How can I measure 0-3V AC using an 8-bit A/D?
Tony_tsi
Posts: 98
I need to measure ac voltage as accurately as possible using an 8-bit a/d controller.
The voltage will be 1-3V AC.
I prefer not to convert to DC unless I can convert it to the same voltage very accurately.
If any one can point me in the right direction i would relay appreciate it.
I am using this to measure current using the current transducer in the link below.
http://www.eio.com/p-23820-velleman-ac97-modular-ac-current-transducer.aspx
The voltage will be 1-3V AC.
I prefer not to convert to DC unless I can convert it to the same voltage very accurately.
If any one can point me in the right direction i would relay appreciate it.
I am using this to measure current using the current transducer in the link below.
http://www.eio.com/p-23820-velleman-ac97-modular-ac-current-transducer.aspx
Comments
Why not ? - That allows you to externally average over time, which will have less reading jitter.
Does this need to be RMS current ?
Alternatives are frequent AC samples, but you need to know Zero - so you could take a lot of samples, ideally over a whole number of cycles (can be a SW decision) and then you can fix Zero, and from there, do a RMS sum.
The only way I know to convert to ac to dc is by using a bridge rectifier. Bridge rectifiers do not output clean dc signals so I will need a smoothing capacitor. The output from the amperage clamp is liner ( 1mV AC per .1Amp AC). If i run this signal through a bridge rectifier and use a smoothing capacitor the capacitor will skew the reading and it will no longer be liner. As far as i know a bridge rectifier is made up of 4 diodes. A diode needs something like.5-.7 volts to work. so if i were trying to read a 1 amp line current the output voltage of the clamp would only be 10mV. Not nearly enough to make a diode work.
and follow the links also, for full wave, and half wave.
If you mean that it is actually AC, such that the min voltage is -3V and the max is +3V, then you can use an adder circuit (using op amps) and add 3 volts to the input signal. This will make it DC varying between 0V and 6V. You can then two resistors to scale the voltage to 0-3V.
Play with the resistors on this calculator to see how:
http://www.pulsedpower.net/Applets/Electronics/SigmaDeltaADC/SigmaDeltab.html
The Quickstart has pads for the important resistors and capacitors...
All you need to do is to bias the AC voltage up.
Assuming you are using the propeller with a 3.3V Vdd, I would run the AC thru a 10K resistor and then add a 10K resistor to 3.3V to bias the voltage up.
This should give you +0.15V at the ADC when the AC is at -3V and 3.15V at the ADC when the AC voltage is at +3V.
10K 3.3V ---/\/\/\/\---| 10K |------ to ADC AC ----/\/\/\/\----|Bean
How fast is the AC voltage moving?
If it is a mains voltage, eg 50Hz or 60Hz, and your A to D sampler is sampling 5000x a second, it is almost a DC voltage as far as your sampler is concerned. Just sample many times a second. You can do all the maths in the chip with the samples you have.
And you can probably do better than 8 bits - maybe 10 or 12 bits.
You are correct. In the olden days you might have used and op amp with a couple of diodes back to back such that the op amp cancels out the diode voltage drop. Then it is linear, and with a few resistors you can even do the RMS conversion as well. But these days, why not sample the voltage many times a second in your microprocessor, and then do the clever maths in the micro?
What is the maximum volts of your sample, and the minimum volts, and the frequency?