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 <mateuslu...@gmail.com > <javascript:>> 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, mateuslu...@gmail.com 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 beagleboard...@googlegroups.com. >> 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 beagleboard...@googlegroups.com <javascript:>. > 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 beagleboard+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/21e867c3-717d-4d7b-87c2-2f96862d47b4%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
