Now I can wait and see how long it will take for the opamp to drift into unacceptable values. I have soldered a 200k resistor parallel with it and the gain is now OK.
The gain is controlled by the lower 10k resistor. This still left about -15 mA offset so I soldered a 3M6 resistor over the previous one (they are now parallel) and now the offset is +2 mA which is acceptable.Īfter fixing offset I have found the gain is also way off - about +5%. After some calculations I decided the right value is 262k, I used 270k standard value. But despite this my device showed about 100 mA less. Since there is 330R between IN+ and GND this resistor simply increases the voltage on IN+ by about 2 mV. Chopper stabilized (also called auto-zero) op amps have a VOS which is less than 1 V (e.g. Figure 1: Typical Op Amp Input Offset Voltage. The 510k resistor on the picture is between 3V3 from regulator and IN+. Untrimmed CMOS Op Amps: 5,000-50,000V DigiTrim CMOS Op Amps: <100V-1,000V-+ VOS. What I did as a quick fix (I don't have a precision op amp to replace it) if anyone is interested: It is not my fault the cheapest op amp is used. I did not design the circuit, only bough it in China.
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