Re : Resolution of the analog input of ni 6218

[mahdi2011]

I have got a NI 6218 device and I am reading analog signals from its analog inputs. The problem is with small input signals(in the range of 0.217 volts). The device specification mentions that the analog inputs have a resolution of less than 1mV, but in practice when I give a constant input of 0.217V to one of its analog inputs then I have a lot of fluctuation in the acquired data ( the acquired data fluctuates between 0.180V to 0.260V.) does anyone have any explanation for this? Is this device able to acquire small signals with the resolution mentioned in its data sheet? 

[ramses64]

Hi,

you posted on a french forum...

Just a question, did you change the Max//Min of your configuration task ? The 12 bits of your card are for the range you defined, so if you decrease these parameters, you increase accuracy..

Hope it will help you.

Regards,

[mahdi2011]

Hi there

Thanks for the reply. It was the first time I was using this forum that is why I posted here by mistake.

 The thing is that I have got 3 analogue inputs and I calculated the range of -3V TO 3V suitable for all of them. The data sheet says that even for this range the resolution is less than 1mV, but in practice I do not see such a resolution.  Even I tried to set different ranges for these three different inputs and I found that even if I set a small range for the input that has a small signal, then the accuracy of the measured signal is still not good. (I see the input readings oscillate with an average error of +/-40mV).

The other problem is that when I apply a voltage signal to other ports on the device then it also affects the small signal port reading up to 10mV. (Even I can measure the small voltage drift on that input port by a multimeter)

Anyway, when I take the average of the measured signal for duration of 1 second then it gives me the proper signal value. For my application the average value during one second is enough because it is for temperature sensor and I do not really need to take the measurements with high sampling rate.

At the end of the day, despite the fact that I could find a solution for my measurement problem, I still do not understand why the resolution of the measured data is not as what is specified in the data sheet!

[Romain_P]

Hi Mahdi,

On the 6218, it exist 4 ranges for analogs inputs : -10/10V, -5/5V,-1/1V,-0.2/0.2V.

If you want to measure a signal between -3V and 3V, you have to use the -5/5V range of your card.

For this range, the absolute accuracy is 1,410mV and sensitivity is 91.6uV.

If you want a better accuracy, you have to use the -1/1V range : absolut accuracy is 310 uV and sensibility 10.4uV.

I advise you to follow this link : here.

Regards,