can't solve this voltage measurement problem

[Christopher4187]

Jerson,

I've attached a quick drawing of the circuit. The problem is, I don't have the piece of equipment with me but if I remember correctly, the frequency is 500Hz but it is very erratic. Not only that, not all pieces of equipment have this problem because some are new chargers and some are old. It's not AC mains, it's the output of the battery charger.

[Jerson]

Try something like this

[Christopher4187]

Thanks Jerson. I realized I drew it wrong because I had the diode on the input to the voltage divider. If not, my measurement would be WAYYYY off!!! I'll try the increased cap size. I didn't put it at the input to the voltage divider because the size would have to be much bigger. How important is it to have the cap before the voltage divider instead of after?

[Charles Linquis]

It also wouldn't hurt to take an average. Since 60 Hz has a period of 16.66 milliseconds, and an A/D conversion takes about 30uSecs, you could average 16 samples (a full cycle) something like this:

   ADCAvg = 0            ; Clear the variable
For X = 1 to 16
   ADCIN,x, ADCresult
   ADCAvg = ADCAvg + ADCresult
   Pauseus 1000  
   Next X
ADCAvg = ADCAvg >> 4    ; divide by 16

This should do a pretty good job of smoothing the result.

[Christopher4187]

I've tried all kinds of software routines but nothing seemed to help. I think the AC ripple is not a normal sine wave, maybe that's why.

[Melanie]

Put a 1uF Electrolytic... or even a 10uF... across the 0.1 Capacitor.

It's a Battery Charging circuit - you don't need instantaneous results. If your filters time period is a few hundred mS then goodbye to any 500Hz noise! Make sure you haven't got any noise on your 5v line - that could be throwing +VRef as well.