scb 68 signal reading

[gianotti]

Hi,
I am using SCB-68 to measure the potential difference between six electrodes and a reference electrode  in an  electrochemical cell. Input signal are quite low (about 50 - 100 mV).
Each electrode signal source is floating, i.e. it's not ground referenced. I am performing the measurements in the differential input mode. All six electrode are connected to ACH(+) channel, while the only one wire, coming from the reference electrode, is connected to the ground, and therefore I wired each ACH(-) channel to the others. I also added a differential low-pass filter to each channel to reduce noise from the power lines (50 Hz).
My questions are:

- Is it better connect the ACH(-) channels to the others or connect each channel to the ground?
- Why I read  between ACH7 and ACH15 channel,  even if I have not connected anything?
- Actually I am using shielded cables, but I have not connected the shield. Should I connect the shield of each signal to the ground of the SCB-68?

Thanks

mattia

[devchander]

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- Is it better connect the ACH(-) channels to the others or connect each channel to the ground?
- Why I read  between ACH7 and ACH15 channel,  even if I have not connected anything?
- Actually I am using shielded cables, but I have not connected the shield. Should I connect the shield of each signal to the ground of the SCB-68?

---

1) Since your reference is common for all 6 electrodes, its ok if you connect channel to channel

2) What sortof value are you reading at ACH7 and ACH15 channels??

3) Are you experiencing any noise in your measured signals?? If not, this connection is not necssary

[carlo>]

Hi mattia,

I only want to add to devchander post a link that should be useful about Analog signal and noise consideration. It's a bit long but I think it collects a series of good hints that can help you.

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

Have a nice day

carlo>

[gianotti]

First of all, thank you for your post.

I begin from your question 2): between channel 7 and 15 I read the  same value of the highest signal. It's a problem of "shadowing" or "ghosting" due to multiplexing on the board?

Thanks a lot

mattia

[carlo>]

Hi Mattia,

I think you're right even if it sound "strange" that you see the highest value, I mean, I expect to see the value of the previous (last) channel multiplexed, because the capacitor of the sample and hold of the nultiplexer itself, should be still charge with the previous (last) value.

However, the best thing you can do is to shunt the pins of the unused channel in order to discharge the capacitor and see 0 V as requested.

I hope I've been of some help

carlo>     

[gianotti]

Hi carlo,

I have tried to short each channel ACH(-) and the unused channel to the ground of the board.

Sorry, I have another question. I believe that there is crosstalk between different channels, because the trend of a measured signal is sometimes (but not always) too much similar to the others.
As I said before the source of signal is made of two electrodes dipped into a saline solution, wihout any common reference . I think that impedance of this system is too much high, but actually I can't measure it.  In your opinion, what is an upper limit of the input impedance?

Thanks a lot.

mattia

[carlo>]

Hi Mattia,

In your first post you speak of a differential measure but at the same time you say that the only reference electrode is connected to ground: if so, you are not doing a differential measure but a Reference Single Ended (RSE) one instead.

So, what I suggest you to do is to connect the wire coming from the reference electrode to each ACH(-) without connecting it to the ground. While you do that, be sure that on the SCB-68 the switches S1-S5 are configured for a differential measure: S5 and S4 up; S3 down; S1 and S2 left. Moreover, consider that differential measure must be made for each channel between couple ACH(+) - ACH(-) separated by 8 input (0-8; 1-9; ... 7-15).

I linked you a couple of document about signal acquisition that should help you (in the second one in particular, take a look to table 1 under figure 12):

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/analog-fundamentals/grounding-c...

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

Have a nice day.

carlo>