I have been using the ADALM1000 to work through the labs in the Engineering Discovery set. While doing the first two Class A amplifier labs I noticed my output is not centered around 2.5 V as per the reference voltage used but rather just under 2.4 V which seems to be lower than what is expected given the tolerances of the parts used when building the amps. I checked the 2.5 V output with a Keysight U1232A and am seeing 2.434 V. The hardware design indicates the 2.5 V output comes from an ADR381 precision voltage reference with a range of +- 6 mV at room temperature. I seem to be getting about 10 times worse. Is this normal for the ADALM1000, is something not quite right with my ADALM1000, or have I improperly read the datasheet/measured wrong?
The original Bill of Materials for the production run of ALM1000 has the feedback resistor for the AD8018 buffer used for the fixed 2.5 V output at 24.9K which causes the output of the buffer to be lower than the desired 2.5 V by around 50 mV due to the large input bias current of that amplifier. This error will vary from board to board and could be as much as 100 mV.
For future production of the ALM1000 this resistor will be changed to 2.49 K which will lower the error but there will always be a small negative error on both the 2.5 and 5 volt outputs.
The large feedback resistor was originally chosen to lower the bandwidth of the buffer and insure stability with large capacitor loads. The smaller future value was found to have similar stability but with the improved output voltage error.