c series module 9205 erratic analog readings

[KWaris]

I started doing projects to acquire analog signals now about a year ago. First time, I read the signals using NI 9205, I wondered why the readings were not stable. ( I was expecting study signal I observe using Digital Multimeter).We minimized the losses completely by grounding properly, using both single ended and differential configuration. This issue however still is outstanding.

I think I might have not used some proper programming to acquire the analog data. I might be oversampling the signals etc. So I thought lets put the question in writing.

The while loop in FPGA of the compact rio includes wait functions wired to 100us. Minimum time between conversion = 6.25us.

Rest assured the signals are correct, 10 points calibration returned the correct results. These errors are withing the Full scale deflection maximum error specification. These however look very bad on the front panel. I am not in the position to say that we can design very precise analog based control systems. We have been questioned in the past about our wiring techniques, grounding methods after our customers observed these erratic readings.

I wonder how everybody else is doing this? Thanks for your time

Kind Regards

Austin

[Tim_S]

What is not stable? Is it the third or fourth decimal place? Are you doing any averaging? Could you be looking in the bit noise?

[JamesMc86]

One thing to be aware of with the 9205 is that it is isolated. This means everything is referenced to ground on the connector not the chassis. Try tying this to a stable ground and see if it improves

(null)

Also just be aware the DMMs are slow and often don't show noise, not that I doubt you but it alone doesn't rule out noise on the line

(null)

[KWaris]

yes it is the third decimal and I am not doing averaging.

This is strange that the differential wiring configuration for this module looks very similar to single ended configuration.

According to instructions, all -ve ends and the common pins must be grounded to a stable ground. In my case, the electrical enclosure has got three grounds. ( Mains supply 230 V for the pc, 24V for Compact RIO and analogue ground for analogues). Compact RIO is fed from a linear regulated power supply. The analogue ground commons up all the negatives of the analogue signal and common terminals on 9205 card. I dont think the analogue ground and compactRIO ground are different.

But really, I cant produce another ground. 230V supply is single phase power cable with the external ground. So there are three cores coming inside this electrical

[Rolf Kalbermatter]

yes it is the third decimal and I am not doing averaging.

This is strange that the differential wiring configuration for this module looks very similar to single ended configuration.

According to instructions, all -ve ends and the common pins must be grounded to a stable ground. In my case, the electrical enclosure has got three grounds. ( Mains supply 230 V for the pc, 24V for Compact RIO and analogue ground for analogues). Compact RIO is fed from a linear regulated power supply. The analogue ground commons up all the negatives of the analogue signal and common terminals on 9205 card. I dont think the analogue ground and compactRIO ground are different.

But really, I cant produce another ground. 230V supply is single phase power cable with the external ground. So there are three cores coming inside this electrical

Well a circuit always needs to have some reference somehow. For isolated modules like this it means that either your sensor or the analog input needs a reference to some form of GND. This can be the GND of the power supply to power your cRIO. Without ground reference, the entire circuit is floating and static charges eventually can get it to a level that exceeds the common mode range of the input amplifier, resulting in saturation and according misbehaviour of the amplifier. The advantages of isolated amplifiers is that they can measure signals that are referenced to a different potential than the actual measurement system uses (of course the difference of reference needs to be lower than the isolation voltage or you get a breakdown). In case the measurement signal is not referenced in any way you need to provide a reference yourself. This may seem like using an expensive isolation amplifier and making that isolation undone by the common reference, but the isolation amplifier still serves as protection for your measurement device (and the operator sitting at the computer connected to it) from high voltage surges such as what is caused by lightning, provided the ground reference is strong enough to dissipate those surges.

[JamesMc86]

According to instructions, all -ve ends and the common pins must be grounded to a stable ground.

I think you do not need to tie all -ve. I believe what the diagram is showing is using one -ve as your ground reference. Vcm is then a symbolic voltage that -ve2 is not equal to -ve1, otherwise you have RFSE/RSE

[Tim_S]

yes it is the third decimal and I am not doing averaging.

I would expect the third digit after the decimal to change from bit and electrical noise. The bit resolution with the input set up for +/- 5V will be 0.15 mV, so the bit noise is in the 4th place after the decimal (assuming you're not gaining in software by 10 or more). Chances are you're seeing electrical noise.

Multimeters have some time averaging in them that will eliminate what you're seeing through the 9205.

It's certainly worth checking over your wiring, perhaps verifying with a battery, and looking for ground loops (evil things!). You've not mentioned behavior that would lead me to think you have a wiring problem. (A lack of understanding, yes, but wiring problem no.)