4-20ma calibration

[Anniyan_x]

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

 

[JimB]

Without seeing the details of your whole system and the calibrator which you are using, my best guess is that the analogue inputs are responding to the different common-mode voltages in the two set-ups.

What I suggest is that you try calibrating again, using both pasive and active modes, and in each mode measure the voltage between the "transmitter" connections at P4 (difficult to read on the schematic) and analogue ground (AGND).
If those voltages are different, rearrange your connections to make them the same, and then try calibrating again.

Also, if you have unused inputs, dont let them "float", connect them to AGND.
I have seen this cause some very strange effects on systems I have worked on in the past.

JimB

 

[Anniyan_x]

**broken link removed**

HI, i just a zoomed pic for clearer.

Im using a FLUKE 787 processmeter as the calibrater which have a SOURCE and STIMULATE options for current loop systems.So first i set my board in active mode by enabling the internal isolated 24v power supply. after I configured jumper setting for active mode, then connect the FLUKE (set to STIMULATE mode) probe into the P4.Then i calibrated the board(active mode). after that i stimulated the current levels using the FLUKE meter. i measured the voltage from fluke P4 to the board AGND. the voltage is same for all the current level stimulated, which is from 4ma,8ma,12ma,16ma,20ma, i get aprox 9.33v~9.26v. beginning was high then slowly drops to the said voltage.the ADC reading was accurate, i get the ADC reading in hex. for eg. min is 0 for 4ma and max is 3FFF for 20ma.

after that i switch to passive mode, i disable the 24v internal voltage, adjust the board input jumper setting, set the FLUKE in SOURCE mode and then i injected the current from 4ma~20ma, now i get the voltage as measured is 1v then gradually drops to milivolts, then it remains in milivolts. So the ADC reading not accurate.for e.g 4ma i get 0014h, it shud be 0000h.

so what u suggest above is that i need to make sure the voltage is always same in both modes when i measure from P4->AGND,if i just wanted to calibrate once only in either mode (active or passive). meaning the Vin of the transmitter should be the same?? for now when i use active mode the voltage source is 24vdc from the board itself. but when i switch to passive mode im using FLUKE's supply voltage to SOURCE the currents. anyway not yet test to see the results of using same voltage.

 

[Anniyan_x]

HI all after some tweaks i found that the the voltages difference between both the modes (passive&active) was because of the shifted offset voltages when it is active mode (using internal supply) the 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.

so the difference in reading was because this offset is shifted (+0.003v) when using in active mode where by it should not have been shifted. it should be same in both modes.

the circuit is as below:-

View attachment 67268

the results of the measurements comparing the both modes is as below:-

View attachment 67269

from the results we can see the difference in output of the newboard (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. the above circuit is only the virtual ground and it is output voltage (1.4Off) supplied to channel 1 of the next amp circuit.

 

[Anniyan_x]

Without seeing the details of your whole system and the calibrator which you are using, my best guess is that the analogue inputs are responding to the different common-mode voltages in the two set-ups.

What I suggest is that you try calibrating again, using both pasive and active modes, and in each mode measure the voltage between the "transmitter" connections at P4 (difficult to read on the schematic) and analogue ground (AGND).
If those voltages are different, rearrange your connections to make them the same, and then try calibrating again.

Also, if you have unused inputs, dont let them "float", connect them to AGND.
I have seen this cause some very strange effects on systems I have worked on in the past.

JimB

hi all the root was finded out, in the board design there was a layout issue on the ground plane.... in sense tht a part of ground plane poligon was removed by the layout designer which cause this instability issue... so when in active mode the on board 24v isolated supply cause interferences to the analogue reading.... when in passive mode, it works fine coz the 24v isolated supply is off now

 

[JimB]

Good to know that you have found your problem.

JimB