AC input to an ADC...please help

[srivikas89]

ac input to adc

hello there..
i m using a microcontroller which has a built-in ADC in it.. now i have to measure an AC signal using dat ADC...can u please tell if the ADC can measure the negative half-cycles of an AC signal..? or shud i give the signal thru some external circuitry first? the ADC needs positive-going zero-crossings to measure the line frequency..

can i use a rectifier - half or full wave...? or shud i shift the reference voltage of the ADC? please do reply soon...

thanks..

 

[medap]

ac input adc

What is Your real needed measure range and accuracy?

 

[srivikas89]

range which can be applied to the ADC in the controller is 1.024V full scale.. n the range to be measured is between 200 to 400V.. so..to scale down the voltage range m using voltage transformer of 600:1 turns ratio.....

 

[daviddlc]

You need to acondition your signal to the ADC input.

What you need to do is to condition the miminum and maximum values of the signal to be measured to the corresponding minimum and maximum signal that you can input on the ADC.

For example you want to measure a signal which the lower value is 50 and the maximum value is 100, and let's say your ADC range is from 0 to 1.2 volts you need to create a circuit that will transfer this values.

When your input singal is 50 the output of this circuit you need to create should be 0 and when the input signal is 100 the output of this signal should be 1.2

You need to consider all the details related to linearity, noise, temperature changes, etc.

I hope this can help you out.

If you are using a AC signal you can mount it on a DC component to shift the values to be measured on the ADC

 

[Petre Petrov]

Hi!
Several useful notes:
Filter the signal before the transformer
Limit the signal before the transformer
Filter the signal after the transformer
Limit the signal after the transformer
Pay attention about possible DC component in the input voltage for the transformer.
If you use a simple rectifier you will have degradation in the accuracy of the measurement.
What is your MCU+ADC?
Usually Vfull scale/2 I taken as “zero” when bipolar signals are measured, but this is not obligatory.
Could provide you schematic diagrams and more detailed requirements?
What will be measured?
What safety standards should be met?
BR

 

[srivikas89]

dat is all very helpful thanks a lot..

i have a few more qns...
if i shift the "zero" level of the ac signal,then how will the controller measure the line frequency...? the datasheet of the controller says dat it measures the line frequency by counting the "positive going zero crossing" of the line voltage... so if the "zero" is shifted then how will the controller know abt the zero crossing??

 

[daviddlc]

Well, you know the value of the signal shifted, so that will be your reference