Re: Line-to-Line Sensing: Analog Vs Digital?
- From: Mitja Nemec <korenje@xxxxxxxxxxx>
- Date: Mon, 9 May 2011 03:28:45 -0700 (PDT)
On May 9, 5:34 pm, Randy Yates <ya...@xxxxxxxx> wrote:
On 05/08/2011 09:21 PM, Vladimir Vassilevsky wrote:
Randy Yates wrote:
My current client has a requirement to obtain the the line-line
voltages of a three-phase power source inside a DSP given the
line-neutral inputs. He's thinking of doing it in the analog domain
using opamps to do the differencing, and feeding the resulting
line-line into the ADC (we're using the 2809, BTW).
Ouch. The built-in ADC of 28xx series is *horrible*. I wouldn't rely on it for any accuracy.
Oh really? I wasn't aware of that. (12 bits going on 8 - LOL!)
Further, there are at least two opamp architectures under consideration.
One is the standard difference amplifier between each of A-B, A-C, and B-C.
The other uses just three opamps to determine A-B and B-C, then relies on the fact that A-C can be derived from those two.
The other alternative is to just input the line-neutral voltages directly
(after appropriate voltage dividers) into the ADC.
I'd be curious to learn what folks here think the "best" method would be
given considerations of 1) noise, and 2) common-mode (the 3-phase center
isn't necessarily the same as the ADC / opamp circuit ground).
I would use "star" connection and do all differences in the DSP. Since you are interested in the differences, it doesn't matter
where the center point is.
What do you mean by "star" connection? Do you mean using a Y of three
caps, one to each phase, and taking the center point? We've thought
about that too but one thing I don't like about that is there will be
some ripple. Or is that what you mean? That any ripple is irrelevent
if you're taking differences?
And where would you use this star connection? You can't just connect
it to circuit ground - thinking (possible) ground currents.
Just worried about common mode. Sticking 10 volts into a 3V referenced
ADC isn't going to go over very well.
Randy Yates % "So now it's getting late,
Digital Signal Labs % and those who hesitate
mailto://ya...@xxxxxxxx % got no one..."http://www.digitalsignallabs.com% 'Waterfall', *Face The Music*, ELO
He might have meant star connection of voltage dividers and the middle
point goes to the ground of your circuit.
Assuming your circuit is otherwise isolated, the ground of the circuit
becomes "virtual" neutral point (I really don't know how to translate
the term we use back here). You might want to measure also the voltage
between "virtual" neutral point and neutral point of the grid in order
to determine if there is any unbalance. You can avoid this additional
measurement if the ground of your circuit (middle point of star
connection of voltage dividers) is connected to the grid neutral
point. But then you loose the advantage of having the circuit
This is probably redundant, but never assume grid neutral point to be
close or equal to grounding point.
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