Hi everyone,
Due to its unavailability a while ago I've built my own µCurrent for measuring very low power microcontroller power consumption. The µCurrent is an open source design, and I sourced all parts from Digikey because the order codes are also made available in the BOM.
The only part I didn't source from DigiKey are the MAX4239 opamps because they weren't (and still aren't) available in SOT23-6 package. So instead I ordered them from AliExpress (
http://www.aliexpress.com/item/New-MAX4239AUT-T-IC-AMP-PREC-LDRIFT-LN-SOT23-6-best-pirce-IC-supply-chain/1992017520.html). After assembling the µCurrent it shows a significant offset of -700 mV at the outputs when the current inputs are shorted.
When tested on a breakout board, the amplifier does behave as one would expect from an opamp, except that the output voltage is 28 mV when both inputs are grounded. For a low offset, low drift opamp I find this on the high side since this offset is also amplified by the second stage, thus resulting in a cumulative error.
Does anyone have any thoughts about this behavior? I don't have much experience with correcting opamp offsets, but the design doesn't include any compensation circuitry nor does the opamp provide offset compensation inputs. How do I get rid of this offset? Could it be that I have been scammed and sold inferior opamps in an identical package?
Thanks in advance!