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Metrology / Re: Analog frontends for DMMs approaching 8.5 digits - Discussions
« Last post by Kleinstein on Today at 09:44:20 am »The input leakage and average current from charge injection looks really good. Also the input inpedance / conductance looks really good: some 2-3 pA of change in the input current when going from -10 V to 10 V would be an input resistance in the 10 Tohm range and this is even with 1 PLC mode. It is cool to have some kind of self test for the input leakage.
For the sum test it would also be good to include a 4 th measurement (BB) for the offsets. The BB rading would be subtraced from the result. This way one has the low side at A and B twice each and the high side at B and C twice each and int the resut each point once positive and once negative. This should give a quite good compensation of offsets (e.g. from the protection part).
For the sum of 2 voltages one needs the test voltages to be floating to have the ability to connect the center point to the low side (GND) for 2 of the readings. This makes is tricky to use the same reference nad DAC. One would need some charge pump floating capacitor system (like the 10 µF film caps) or similar system with current source and series of resistors. One than still has the problem that the error can be from the ADC or the charge-pump (or current source output resistance). With the capacitor the issues are leakage, switch charge injection and also parasitic capacitance that can pump out some charge.
The result from the sum test still show quite some scattering. Part of this may be from drift in the battery voltages. At least I see this as a weak point with batteries. The dift can give a systematic error and just simple averaging is not enough. One can compensate at least for the linear drift part, by interpolation to the same time from more readings in a row.
Depending of the voltage reference used (especially a LM399) there can also be an error from popcorn type jumps in the reference voltage. Here just averaging over enough reading could help.
With the scattering and possible error from drift of the battery voltages it is hard to say what the results mean.
For the sum test it would also be good to include a 4 th measurement (BB) for the offsets. The BB rading would be subtraced from the result. This way one has the low side at A and B twice each and the high side at B and C twice each and int the resut each point once positive and once negative. This should give a quite good compensation of offsets (e.g. from the protection part).
For the sum of 2 voltages one needs the test voltages to be floating to have the ability to connect the center point to the low side (GND) for 2 of the readings. This makes is tricky to use the same reference nad DAC. One would need some charge pump floating capacitor system (like the 10 µF film caps) or similar system with current source and series of resistors. One than still has the problem that the error can be from the ADC or the charge-pump (or current source output resistance). With the capacitor the issues are leakage, switch charge injection and also parasitic capacitance that can pump out some charge.
The result from the sum test still show quite some scattering. Part of this may be from drift in the battery voltages. At least I see this as a weak point with batteries. The dift can give a systematic error and just simple averaging is not enough. One can compensate at least for the linear drift part, by interpolation to the same time from more readings in a row.
Depending of the voltage reference used (especially a LM399) there can also be an error from popcorn type jumps in the reference voltage. Here just averaging over enough reading could help.
With the scattering and possible error from drift of the battery voltages it is hard to say what the results mean.