ADC interfacing Problem

RE: ADC interfacing Problem

Stefan Birnir Sverrisson — Fri, 17 Oct 2014 08:51:00 GMT

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RE: ADC interfacing Problem

hrg — Fri, 17 Oct 2014 04:42:24 GMT

Thankyou Stefan !

RE: ADC interfacing Problem

hrg — Tue, 14 Oct 2014 09:37:40 GMT

And the analog input without any prescaling ??

RE: ADC interfacing Problem

Stefan Birnir Sverrisson — Tue, 14 Oct 2014 09:04:05 GMT

@ Krzysztof: Thanks for your input and observation.

In addition to Krzysztof's input, it could be better to choose VDD/2 as reference voltage. That way, you will get a steady voltage measurement on the ADC AIN input pin when the battery drains and the supply voltage drops.

Update 16.10.2014 If you have a battery voltage of 3.0V and choose VDD/2 as reference, your reference voltage is 1.5V. This means that you should not have higher input voltage than 1.5V, because that is the saturation point of the ADC where you will see value 1023 on the output when using 10 bit resolution. So if you have 1.5V as the source input to your voltage divider, and you have the resistor at 10kohm and thermistor at 10kohm then you will get 0.75V at the input and that should give you value 512 when you have no prescaler (option ADC_CONFIG_INPSEL_AnalogInputNoPrescaling). So with this setup, no prescaler would be needed on the input.

Anyway, in order to utilize the range of the ADC in the best way and to maximize accuracy, the input voltage range should span as much of the reference voltage as possible. For example, if the reference voltage is 1.5V, then it would be best to dimension the voltage divider on the input so that temperature changes will make the input voltage vary between 0V and 1.5V

RE: ADC interfacing Problem

hrg — Tue, 14 Oct 2014 08:55:50 GMT

Also how to use the voltage input prescalar for the supply voltage of 3.0 volt ???

RE: ADC interfacing Problem

hrg — Tue, 14 Oct 2014 08:44:21 GMT

Thankyou i got to know that ref voltage make sense much here ! is it correct if we suply external ref voltage as the supply voltage ?

RE: ADC interfacing Problem

hrg — Tue, 14 Oct 2014 08:27:01 GMT

no ,it is an NTC thermistor with 10k at 25C . The resistor R_1 is connected to the source side & the themistor to the ground side. so at normal temperature 25C i need to dislay the adc value to be 511 range ! so should i take the supply voltage as reference ??

RE: ADC interfacing Problem

Stefan Birnir Sverrisson — Tue, 14 Oct 2014 08:17:25 GMT

Hi. I have questions regarding your setup. Is the range of your thermistor 0kohm-10kohm? How are your schematics, are they similar to the schematics on this blog, where you have your resistor as R_1 and your thermistor as R_2?

RE: ADC interfacing Problem

Krzysztof Rosinski — Tue, 14 Oct 2014 07:28:40 GMT

Hi,

Please check the reference manual page 176 and product specification on page 49.

You're using internal ADC reference which is 1.2V (ADC_CONFIG_REFSEL_VBG) so you're 1024 ADC is 1.2V, then you're using 1/3 voltage divider for input (ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling). I believe that you should use the following equation:

ADC = (Vin / 3) * 1024 / 1.2

e.g. for 0.75 Vin is:

ADC = (0.75 / 3) * 1024 / 1.2 = 213

which is roughly what you get.

Krzysztof