed for specific devices and specific situations.

2. Usually the source impedance of analog signals is high (greater than 1K ohm), so the source impedance output current will affect the accuracy of charging the sampling capacitor. If the input signal does not change too fast, try connecting a 0.1uF capacitor to the analog input. This capacitor will charge to the sampled analog voltage and provide the instantaneous current required to charge the 52pf internal sampling capacitor.

3. On the PIC16C71, an analog input pin is right next to an oscillator pin. Generally, when their traces are adjacent to each other, the noise on the oscillator will couple to the analog circuit. This is especially true if the clock source is an external oscillator box, because unlike the slowly rising sine wave provided by the crystal oscillator circuit, the external oscillator produces a square wave with steep edges and high-frequency components. Of course, decoupling the analog pin will help improve noise coupling, and if you can leave the pin empty, turn it into an output and drive it low, you can actually eliminate cross-coupling to the analog circuit.

4. Finally, a quote from the data sheet: "In systems where the device frequency is lower, it is better to use the A/D clock from the device oscillator... This will greatly reduce the effects of digital switching noise." And, "In systems where the device goes to sleep after starting the A/D conversion, it is required to select an RC clock source... It will provide the highest accuracy."