add a 10uF or 100uF across the 4-20mA lines. it should reduce the noise if there is any.
@Bhaskar We seem to have a similar issue in regards to the initial post. Can you confirm that there were modification completed and what date. We have purchased ADC last year and are having this issue and need to find a solution.
Do we still need to add these Capacitors to the new Rev board?
What is your order number?
Hi @Bhaskar – coming back to this topic here as I’m having the same issue as described in the original post by Joey.
All my boards were ordered on Jan 27, 2021 and according to your suggestion back then, I ordered 2 channel PR33-8 boards instead of 4 channel version to avoid the noise issue. However, as it appears, the same issue exists also on my 2 channel boards. Were these boards mfgd with some changes applied to remove this issue? Is there anything I can do to solve the problem?
Whenever the sensors collectively draw ~21 mA, I’m getting false readings. Please see attached graph where there is a steady pressure drop read by 2 x 4-20mA sensors and the orange line shows the sum of mA from those 2 sensors (the ADC output from the board).
Whats your input supply voltage?
I’m powering the Spark shield with this Mean Well OWA-60E-12
The 4-20mA boards are then connected to the Spark shield via I2C
How much current does the spark shield output at 5V?
I’m not sure about that, can I measure it with a multimeter for example? If it’s current related, should the issue be visible also when using 1 sensor drawing the same current, i.e a 4-20mA sensor slightly above the 20mA top end of the range?
power consumption is like this
16V/20mA ch1 and 16V/20mA ch2, so we need to ensure the input source can supply this power.
Yes, 20mA is the max.
Ok, but in this case, as I actually have 2 boards connected in chain + additionally 2 x 4-20mA transmitters also in the same chain, wouldn’t the issue be visible when a sensor or combination of sensors on both boards collectively draw this 21 mA current? To be clear, that’s not the case. When either of the boards independently reach this 21 mA region, they have the issue, regardless of the other board or what the transmitters are doing…
i will take a look into it. i can’t think of anything else which can cause it.
the buck-boost is this one
https://www.ti.com/lit/ds/symlink/tps61041.pdf?ts=1679337902089&ref_url=https%253A%252F%252Fwww.google.com%252F#:~:text=The%20TPS61040%20device%20has%20an,inductor%20for%20lower%20power%20applications.
Thanks @Bhaskar – maybe you can test with one of your boards by using 2 sensors and bringing them gradually through the 4-20 mA scale – you should see the reading errors when they collectively draw 20-21 mA. It seems to be exactly the same issue as explained in the original post by Joey. I don’t know if you have since then modified the boards to fix it and confirmed that it has been fixed. In that case, is it possible that for some reason I got older boards?
On the old board, there was only one power supply to power all 4 channels, we now have two power supplies.
can you share a picture of the board you have ?
Hi @Bhaskar – as per suggestion, I took the 2 channel boards instead of 4 channels to avoid the noise issue. But it’s there also for 2 channel boards. Reading Joey’s original post, it seems that the underlying issue is there for both the 2 and 4 channel boards but the 4 channel board had additional issues (which maybe are fixed in new rev, but I don’t have the 4 channel board so I cannot test that). If I understood correctly, Joey got his board to read correctly only thanks to his modifications and those mods would be required on both the 2 as well 4 channel board. But I’m not a circuit designer so I’m not educated enough to say anything here and hope you can help me to find a solution.
In the two-channel units, there are two separate ADS IC and two power supplies.
You can send NCD Login - ncd.io
Hi @Bhaskar – are you suggesting that I should return the products and there is no solution? I’ve built quite a large system around these boards and that’s not an option right now for me, I really hope there is something I can do to fix it, but I just don’t know what. Note, that the same issue exists also with 1 sensor, so it doesn’t seem to be caused with something when 2 sensors are connected. The issue exists on other currents as well, which seem to be related to some frequency. For example, around 6 mA and I think was around 11 mA. If it helps, I can do specific test for this and give you the details? I do not have special electric equipment tho, to measure waveforms or frequencies or exact currents.
Hi,
The 4-20mA IC we are using is INA196 and ADC is ADS1115. No other users reported similar issues.
I will recommend reaching out to TI.
Thanks
