Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[Mateus Lucas]

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 Let’s 
> say ADC0 is 3.5V and ADC1 is 0.2V and ADC2 is 2V. The S 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 cap will not 
> charge/discharge fully before the ADC does the conversion. The time 
> constant T=RC where T is the S window, R is the source impedance and C is 
> the S 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  > 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 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  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) 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.
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>>>  
>>> 
>>> .
>>> For more options, visit https://groups.google.com/d/optout.
>>>
>>>
>>>
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>>  
>> 
>> .
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>>
>>
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> 

Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[John Syne]

Yes, the problem is a combination of the multiplexor and the S Let’s say 
ADC0 is 3.5V and ADC1 is 0.2V and ADC2 is 2V. The S 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 cap will not charge/discharge fully 
before the ADC does the conversion. The time constant T=RC where T is the S 
window, R is the source impedance and C is the S 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  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 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 > 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) 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
>>>  
>>> .
>>> For more options, visit https://groups.google.com/d/optout 
>>> .
>> 
>> 
>> -- 
>> For more options, visit http://beagleboard.org/discuss 
>> 
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>>  
>> .
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>> .
> 
> 
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Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[Mateus Lucas]

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 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  > 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) 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
>>  
>> 
>> .
>> For more options, visit https://groups.google.com/d/optout.
>>
>>
>>
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>  
> 
> .
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>
>

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Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[John Syne]

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  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) 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
>>  
>> .
>> For more options, visit https://groups.google.com/d/optout 
>> .
> 
> 
> -- 
> For more options, visit http://beagleboard.org/discuss 
> 
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Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[danilodomdealm]

Não gracinha, tá falando da pqp

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Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[Mateus Lucas]

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) 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.
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>  
> 
> .
> For more options, visit https://groups.google.com/d/optout.
>
>
>

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Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[John Syne]

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) and prevent the bleed through from one channel to the next.

Regards,
John




> On Jan 18, 2018, at 7:34 PM, mateuslucas.l7...@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.
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>  
> .
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> .

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Re: [beagleboard] Using two analog inputs and occurs distortion on the measure

[evilwulfie]

Ain3 has a maximum input voltage of 1.8v
the lm35 is being powered from a +5v supply
your going to need to do something a bit different with the output of
the LM35 so you dont damage the adc input.

On 1/19/2018 6:17 AM, Mateus Lucas wrote:
> Oh, sorry. I have sent the wrong image.
> Thanks for your attention.
>
>
>
> 2018-01-19 9:49 GMT-03:00 evilwulfie  >:
>
> Would love to try to help but even when i saved your schematic
> picture and enlarged it
> i was unable to make out anything in it.
>
> On 1/18/2018 8:34 PM, mateuslucas.l7...@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.
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>> send an email to beagleboard+unsubscr...@googlegroups.com
>> .
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>> 
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>> 
>> .
>> For more options, visit https://groups.google.com/d/optout
>> .
>
>
>
>
> -- 
> Mateus Lucas de Campos e Silva
> Técnico em Eletrônica pelo IFPB - Campus João Pessoa
> Bacharelando em Engenharia Elétrica pelo IFPB
> Celular: (83) 988335074

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