PIC Microcontroller Input Methods

for Analog to Digital Conversion

• Averaging A2D results
• Averaging A2D results with a median filter by Louis Nel
• Avoiding constant current draw in RC A2C converters that use a voltage divider to measure higher than the uP's Vdd.
• Seperating the Vcc and Gnd for the A/D converter
• A/D sample programs for 16f877 and 16f873
• SPI interface to AD7730 by Tony Kübek

See:

• PWM@
• /techref/piclist/projects.htm PICList projects
• PIC Microcontoller DSP Math Method : Median filter and sort
• The "Bill & Pookie" 10 bit A2D binary to decimal w/scaleing code generator

See also:

• http://www.ise.pw.edu.pl/~wzab/picadc/picadc.html PICADC - a free, PIC based "intelligent" A/D converter
• http://www.dattalo.com/technical/software/software.html Scott Dattalo's  ADC

Questions:

• slvfd84@windstream.net asks:

  RE:BILL&POOKIE CODE GEN WORK OR NOT FOR ME?
  I am trying to use a 16f87x for adc of acs750 lca-050 corrent>voltage sensor, convert the adc to decimal 0.000-50.000, and display it on led/lcd. i want rms values for the current sampling.(1/sec or so maybe) (readable display) i have several pics, several c>v sensors, and very little understanding of the code req'd. i prefer 5 digit display, with leading zero's blanked before dp, dp after 2'nd digit(like this [xx.xxx]) sensor is 0v out for 0amp, 2.5v out for 50 amps. my supply is std +5vdc. kinda new to assembly, seriously interested. i use mplab & mpasm. plan to start with led, then port to lcd after it works. then maybe a volt meter with 50 amp current. should be a very handy instrument. so with that in mind.....bring it on.
  
  KB3IFS

  James Newton replies: Try it an see.

• aneeshcheriammal@yahoo.co.in asks:

  what is the calculation to be done to get the digital output of an analog input in an adc in a microcontroller using adc inut voltage,adc reference voltage etc

  James Newton replies: See The "Bill & Pookie" 10 bit A2D binary to decimal w/scaleing code generator

• kkouinis@yahoo.gr asks: " i use pic 16f877 and i want to make 8-bit A/D conversion. Can i setup the internal A/D converter to take samples of 8-bit and not of 10-bit?" Sure. Just throw away the bottom 2 bits. "thanks.but then have to multiply the result with 4 factor?" Err... are you saying that you need to multiply by four for some reason of your own or are you asking if disreguarding the last two bits requires a multiply by four?
  
  If you just want an 8 bit A/D and you have a 10 bit A/D you just shift all 10 bits right two bit places. That is all. No need to multiply by 4. The original range of values is 0 to 2^10 or 1024. The new range of values is 0 to 2^8 or 256 which is exactly what you would get from an 8 bit A/D
  
  If you want to multiply by four, well: *4 = left shift twice but in your case, just use the lower 8 bits. That is the same as takeing all 10 bits, shifting right two bits and then back left two bits.
   
• bigdonahoe@yahoo.com asks:

  I am working with the PIC16C745 and I can successfully get a2d to work, but only on 1 channel at a time. Is there a way to switch to a channel, do a single conversion, and then switch to another channel and repeat? When I try this the channels interfere with each other and I get bad results.

  Yes, "channels interfering" is a common problem. I'm pretty sure you have a "settling time" problem. When you switch from one channel to another, you *must* wait a certain amount of "settling time" before starting the next conversion. Please do *all 3* of these: (1) After the code switches from one channel to another, *delay* the full Tacq time, and only then start the conversion. (Typically the ADC routine does: Start ADC, wait for end of conversion, Switch to next channel, Read ADC, Store value. Then return and let the rest of the program go while waiting for this new channel to settle). (2) Buffer your analog signals with an op amp (running off the same power supply as the PIC), so the output of the op amp is directly connected to the PIC analog input pin. And (3) Read the "Analog to Digital" section of the data sheet one more time to make sure I didn't miss anything. -- David Cary

Comments:

• One thing I found in trying to switch between multiple analog inputs with a 16F690 was that some of the example files from microchip had a mistake in the wait for conversion complete -- btfss instead of btfsc when checking for the clear of the "GO" bit.

PICList post "LM335" Interesting idea for expanding A2D resolution.