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150V opamp feedback resistor suggestions
TinyMirrors:
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!
magic:
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?
Kleinstein:
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.
SiliconWizard:
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.
mzzj:
0805 resistor has thermal resistance in order of 500K/W from resistive layer to ambient. https://www.vishay.com/docs/53048/pprachp.pdf
You 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.
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