| Electronics > Projects, Designs, and Technical Stuff |
| Nulling Input Offset Voltage |
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| TinyMirrors:
I have an opamp which drifts slowly throughout 10 hours. It's drifting around by 10mV with a 100V output. It's a high voltage amplifier that I can't just change. After a lot of testing I believe it's most likely input offset voltage as it needs to only drift by 75uV to cause the drift on the output. So what are you favorite ways to nullify input offset drift? The opamp is a 4 channels, but only 3 channels are used. I believe the opamps are on a single die in the package. Thanks! |
| jbb:
How low do you need it? Do you need it low on all channels or just 1? Have you checked whether it could be due to temperature coefficients in the feedback divider? |
| DaJMasta:
Especially with such high voltage, I would also suspect tempco issues with other parts, but perhaps even with air currents over the opamp. Some amount of input offset is normal, but can often be nulled out by a trimmer pot in a divider, so the rest is probably coming from thermal drift of something involved. Do you really need 6 digits of accuracy in this circuit, though? I know you say that you can't really change it, but if you're really looking for that kind of stability, you may need to rework your component choice at least, and maybe your thermal design as well, because that sort of stability is challenging without considering them in the design. I'm sure they exist, but I can't figure an application where 10mV swing on 100V makes much of a practical difference. |
| TinyMirrors:
Hi jbb One channel would be enough as the other two channel aren't are critical. The spec I would like to meet is around 0.5mV or lower, but I'm sure a bit more would be okay. Currently there exists a system that meets this spec, but is no longer in production so this is a replacement. I have a lot of requirements around this device so I don't have many options available to change to. I don't suspect this is due to temperature as the temperature and humidity were controlled and monitored but found no correlation to the drift. The temperature fluctuation was less than 0.5 degrees. Relative humidity is around 5%. There are no major sources of heat on the board. Using a thermal camera the hot spot of a regulator is about 3 inches away and is about 4 degrees warmer. There's no change to the board's temperature during the test. I've tried to ensure that the environment is changing as little as possible, no dust is falling on the board, and the no sources of noise near the board. Thought I haven't yet put it in a cookie tin. Hope that gives a few clues to help figure this out. |
| DaJMasta:
The plot thickens! To be sure, have you checked the voltage rails for fluctuation as well? If they're drifting around, any virtual ground could be moving. Also, you've verified that the drift is not present on the 100V measured with a 6+ digit DMM? |
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