Re: [volt-nuts] Matched resistors
Tony, Sorry for not reading further. I will give it some thought. Interesting idea. Randy On Thu, Jul 24, 2014 at 5:13 AM, Tony Holt wrote: > Randy, > > > On 24/07/2014 04:22, Randy Evans wrote: > >> Tony, >> >> Your improvement factor of SQRT(n) assumes that each resistor in the group >> has random changes uncorrelated to all others in the group. For similar >> type resistors, I would think that is not likely to be true. >> > > Yes/,/ I'm well aware of that which is why I discussed that point further > down in my post. It was a long post though so I don't blame you for getting > bored and not getting that far! > > > For shelf life >> stability it is likely that they all "age" in a similar way. Unless the >> resistors are in a hermetic package, humidity would impact all the >> resistors in a similar manner. >> >> Randy >> > Exactly. Since they are being used in a 1:1 divider configuration, if they > age in a similar way, the tracking ratio stability will be good. The > reality however is that there will be some variance between components, and > using multiple resisters will reduce that overall variance. Part of the > variance between individual resisters will likely follow a Guassian > distribution and thus the improvement factor for that element will be > SQRT(N). > > Some of the variance will likely be due to random factors which have a > rather different distribution, probably highly skewed with long tails, and > thus the improvement probably won't be SQRT(N). My conjecture (ok random > speculation) is that factors such as stress differences due to > microcracking in the ceramic substrate or at the terminations may cause > some of the latter. Nevertheless, even though part of the variance doesn't > follow SQRT(N) the variance will still reduce by using multiple identical > resisters (if there are enough*). The problem is knowing how much - it > probably can only be determined by lengthy experimentation, unless some > good empirical data can be obtained from manufacturers or research papers. > > Another complication is that I believe that thin film resistor stability > and TCR characteristics improve as the resistance reduces. This is not > usually reflected in the datasheet but using multiple resistors in series > allows lower values to be used which may perform better. On the other hand, > thermal EMF problems may increase proportionally. > > TCR tracking is much easier to measure, so it might be interesting to see > how it improves with increasing numbers of resisters. However, I understand > that ratio stability is likely to be a bigger problem than TCR tracking. > > The other end of the spectrum, using a single Vishay VHD foil divider is > certainly the simplest; however bear in mind that Vishay's stated typical > tracking TCR of < .1ppm is just that, and the one that you buy may be > anything but typical. And if you can work out the maximum tracking TCR from > the VHD144/200 datasheet, you're a better man than I. My guess is that its > probably better than .5ppm which is likely good enough for your > application. But would it perform better than, say $30 worth of Vishay DFN, > 3ppm 4-resister networks, 1 year shelf life ratio stability < 20ppm? I > don't know. > > Of course there's nothing to stop you using multiple VHDs if you can get > them at a good price. Ebay maybe? > > *) If all resisters are identical expcept that 1 in a 100 is markedly > different, then any 10 will have a good chance (90%) of being identical; > using 100 will have a good chance (64%) that at least one is different and > thus the overall error would be at least 1/100 of the difference. > > Tony H > > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Datron 1281 Repair Help
Михаил writes: > > M202 (DG211) switchs, pins 6-8 and 1-3. > > Friday, March 21, 2014, 2:45:46 PM, you wrote: > sun> I am working on a Datron 1281 meter. > sun> DC voltage calibration at 10v functions ok, but the Internal source > sun> calibration failed. > sun> This Meter has the following errors after Internal Source Calibration. > sun> Error codes: > sun> 2202 1KVRange zero magnitude test (P028). > sun> 2192 100V Range zero magnitude test (P021). > sun> 2162 10VRange zero magnitude test (P006). > sun> What would cause these errors? > > I have the same issue and prior to seeing this blog i checked the m202 dg11 switchs and did not find an issue. What i did see was noise on the guard around that line which goes to the non-inverting side of the dc amp, where that noise is coming from possibly the bootstrap i am not 100% or i would have fixed it. But it seems like this is a common issue this is the second post i have found on this issue which makes me believe it not a cold solder issue. Ive checked most caps in power supply and dc board if you have any other possibilities please let me know. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Matched resistors
Randy, On 24/07/2014 04:22, Randy Evans wrote: Tony, Your improvement factor of SQRT(n) assumes that each resistor in the group has random changes uncorrelated to all others in the group. For similar type resistors, I would think that is not likely to be true. Yes/,/ I'm well aware of that which is why I discussed that point further down in my post. It was a long post though so I don't blame you for getting bored and not getting that far! For shelf life stability it is likely that they all "age" in a similar way. Unless the resistors are in a hermetic package, humidity would impact all the resistors in a similar manner. Randy Exactly. Since they are being used in a 1:1 divider configuration, if they age in a similar way, the tracking ratio stability will be good. The reality however is that there will be some variance between components, and using multiple resisters will reduce that overall variance. Part of the variance between individual resisters will likely follow a Guassian distribution and thus the improvement factor for that element will be SQRT(N). Some of the variance will likely be due to random factors which have a rather different distribution, probably highly skewed with long tails, and thus the improvement probably won't be SQRT(N). My conjecture (ok random speculation) is that factors such as stress differences due to microcracking in the ceramic substrate or at the terminations may cause some of the latter. Nevertheless, even though part of the variance doesn't follow SQRT(N) the variance will still reduce by using multiple identical resisters (if there are enough*). The problem is knowing how much - it probably can only be determined by lengthy experimentation, unless some good empirical data can be obtained from manufacturers or research papers. Another complication is that I believe that thin film resistor stability and TCR characteristics improve as the resistance reduces. This is not usually reflected in the datasheet but using multiple resistors in series allows lower values to be used which may perform better. On the other hand, thermal EMF problems may increase proportionally. TCR tracking is much easier to measure, so it might be interesting to see how it improves with increasing numbers of resisters. However, I understand that ratio stability is likely to be a bigger problem than TCR tracking. The other end of the spectrum, using a single Vishay VHD foil divider is certainly the simplest; however bear in mind that Vishay's stated typical tracking TCR of < .1ppm is just that, and the one that you buy may be anything but typical. And if you can work out the maximum tracking TCR from the VHD144/200 datasheet, you're a better man than I. My guess is that its probably better than .5ppm which is likely good enough for your application. But would it perform better than, say $30 worth of Vishay DFN, 3ppm 4-resister networks, 1 year shelf life ratio stability < 20ppm? I don't know. Of course there's nothing to stop you using multiple VHDs if you can get them at a good price. Ebay maybe? *) If all resisters are identical expcept that 1 in a 100 is markedly different, then any 10 will have a good chance (90%) of being identical; using 100 will have a good chance (64%) that at least one is different and thus the overall error would be at least 1/100 of the difference. Tony H ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
