Hello comp.dsp 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). 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). -- Randy Yates % "So now it's getting late, Digital Signal Labs % and those who hesitate mailto://yates@ieee.org % got no one..." http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO
Line-to-Line Sensing: Analog Vs Digital?
Started by ●May 8, 2011
Reply by ●May 8, 20112011-05-08
Randy Yates wrote:> Hello comp.dsp > > 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.> 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. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●May 8, 20112011-05-08
>Hello comp.dsp > >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). > >Further, there are at least two opamp architectures under consideration. >One is the standard difference amplifier between each of A-B, A-C, andB-C.>The other uses just three opamps to determine A-B and B-C, then relies onthe>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).If you have a neutral connection, why not do things digitally? If you are using a single ADC there will usually be a phase difference between samples. However, the C2000 ADC has two sample and holds, so you can simultaneously sample two of the phases, and simply take the difference of the results. As Vlad said, the ADC in the older C2000 parts isn't brilliant, so if you need high precision, its probably not the best choice. You mentioned noise, which suggests they may be looking for a fairly precise result. Steve
Reply by ●May 9, 20112011-05-09
Randy Yates wrote:> Hello comp.dsp > > 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). > > 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).*MISSING* info "What is client's goal?"
Reply by ●May 9, 20112011-05-09
On May 8, 8:34�pm, Randy Yates <ya...@ieee.org> wrote:> Hello comp.dsp > > 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). > > 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).Draw the phasor diagrams and you will see that knowledge of phase is critical. Phase is implicit with independent ADCs making simultaneous measurements. Assunptions about phase symmetry are useless. If they held, a single line-to-neutral RMS or peak amplitude would suffice. Jerry -- Engineering is the art of making what you want from things you can get.
Reply by ●May 9, 20112011-05-09
On May 9, 2:34�am, Randy Yates <ya...@ieee.org> wrote:> Hello comp.dsp > > 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). > > 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.Is that a *fact*? Sure, the theory says so *provided* you know all there is to know about the system. In my experience, that assumption tends to break down when stuff gets interesting, like when there is a mechanical faliure somewhere. Like a short to ground, or some impedance straying off spec somewhere. Nah, measure all three phases relative to some earth locally at the DSP board. That way you cover all the bases - you might even be able to detect problems with the earth cable that you would miss if you only measured differences between phases. Rune
Reply by ●May 9, 20112011-05-09
On May 9, 12:56�pm, Rune Allnor <all...@tele.ntnu.no> wrote:> On May 9, 2:34�am, Randy Yates <ya...@ieee.org> wrote: > > > Hello comp.dsp > > > 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). > > > 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. > > Is that a *fact*? > > Sure, the theory says so *provided* you know all there is > to know about the system. > > In my experience, that assumption tends to break down when > stuff gets interesting, like when there is a mechanical > faliure somewhere. Like a short to ground, or some impedance > straying off spec somewhere. > > Nah, measure all three phases relative to some earth locally > at the DSP board. That way you cover all the bases - you might > even be able to detect problems with the earth cable that you > would miss if you only measured differences between phases. > > RuneI'd also go this way, unless you have high harmonics present, and sequential sampling would influence the results. In that case measuring A-B and B-C, sampling them simultaneously and deriving A-C is probably the only way with internal ADC. Mitja
Reply by ●May 9, 20112011-05-09
On 05/08/2011 06:21 PM, Vladimir Vassilevsky wrote:> > > Randy Yates wrote: >> Hello comp.dsp >> >> 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.The best characterization of the ADC in the 2812 was "twelve bits going on eight".>> 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.I can only second this. Even if the math told you that you could squeeze another bit out of the system by doing the subtraction in the analog domain, my inclination would still be to put the signals into the ADC with as little analog folderol as possible. I would definitely want to monitor all three legs, though -- redundancy is nice, and if you get all the neutral-leg readings, you'll have information for testing imbalances later, as the product requirements grow. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●May 9, 20112011-05-09
On 05/09/2011 12:56 AM, Rune Allnor wrote:> On May 9, 2:34 am, Randy Yates<ya...@ieee.org> wrote: >> Hello comp.dsp >> >> 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). >> >> 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. > > Is that a *fact*? > > Sure, the theory says so *provided* you know all there is > to know about the system. > > In my experience, that assumption tends to break down when > stuff gets interesting, like when there is a mechanical > faliure somewhere. Like a short to ground, or some impedance > straying off spec somewhere.Hi Rune, Thanks for your challenge! I can't think of a specific scenario in which this assumption would break down, namely that, (A - B) - (B - C) would not be equal to (A - C). Can you give me a concrete example? -- Randy Yates % "So now it's getting late, Digital Signal Labs % and those who hesitate mailto://yates@ieee.org % got no one..." http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO
Reply by ●May 9, 20112011-05-09
On 05/08/2011 09:21 PM, Vladimir Vassilevsky wrote:> > > Randy Yates wrote: >> Hello comp.dsp >> >> 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://yates@ieee.org % got no one..." http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO
