Anniyan_x
Member
HI i have a analog input board tht can receive 4~20ma current and it is used for sensor current loop communications. the board design have 2 modes active and passive where in active mode the board itself supplies 24v (internal isolated power supply) to the sensors and in passive mode sensor must has own power supply. the image below is the connections:-
View attachment 64633
now the problem is when i use a loop calibrator to calibrate the board, let say i connect it in passive mode, after calibrations , the board reading results are accurate and good but when switch to active mode, the reading results of the board have errors, and only can be accurate if i re-calibrate it in active mode. so each time i switch mode i need to re-calibrate. why cant i just calibrate once in one of the mode?? what might cause the error because when i check the current that flows in both the modes, it almost same for .e.g 3.99ma in active mode, and 3.98ma in passive but the board reading is different, sometime both current are same 3.99ma, 3.99ma but still the reading from the board is different. is this because of the analog circuitry?? or the current now sinks to ground in different place so it cause the variations.
the analog front end circuit is as below:-
View attachment 64635
View attachment 64633
now the problem is when i use a loop calibrator to calibrate the board, let say i connect it in passive mode, after calibrations , the board reading results are accurate and good but when switch to active mode, the reading results of the board have errors, and only can be accurate if i re-calibrate it in active mode. so each time i switch mode i need to re-calibrate. why cant i just calibrate once in one of the mode?? what might cause the error because when i check the current that flows in both the modes, it almost same for .e.g 3.99ma in active mode, and 3.98ma in passive but the board reading is different, sometime both current are same 3.99ma, 3.99ma but still the reading from the board is different. is this because of the analog circuitry?? or the current now sinks to ground in different place so it cause the variations.
the analog front end circuit is as below:-
View attachment 64635
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