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'Doing ADC on Differential AC Data?'
2000\02\05@032834 by Darren King

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I'm trying to read information from an alternating type current.  It starts
positive ends up negative and then repeats and some crazy frequency.  Does
anybody know how I'm supposed to hook up 2 Analog lines to my Pic16F788.
When neither is ground?  And then after that how would I read it to get the
relevent Data something like 2,1,0,-1,-2,-1,0,1,2?

This ones been killing me.  Thanks

Darren King

2000\02\08@160613 by Mark Willis

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I'm guessing here - using an op amp as a differential amplifier and
level shifter, would possibly help you a lot here.  (Assuming your AC
voltage dips below Ground.)

Set the op amp gain up to give you a reasonable swing on the output,
resistor or zener from that output to PIC input pins, pull-up resistor
(weak) to +V from there;  you want to set things up so that you get 2.5V
at the PIC input pin for a "Zero" input, and the gain of the op amp is
set to map the maximum input voltages to +-2.0V or so output on the
output end of the op amp.  IIRC I just used a resistor - use a Zener if
that +V is unregulated, and I think there's a trick to THAT one that I'm
not remembering still - I'm not a linear guru, the last time I did this
I
did it something like that, it's been a while and "If you don't use it,
you lose it" partially at least.  Hoping this gets you unstuck.

How accurate of ADC reading do you need?  Do you need differential AC
voltage readings, to track each voltage independently, RMS readings, ???

 Mark

Darren King wrote:
> I'm trying to read information from an alternating type current.  It starts
> positive ends up negative and then repeats and some crazy frequency.  Does
> anybody know how I'm supposed to hook up 2 Analog lines to my Pic16F788.
> When neither is ground?  And then after that how would I read it to get the
> relevent Data something like 2,1,0,-1,-2,-1,0,1,2?
>
> This ones been killing me.  Thanks
>
> Darren King

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2000\02\09@044606 by Darren King

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> I'm guessing here - using an op amp as a differential amplifier and
> level shifter, would possibly help you a lot here.  (Assuming your AC
> voltage dips below Ground.)
>
> Set the op amp gain up to give you a reasonable swing on the output,
> resistor or zener from that output to PIC input pins, pull-up resistor
> (weak) to +V from there;  you want to set things up so that you get 2.5V
> at the PIC input pin for a "Zero" input, and the gain of the op amp is
> set to map the maximum input voltages to +-2.0V or so output on the
> output end of the op amp.  IIRC I just used a resistor - use a Zener if
> that +V is unregulated, and I think there's a trick to THAT one that I'm
> not remembering still - I'm not a linear guru, the last time I did this
> I
> did it something like that, it's been a while and "If you don't use it,
> you lose it" partially at least.  Hoping this gets you unstuck.

I guess I should pick up some books on doing the Analog side of things.  I'm
getting more and more out of time.  This kind of circuit you described is
better known as what?  Analog Gain.  Can you recomend any website or reading
to quickly get me upto speed?

> How accurate of ADC reading do you need?  Do you need differential AC
> voltage readings, to track each voltage independently, RMS readings, ???

I'm going to use a 12bit ADC from Microchip.  I don't really follow the rest
of the question.  I just need to know when its positive how much and then
when its negative how much.  I would use 2 ADC channels for this and would I
hook them up completely seperate and do the ADC logging and combine the 2
results in software?

Darren King

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