On Thu, May 1, 2025 at 02:20 PM, thet wrote:
I cleaned the board better, paying special attention to the gap between pins 3 and 4 which are +In and V- and they are very close together on this smd opamp.
that helped! there must have been some flux still right under the edge of the opamp - the virtual ground is now at 13.5V with the 1G input resistor, so only a volt or so too high.
A friend of mine recently had exactly the same problem you described here and also solved it by thoroughly cleaning the PCBA with IPA. I gave him the advice to add a guardring. The guardring connects to the inverting input of the OPA and encircles the high-Z node on the non-inverting input. Ideally, the guardring should be placed on both sides of the PCB and, connected with vias and should be left free from solder mask, also on the area inside the guardring. This prevents the DC drift when there are flux residues on the board or when the board is used in a high humidity environment. But you still want to thoroughly clean the board as the resistive flux will decrease SNR.
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The close spacing of the conductors that is inherant to SMT OPAs makes it hard to design PCBs that are free of noise and drift issues in high-Z circuits. TI should have given it another package, leaving out some pins as in some High Voltage ICs. This would allow the use of isolating slots, in combination with a guardring and prevent noise and drift issues.
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The low-melt solder that you use could be part of the problem, as you already reasoned. The flux has to do its work at lower temperatures, so I assume it has to be of a more aggressive type which possibly lowers the Surface Insulation Resistance. You could check its datasheet or just use regular solder from a name brand like Kester.
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Jan
