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ADC input fluctuation issue

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Veketti:
Dear All,

I'm banging my head on wall with issue of ADC fluctuating, I really don't understand why so, so I hope someone could help me. I'm trying to read 4-20mA sensor. As an ADC chip I’m using AD7992 and for that reference voltage REF195. This works well in breadboard when voltage passed by potentiometer.

For the ADC input I added BAT54S for over/undervoltage protection and low pass filter to filter out any fast oscillations. I wonder does this circuitry cause the fluctuation issue?


Thank you in advance.

RoGeorge:
I don't think the diodes would create fluctuations, thought 20mA on 250 ohms is 5V, any voltage or current errors might open a diode.  Too few details to say what might be happening.

- What do you mean by "fluctuations"? 
- Is there a plot with those fluctuations in time, so to see their nature and amplitude? 
- Does the fluctuations appears on an oscilloscope, too, or are they seen only by the ADC?
- Is the ADC in an industrial environment, or in a lab?
- Is it properly shielded and isolated against interference?
- Are the 24V and 5V well stabilized?
- Is it all wired properly?

Veketti:
Good to hear that at least there is nothing inherently wrong with this part of circuitry. I’ve scoped the 4-20mA sensor separately with lab power and 250ohm resistor and it had noise ~9mV peak to peak. I’m supplying the 24V in this PCB with RO-0524S converter. I’ve scoped that also and it was ok. Next I’ll scope the AD7992 ADC input, whether it is stable or not.

From my calculations the fluctuations are around 19mV. What I mean by the fluctuations is that the voltage readings are not stable wandering up and down. I’m testing this in my “lab”, so no industrial environment. Isolation and shielding is bit gray area for me. At least the cable for the sensor is not shielded or twisted pair. Just standard M12 connector readymade commercial sensor cable. I’ve understood that these 4-20mA current sensors are relatively interference free due to the nature of current. And on the pcb I’ve tried to route the adc signal short and far from oscillating components, like crystal.

I’d like to hear some thoughts about the shielding and isolating, what should I take into account?

Anyhow, I’ll try to measure those things you mentioned and get back. Btw. what kind of performance should I expect from the voltage stability? I’ve no idea whether my expectations are too high..

For the software side, I’ve done averaging for 80 samples and then disregard + and – 5 readings from that ADC averaged value and latest now added filter which takes three of these averaged samples and compare that if all of these samples are gradually to same direction, only then take the value into account. Otherwise disregard readings which are up and down. Like said, readings are super stable on breadboard when ADC supplied through potentiometer. But on PCB and real sensor all over the place.. Wirings are properly seated.

Thank you and sorry for my ignorance..

Zero999:
The only concern I have with the circuit is if the sensor does fail short circuit, a large current will flow via D2, into the 5V rail. If you're lucky, D2 will simply fail open circuit, but what's more likely to happen is the 5V rail's voltage will increase, destroying the ADC and microcontroller.

Put the protection diodes after R1, which will limit the current into the 5V rail, if anything bad happens, ancrease the value of R1 to something like 100k, in order to restrict the fault current to a sane level and reduce C1 to 100pF, to compensate. Connecting a 5.6V zener across the 5V rail might also be a good idea.

Veketti:
I was wrong with the fluctuation amplitude. It's 137mV and frequency between 4.2-5.1kHz. This was measured from the R1 ADC pin side. So confusing, what might cause this? Attached is the picture of the scope shot.

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