ADS114S08B: Capacitive coupling due to 240volts supply causing absolutely wrong measurement.

Hi Sarosh,

Are there any circumstances under which the system works properly when the shield is connected and is powered by 240VAC?

If you ground the shield, does that improve the performance? It seems like now you are connecting the shield to T3 (is that 1T3 then?), which is to one of the ADC analog inputs.

I assume the 24VAC has the same line-cycle frequency as the 240VAC (either 50 or 60Hz) - is that correct?

How is the ADC powered from the 240VAC supply? Through an isolated transformer + step-down converter?

Let me know and we will work to support this issue

-Bryan

Hi Bryan,
Thanks for your response. Between the line and shield, we have placed test capacitance. If the shield is either floating (not connected to anything) or if the shield is connected to T3 (in my case yes 1T3) this problem will occur but if I connect the shield to the ground no coupling problem or effect on the measurement takes place. With 24V AC I haven't tested yet. But with 24V DC this problem doesn't appear.

Supply to ADS114S08B is always fixed  AVDD +5V and AVSS GND, regardless of how our device is powered.

And yes 240 V is stepped down through an isolated transformer. Even complete power circuitry is placed on a different PCB. Measurement PCB is powered by connecters from supply PCB. 

the coupling effect happens outside the device when the shielded sensor cable is placed near the power line. And that is what we tried to simulate by connecting capacitance between line and shield. If we don't have capacitance between line and sensor shield we also don't see the problem.

Regarding the 24V AC question, I will get back to you in few hours.

PS: I am also sending you in private message the schematic of circuitry around ADS114S08B.

BR
Sarosh

Hi Bryan,

I’m a colleague of Sarosh. We work on same project.

I want to add some comments to your questions.

The system works correct with mains power supply of 230 VAC:

• when there is no test capacity between L1 and the shielded cable (see “Ctest” in drawing “shield problem.pdf” from Sarosh) and
• when the shield is floating or when the shield is connected to terminal“1T3” (1T3 = 2T3 = 3T3)

In the attached excel file “measuring values” you can see the measured temperature values.

When we ground the shielded cable, we get wrong measuring values. Changing the value of  the capacitance has no influence. The value is always wrong and fixed (see attached excel file)

Our line-cycle frequency is always 50 Hz (60 Hz).

The ADC is powered through an isolated transformer (switching power supply). AVDD pin of the ADC is powered with +5 VDC (LDO). DVDD pin is powered with +3,3 VDC (DC/DC converter).

Summary:

We have problems with AC power supplies (50/60 Hz). If we use adjustable, isolated AC power supplies the deviation is smaller. DC power supplies causes no problems.

• the higher the AC voltage, the greater the measurement deviation
• the longer the shielded cable, the greater the measurement deviation

If you have any further questions, please feel free to ask.

Best regards

Mike

measuring values.xlsx

Hi Sarosh, Hi Mike,

Let me dig into this a bit and get back to you.

-Bryan

Hi Sarosh, Hi Mike,

I was looking this information over with some of my colleagues and we came up with a list of questions. Feel free to post any answers you want here or via the direct message system, depending on how proprietary the information is:

1. Can you send a picture of the system? We were having difficulty understanding how everything is being connected. It would be ideal to see how the shield is connected to the terminal block, how the capacitor is connected to the shield, and how the power supply is connected to the cap. The more detail the better
2. Similarly, can you send us a picture of the cable you are using? The image in Sarosh's original post showed some twisted-pair wiring, but this does not appear to be standard for bifilar cable
3. Are you using a fixed resistor for these measurements or actual RTDs? It would make sense to use a fixed resistor for now just to eliminate any possible discrepancies due to temperature changes
4. Can you put an oscilloscope probe on each input so we can potentially see how the AC signal is coupling into the ADC?
5. Can you provide the measured parameter in voltage instead of degrees C? This way we can better understand the magnitude of the voltage error being seen (we could not estimate this without knowing the PGA setting and IDAC value, so it would be easier to just get the data directly from you). Then we can back calculate how much error (in volts) is being introduced into the system
6. In the provided test data, the error seems to be occurring even on the wiring that does not have a shield. How are the tests being performed in this case? What does the cap connect to if the wire is not shielded? And are the tests performed simultaneously, or do you have to stop and start for each set of terminal blocks? As per the other requests, the more detail the better

-Bryan

Hi Bryan,
I have prepared 2 documents.
1-word document that contains precise information regarding our setup and scenarios of a problem with pictures/ more understandable short table and Oscilloscope images.
2nd document answers your point 5

As the images are sensitive so unfortunately, I have to send them as a private message. kindly give me short feedback when you have received them.

BR
Sarosh

Thanks Sarosh, I will review and get back to you as soon as possible.

-Bryan