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I am trying to measure 10-15volts to 0.01 volt accuracy. I am using the built-in 10bit ADC on the 12F675 with 1% tolerance resistor voltage divider. I find that the values fluctuate way more than expected, +/- 0.03 volts. The voltage is coming from a regulated power supply and I have verified it with 2 DVMs, and it seems quite steady. Here are some things I have tried: 1. changed power supplies - no difference 2. put up to a 1 second sleep after going into AD read mode - no difference. 3. averaged 5, 10 and 50 samples - this has reduced the quantity of fluctuations, but not the magnitude. 4. Tried a different development board - no change. 5. Changed the sample rate of the ADC to its max (250us) no change. Any comments or suggestions? thanks d1 |
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AD with 12F675 fluctuations
Started by ●April 8, 2004
Reply by ●April 9, 20042004-04-09
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There are some app notes for the 12F675 for doing higher accuracy A/D conversions here: http://www.microchip.com/download/lit/pline/picmicro/families/12f6xx/40040b.pdf Perhaps they might give you some ideas... - Don d1camero wrote: >I am trying to measure 10-15volts to 0.01 volt accuracy. I am >using the built-in 10bit ADC on the 12F675 with 1% tolerance >resistor voltage divider. > >I find that the values fluctuate way more than expected, +/- 0.03 >volts. The voltage is coming from a regulated power supply and I >have verified it with 2 DVMs, and it seems quite steady. > >Here are some things I have tried: > >1. changed power supplies - no difference > >2. put up to a 1 second sleep after going into AD read mode - no >difference. > >3. averaged 5, 10 and 50 samples - this has reduced the quantity of >fluctuations, but not the magnitude. > >4. Tried a different development board - no change. >5. Changed the sample rate of the ADC to its max (250us) no change. >Any comments or suggestions? > >thanks >d1 >to unsubscribe, go to http://www.yahoogroups.com and follow the instructions >Yahoo! Groups Links > > |
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Reply by ●April 9, 20042004-04-09
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Hello, 1k resistor and 0.01uF capacitor useally helps. Connect resistor to voltage source and to A/D pin. Connect capacitor to A/D pin and to ground. That's it. Make experiments with different values for both resistor and capacitor. Make sure that total resistance of source and resistor don't exceed 10k (see Data Sheet). Regards, Igor ----- Original Message ----- From: "d1camero" <> To: <> Sent: 09.04.2004. 3:46 AM Subject: [piclist] AD with 12F675 fluctuations > I am trying to measure 10-15volts to 0.01 volt accuracy. I am > using the built-in 10bit ADC on the 12F675 with 1% tolerance > resistor voltage divider. > > I find that the values fluctuate way more than expected, +/- 0.03 > volts. The voltage is coming from a regulated power supply and I > have verified it with 2 DVMs, and it seems quite steady. > > Here are some things I have tried: > > 1. changed power supplies - no difference > > 2. put up to a 1 second sleep after going into AD read mode - no > difference. > > 3. averaged 5, 10 and 50 samples - this has reduced the quantity of > fluctuations, but not the magnitude. > > 4. Tried a different development board - no change. > 5. Changed the sample rate of the ADC to its max (250us) no change. > Any comments or suggestions? > > thanks > d1 > to unsubscribe, go to http://www.yahoogroups.com and follow the instructions > Yahoo! Groups Links |
Reply by ●April 9, 20042004-04-09
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Hello, ...and another reply: > I am trying to measure 10-15volts to 0.01 volt accuracy. I am > using the built-in 10bit ADC on the 12F675 with 1% tolerance > resistor voltage divider. OK, so you have 15V maximum divided by 3 in order to have 0-5V range for AD, right? > I find that the values fluctuate way more than expected, +/- 0.03 > volts. The voltage is coming from a regulated power supply and I > have verified it with 2 DVMs, and it seems quite steady. OK, now lets do some math... 2^10 = 1024 5V / 1024 = 0.00488V Now, we multiply that with 3 because of that divider: 0.00488V * 3 = 0.0146V Now, we add AD error which is probably +/-1LSB and that gives us: +/-0.0146V or approx. 0.03Vpp fluctuation What it means is that fluctuation is quite normal... if I did this math right :) Regards, Igor |
