150V opamp feedback resistor suggestions - Page 1
Forum section: electronics / projects

author: TinyMirrors
author_rank: Contributor
timestamp: October 24, 2019, 03:36:57 pm
content: So we have a problem in which the feedback resistors we're using for a single stage gain opamp are warming up and causing drift in our output. It does stabilize, but the issue is that the output becomes a function of the previous outputs. It's a basic non-inverting opamp setup with a gain of about 66. Using 1Meg and 15k resistors which are both in an 0805 package. The output can be as high as 150V, but the future may be as high as 250V. I need help brain storming ideas to solve the drift issue.The first thought is to use ultra low ppm resistors which are big and cost around $30 each. Another idea is to increase the 1Meg resistor to 20Meg or so. A third option is to pot everything in thermal epoxy, but that might only delay the effect. What are some other strategies to try?Thanks!

author: magic
author_rank: Super Contributor
timestamp: October 24, 2019, 04:21:27 pm
content: If there is space for that, larger package for lower thermal resistance and/or multiple resistors in series to spread the heat. More PCB copper in the area.Also, resistors come in all kinds of varieties between $30 and $.03. Try thin film SMD if you haven't already, they are an order of magnitude better than the generic stuff.Is a single 0805 package even rated to withstand 150 or 250 volts?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2019, 04:35:12 pm
content: One does not really need low ppm resistors. The important point would be good TC tracking. A possible way could be using many resistors in series / parallel and allow for thermal coupling. At lower voltages the normal way would be a resistors array, but not many really like 150 / 250 V.Something like 13 in series and 5 in parallel would give a x 66 gain.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 24, 2019, 04:40:01 pm
content: You could also use resistor networks, such as the LT5400 series. Pretty good matching (0.2ppm/°C), so nice for feedback networks. The problem you'll have is the limited number of configurations. With the LT5400, you could typically get a non-inverting gain of 11 (100k/10k version). With a slightly degraded accuracy, you could combine the resistors (it has 2 of each): 100k+100k, 10k//10k => gain = 41.Price is reasonable.(Note: the +/-80V max across the resistors could be a problem in your application.)You can look up other matched resistor networks though.

author: mzzj
author_rank: Super Contributor
timestamp: October 24, 2019, 06:22:15 pm
content: 0805 resistor has thermal resistance in order of 500K/W from resistive layer to ambient.https://www.vishay.com/docs/53048/pprachp.pdfYou are dissipating 150v*150uA = 22mW of power , 500K/W --> 11 degree temp rise --> 1100ppm or 0.11% dritft with 100ppm/K resistors.Changing to 10 meg resistors drops the power dissipation to 1/100 and same goes for temp rise and resistance drift.Or use larger resistor than 0805, or several resistors in series.

author: jbb
author_rank: Super Contributor
timestamp: October 24, 2019, 07:38:57 pm
content: Maybe you should post a a bit of schematic and a photo of the board? There are lots of keen people around here who might spot stuff for you. Have you looked at whether heat is spreading from the HV opamp to the resistor array? That could be making things worse (plus, opamp heat dissipation doesn't always work out like you expect. It depends on supply rails, output voltage, and load characteristics etc.)Quote from: TinyMirrors on October 24, 2019, 03:36:57 pmWhat resistors exactly are you currently using? You might be able to do something much better without spending $30 each.Quote from: TinyMirrors on October 24, 2019, 03:36:57 pmThis might work, but might lose some bandwidth and gain some noise (don't forget to look at opamp input current noise * input resistance).

author: TimFox
author_rank: Super Contributor
timestamp: October 24, 2019, 08:34:32 pm
content: One thing to watch out for is the voltage co-efficient of resistance, which can be surprisingly high in surface-mount high-resistance cermet or metal-oxide “thick film” units. “Thin film” (similar to through-hole metal film) is better, but hard to find in high-megohm values in small packages. Even in 10 volt circuits, I had to specify 1206 premium parts (instead of 0805) where I needed 50 megohm feedback resistors. It is hard to obtain the value of this spec for high-megohm units, specified as ppm/V. If you need 0.1% linearity at 100 V signal level, you need better than 10 ppm/V. This is in addition to the self-heating plus temperature co-efficient, which is also important, although self-heating improves at higher resistance due to lower power dissipation.At the internal level, for a given resistive material, the deviation from the Ohmic linear approximation is a function of the linear voltage gradient (V/mm) which gets worse for physically short smd packages.

author: floobydust
author_rank: Super Contributor
timestamp: October 24, 2019, 09:31:29 pm
content: 250V across an 0805 is pushing it, it's likely too small for what you are working with and you need to use longer parts or several in series. Have a spotless, clean board with no flux on the board unless you use guard-bands.Also check the capacitor's dielectric if you are seeing hysteresis effects, if leakage is a issue.I would look at the voltage coefficient of resistance (VCR), some parts are over -10ppm/V.The spec is almost never mentioned in resistor datasheetsWelwyn VCR tech note[/url

author: Wolfgang
author_rank: Super Contributor
timestamp: October 24, 2019, 09:34:13 pm
content: ... What about a standard thru-hole HV metal film resistor ? They dont cost a fortune, are temp stable and have a very low voltage coefficient.I dont believe in TC matching because the "upper" resistor always runs at a lot higher dissipation that the lower one. What does matching TCs help if the temperatures dont match ?. You want a very LOW TC.

author: splin
author_rank: Frequent Contributor
timestamp: October 25, 2019, 01:03:06 am
content: Quote from: mzzj on October 24, 2019, 06:22:15 pmThat document quotes 440K/W for an 0805 in pretty much a worst case scenario (minimal copper 'mCu' column in Table 1), but it shows 136K/W for a more typical layout and only 63K/W with lots of 2oz/inch^2 of copper - all for natural convection.It also depends on how many resistors are in close proximity. This document gives some good experimental numbers for various sizes and quantities of resistors:http://www.vishay.com/docs/30380/terminalderating.pdfWhat level of precision are you aiming for over time and temperature?

author: duak
author_rank: Super Contributor
timestamp: October 25, 2019, 03:31:37 am
content: Others have pointed out that the feedback resistor is probably dissipating too much power to meet your stability requirements and have presented solutions.If all gain determining resistors for can be thermally coupled together, the TCs should cancel out. This is where a monolithic or hybrid network on one substrate could work very well. If this is a multichannel assembly, each channel's network should be thermally isolated from its neighbors.A left field idea would be to have another resistor in the low voltage part of the signal path that would dissipate a similar amount of power and be similarly affected and could compensate for the resistor under question. I can't think of a simple circuit that either doesn't use another op-amp or doesn't mess with the noise gain of the existing opamp.Not knowing what op-amp is used or what the actual circuit is, could the op-amp also be contributing to part of the drift? With a supply rail of at least 150 V, the output devices could be dissipating enough power to cause a significant thermal gradient in the die that may affect the input offset.