Thanks John, I will test the circuit with a opamp and post the result here.
Em domingo, 21 de janeiro de 2018 21:27:43 UTC-3, john3909 escreveu: > > Yes, the problem is a combination of the multiplexor and the S&H. Let’s > say ADC0 is 3.5V and ADC1 is 0.2V and ADC2 is 2V. The S&H must first charge > the capacitor to 3.5V for ADC0 and then the multiplexor changes to ADC1 and > discharges the cap to 0.2V and then the mux changes to ADC2 and charges the > cap to 2V. Now, if the source impedance of the circuits that feed ADC0, > ADC1 and ADC2 are not low impedance, then the S&H cap will not > charge/discharge fully before the ADC does the conversion. The time > constant T=RC where T is the S&H window, R is the source impedance and C is > the S&H cap. So if R is not low enough, you will see the accuracy of your > conversion affected by the voltage of the previous channel. Alternatively, > you can slow the sample rate and conversion time in the device tree and > that will improve your accuracy. > > Regards, > John > > > > > On Jan 21, 2018, at 7:37 AM, Mateus Lucas <[email protected] > <javascript:>> wrote: > > Thanks, John. > > But I didn't understand yet how the high impedance on the input may cause > this error. The ADC input usually have a high impedance and the S&H have a > capacitor, the problem is related with the time to charge the capacitor? > > Em domingo, 21 de janeiro de 2018 04:31:36 UTC-3, john3909 escreveu: >> >> Yes, you use the opamp as a low impedance buffer and you can also use it >> to scale the input to a range suitable for the ADC. Have a look at Analog >> Devices, (www.analog.com), they have many application notes on how to >> use an opamp as a signal conditioner suitable for an ADC. >> >> Regards, >> John >> >> >> >> >> On Jan 20, 2018, at 3:13 PM, Mateus Lucas <[email protected]> wrote: >> >> Oh, thank you for the explanation. >> Are you talking about opamp as buffer or other circuit? >> >> Em sábado, 20 de janeiro de 2018 19:43:31 UTC-3, john3909 escreveu: >>> >>> The problem you are experiencing is due to the sample and hold of of the >>> ADC input which is multiplexed to all analog inputs. If you don’t use a low >>> impedance source, you will see bleed through from one ADC channel to the >>> next. What you need is a opamp connected between the circuits you are >>> measuring and the ADC input. The opamp will provide a low impedance source >>> for the sample and hole (S&H) and prevent the bleed through from one >>> channel to the next. >>> >>> Regards, >>> John >>> >>> >>> >>> >>> On Jan 18, 2018, at 7:34 PM, [email protected] wrote: >>> >>> >>> >>> I'm using two analog inputs, the first with a LDR and a resistor of 10k >>> Ohm, connected at P9_39 (AIN0), and the second connected at P9_38 (AIN3). >>> When the lights are down, the measure of LM35 works very well, with a >>> low distorction, +/- 0.2 ºC. >>> But when the lights are on, the measure of LM35 starts to float, about >>> +/- 1 ºC. >>> I have tried to put a 4.7nF between GNDA_ADC and AIN3, but the >>> distorction continues. >>> >>> I asking for help to diminish this distorction when the lights are on. >>> >>> >>> -- >>> For more options, visit http://beagleboard.org/discuss >>> --- >>> You received this message because you are subscribed to the Google >>> Groups "BeagleBoard" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/beagleboard/d5353977-2025-4aa3-b6a0-361742ae9c96%40googlegroups.com >>> >>> <https://groups.google.com/d/msgid/beagleboard/d5353977-2025-4aa3-b6a0-361742ae9c96%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> For more options, visit https://groups.google.com/d/optout. >>> >>> >>> >> -- >> For more options, visit http://beagleboard.org/discuss >> --- >> You received this message because you are subscribed to the Google Groups >> "BeagleBoard" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/beagleboard/5024589e-3e59-43be-b293-98cd5e6c6f6e%40googlegroups.com >> >> <https://groups.google.com/d/msgid/beagleboard/5024589e-3e59-43be-b293-98cd5e6c6f6e%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> For more options, visit https://groups.google.com/d/optout. >> >> >> > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] <javascript:>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/beagleboard/21e867c3-717d-4d7b-87c2-2f96862d47b4%40googlegroups.com > > <https://groups.google.com/d/msgid/beagleboard/21e867c3-717d-4d7b-87c2-2f96862d47b4%40googlegroups.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > > > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. 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