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Reply by ●April 9, 20042004-04-09
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This seems to be a common problem with PICs. I've seen it on the 12F675, 16F88, 16F877 (A and non-A), 16F819. I too have tried the same things that d1camero tried, also to no avail. I've had some success with taking care to isolate the analog section from the rest of the system but the PICs pin outs makes that a bit difficult. There is a Microchip app note on this topic. Also, some sensors produce a lot of noise. I've run tests where I used the center tap on a pot as the input to an ADC pin and it was quite stable. Hook up a sensor and it got 3+ LSBs of noise. I've started looking into conditioning as a way to improve the noise. I got some noise reduction on a temperature sensor by using a cap and a resistor (LM34, as described in the app note). However, the Sharp GP2D12 is one noisy sensor and probably needs it own power. The tips app note is kind of funny in that they seem to be saying that to get higher accuracy, dont use the ADC. Uh, yeah, sure, why did I buy that chip??? --- In , Don Hackler <donh@s...> wrote: > > There are some app notes for the 12F675 for doing higher accuracy A/D > conversions here: > http://www.microchip.com/download/lit/pline/picmicro/families/12f6xx/4 0040b.pdf > Perhaps they might give you some ideas... > - Don > > d1camero wrote: > > >I am trying to measure 10-15volts to 0.01 volt accuracy. I am > >using the built-in 10bit ADC on the 12F675 with 1% tolerance > >resistor voltage divider. > > > >I find that the values fluctuate way more than expected, +/- 0.03 > >volts. The voltage is coming from a regulated power supply and I > >have verified it with 2 DVMs, and it seems quite steady. > > > >Here are some things I have tried: > > > >1. changed power supplies - no difference > > > >2. put up to a 1 second sleep after going into AD read mode - no > >difference. > > > >3. averaged 5, 10 and 50 samples - this has reduced the quantity of > >fluctuations, but not the magnitude. > > > >4. Tried a different development board - no change. > > > > > >5. Changed the sample rate of the ADC to its max (250us) no change. > > > > > >Any comments or suggestions? > > > >thanks > >d1 > > > > > > > > > > > >to unsubscribe, go to http://www.yahoogroups.com and follow the instructions > >Yahoo! Groups Links > > > > > > > > > > > > > > |
Reply by ●April 9, 20042004-04-09
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I have had really good luck with PIC A/Ds by: Bypassing the PIC with 6 or 8 .1uF caps close to the chip on a 4-layer board. Connecting the reference GND directly to the PIC GND pins. Using 2K in series and .1uF directly to GND at the input pin. Making sure that the source impedance (feeding the 2K resistor) is not more than 2K. Waiting 100uSec before beginning conversion after any channel change. Averaging at least 6 samples. When doing the above, I have found that the jitter is generally less than +/- 1 LSB. Charles Linquist -----Original Message----- From: Don Hackler [mailto:] Sent: Thursday, April 08, 2004 11:46 PM To: Subject: Re: [piclist] AD with 12F675 fluctuations There are some app notes for the 12F675 for doing higher accuracy A/D conversions here: http://www.microchip.com/download/lit/pline/picmicro/families/12f6xx/40040b. Perhaps they might give you some ideas... - Don d1camero wrote: >I am trying to measure 10-15volts to 0.01 volt accuracy. I am using >the built-in 10bit ADC on the 12F675 with 1% tolerance resistor voltage >divider. > >I find that the values fluctuate way more than expected, +/- 0.03 >volts. The voltage is coming from a regulated power supply and I have >verified it with 2 DVMs, and it seems quite steady. > >Here are some things I have tried: > >1. changed power supplies - no difference > >2. put up to a 1 second sleep after going into AD read mode - no >difference. > >3. averaged 5, 10 and 50 samples - this has reduced the quantity of >fluctuations, but not the magnitude. > >4. Tried a different development board - no change. >5. Changed the sample rate of the ADC to its max (250us) no change. >Any comments or suggestions? > >thanks >d1 >to unsubscribe, go to http://www.yahoogroups.com and follow the >instructions Yahoo! Groups Links > > to unsubscribe, go to http://www.yahoogroups.com and follow the instructions Yahoo! Groups Links |
