LowPowerLab Forum

Hardware support => CurrentRanger => Topic started by: kgmuzungu on December 28, 2019, 05:31:35 AM

Title: Bias mode - how?
Post by: kgmuzungu on December 28, 2019, 05:31:35 AM

I do not understand how is the bias mode is realized. In the schematic: what MOSFETs are switch on or off for bias mode? The opamps are only have a single supply, right? Are the signals D38_ATN_MA and NMA involved? What are these abbreviations stand for?

Thanks in advance.

Title: Re: Bias mode - how?
Post by: kgmuzungu on February 26, 2020, 01:35:42 AM

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The reason that there is a GND and a GND-ISO in the schematic is that GND-ISO is the output of the DAC of the microprocessor and GND is the battery ground (min) to use the CurrentRanger in normal- or bias-mode.

Taking a non-inverting op amp voltage amplifier configuration and you put the feedback resistor not on ground but on an arbitrary voltage level (here GND-ISO) e.g. to bat/2 volt then the output of the op amp relative to the battery goes also to bat/2 V if there is no diff voltage present at the inputs. You can show that if you analyze the circuit on paper or you try it out or simulate it. With the CR you measure then the output voltage between the op amp output and GND-ISO, which is in bias-mode bat/2 or to whatever voltage you set the DAC output. In bias-mode if there is no input voltage then the output is zero because the op amp output and GND-ISO are on the same level. The output can go higher then bat/2 then you get a positive output voltage or lower then the output appears negative. That's how the bias-mode works.

Note: in the source code the voltage for GND-ISO is set with the variable "offset".

Note: If you have a noise signal then in bias-mode the positive and negative parts are amplified and might average out measured with a multimeter. In normal mode you might only amplify the positive part and create an DC error.