Anniyan_x
Member
hi i have analog current input card that have 2 modes, passive ( need to power the sensor connected to it by external power supply) and active mode, sensor will be power using cards internal supply. the problem is when using in either (active or passive) modes using same sensor the outputs have a slight voltage shift where-byright in both modes it should be the same. this cause different sensor readings and by right the reading should same no matter in what modes the cards is being used. after some analysis i find that, please refer below circuit:-
View attachment 67270
the voltage point labelled 1.4Off has a voltage shift of 0.003v when used in active mode.
This virtual ground circuit (thats what it is called i guess, using opamp) has voltages divider on its +input which divide the Vref into half and provides this voltage as an input to virtual ground opamp's +input. This opamp configurations then generates a stable voltage on the -ve input side which is then used as an offset volatage. and this offset gets shifted (+0.003) when using in active mode(using internal supply to power the sensors connected to the card)
for e.g in passive mode 1.4Off => 1.4volts
in active mode 1.4Off => 1.403
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the above was confirmed when this measurements where taken to indentify the root cause.
the measure results was taken at this circuit(final circuit on the analog signal chain before fed to ADC:-
View attachment 67271
and the results as below:-
View attachment 67272
from the results we can see the difference in output of the (active vs passive) whereby in the active mode the has voltage shift of aproxx 0.003/4 and it is consistent.
does anybody have hints on why this might occur and the tips about common issue regarding using virtual ground in opamp to provide stable voltage output.
View attachment 67270
the voltage point labelled 1.4Off has a voltage shift of 0.003v when used in active mode.
This virtual ground circuit (thats what it is called i guess, using opamp) has voltages divider on its +input which divide the Vref into half and provides this voltage as an input to virtual ground opamp's +input. This opamp configurations then generates a stable voltage on the -ve input side which is then used as an offset volatage. and this offset gets shifted (+0.003) when using in active mode(using internal supply to power the sensors connected to the card)
for e.g in passive mode 1.4Off => 1.4volts
in active mode 1.4Off => 1.403
---------------------------------------------------------------------------------------------------
the above was confirmed when this measurements where taken to indentify the root cause.
the measure results was taken at this circuit(final circuit on the analog signal chain before fed to ADC:-
View attachment 67271
and the results as below:-
View attachment 67272
from the results we can see the difference in output of the (active vs passive) whereby in the active mode the has voltage shift of aproxx 0.003/4 and it is consistent.
does anybody have hints on why this might occur and the tips about common issue regarding using virtual ground in opamp to provide stable voltage output.