Reply by ●April 9, 20042004-04-09
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Igor, Thanks for the quick feedback. I calculate the error: The AD converter is measuring 0-5 volts, hence no division or multiplication by 3 for the AD part. 1 LSB bit fluctuation is simply then: 5v / 1024 = 0.0048v So I expect to see some fluctuation, but not as much as I have been measuring. I will try the other suggestions and repot back. thanks everyone. Don --- In , "Igor Janjatovic" <kodrat@p...> wrote: > Hello, > > ...and another reply: > > > I am trying to measure 10-15volts to 0.01 volt accuracy. I am > > using the built-in 10bit ADC on the 12F675 with 1% tolerance > > resistor voltage divider. > > OK, so you have 15V maximum divided by 3 in order to have 0-5V range for AD, > right? > > > I find that the values fluctuate way more than expected, +/- 0.03 > > volts. The voltage is coming from a regulated power supply and I > > have verified it with 2 DVMs, and it seems quite steady. > > OK, now lets do some math... > > 2^10 = 1024 > 5V / 1024 = 0.00488V > > Now, we multiply that with 3 because of that divider: > > 0.00488V * 3 = 0.0146V > > Now, we add AD error which is probably +/-1LSB and that gives us: > > +/-0.0146V or approx. 0.03Vpp fluctuation > > What it means is that fluctuation is quite normal... if I did this math > right :) > > Regards, > Igor |
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Reply by ●April 9, 20042004-04-09
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--- In , "d1camero" <a8723@c...> wrote: > The AD converter is measuring 0-5 volts, hence no division or > multiplication by 3 for the AD part. But you said you were measuring 10-15V. How are you getting this down to the 0-5V range for measurement? Is your PIC properly decoupled? (That is do you have a .1uf cap as close to Vss as possible?) You said you were sleeping for 1 second after starting the conversion. Why not just turn the interrupt on for A/D and go to sleep? (It will wake up when the A/D is done that way.) If you are doing this in a high level language are you sure that when you say to sleep for a second that it's not just doing a 1 second timing loop? (I always done everything in asm so I don't know anything about HL languages on the PIC -- this is just a stab in the dark.) To truely sleep for 1 second you would need to set a timer so it would expire in 1 second and it would need to be the only interrupt source -- that's why I think it's reasonable that the HL language is simply doing a timing loop. It sounds like you are only using one A/D channel so you would have no need to change channels but if you are changing channels are you waiting the necessary aqcuisition time (Taqc as described in the datasheet) prior to starting the conversion? --Scott |
Reply by ●April 9, 20042004-04-09
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Thanks for all the input. I have tried your suggestions (averaging, capacitors, sleeping) without any appeciable change in fluctuation. BUT, when I removed the resistor divider network, and simply used the 12F675 to measure 0-5volts, the ADC works great, with an acceptable 1 bit fluctuation. So my question has changed: How can I accurately measure 10-15volts with the 675. Obviously a 1% resistor divider network introduces too much noise. Other ideas? thanks d1 |
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Reply by ●April 9, 20042004-04-09
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You don't mention what values of 1% resistors you use. It is important that the parallel combination (thevenin equivalent) of these two resistors is less than 10K. Actually I prefer to keep it below 4K. Charles Linquist ----- Original Message ----- From: "d1camero" <> To: <> Sent: Friday, April 09, 2004 11:44 AM Subject: [piclist] Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fluctuations) > Thanks for all the input. I have tried your suggestions (averaging, > capacitors, sleeping) without any appeciable change in fluctuation. > > BUT, when I removed the resistor divider network, and simply used > the 12F675 to measure 0-5volts, the ADC works great, with an > acceptable 1 bit fluctuation. > > So my question has changed: How can I accurately measure 10-15volts > with the 675. Obviously a 1% resistor divider network introduces > too much noise. Other ideas? > > thanks > d1 > > > to unsubscribe, go to http://www.yahoogroups.com and follow the instructions > Yahoo! Groups Links |
