Labview enquiry : Circuit to control relay and maybe temporary short circuit

[uranus999]

Hi there. I don't really write programs using Labview, I really appreciate some advice on how to prepare the circuit as mentioned in the title.

Right now, I have a Labview program to control the relay, one connecting it to a voltage preamplifier and when switching occurs, it will connect to the current preamplifier. However, I notice one problem, when the switching to current preamplifier occurs, the voltage preamplier (not connected to anything), seem to be self-charging as I notice that there is an increasing set of values shown on the voltage graph. Thus, I was suggested to try to short the voltage preamplier when this situation occurs, allowing only measurements of the value to be taken when it is the circuit is connected to the voltage preamplifier. I would really appreciate if anyone can provide me some advice on what to do on the Labview program to obtain the mentioned situation.

Thank you very much in advance.

Regards,

uranus999

[Ángeniero]

Hi uranus999,

which port or device are you using to connect the pc and the relay? maybe is noise what you are reading there?

[uranus999]

Hi Angeniero!

Do you know Nanonis system? I am using the SC4 unit to generate a pulse so that switching can occur. 

Anyway, what do you think? Is it a little strange that the reading gradually increases and not remain at a stable /fix value?

Looking forward to any input to this problem I am having here. Thank you in advance.

Regards,

uranus999

[Ben]

Plese post some circuit schematics along with what you see plotted to get more suggestions.

Ben

[uranus999]

Dear Ben, 

As requested, I have attached two files to this mail, one showing the Labview program/circuit and the other one showing the graph (increase in voltage value).

I appreciate to have suggestions from anyone of you, on how to modify the circuit and even discussion on the behavior of the voltage increase.

Thank you in advance.

Regards,

uranus999

[RavensFan]

You've attached your graph and your LabVIEW program.  But you did not attach a sketch of your circuit.

[uranus999]

Dear Ravens Fan,

I have attached a sketch of my circuit. I hope that it is understandable.

Thank you in advance.

Regards,

uranus999

[Ben]

Reading between the lines...

The output of many of the pre-amps I have woked with will "float toward rail" if the input is open. I high value resistor betwen the input and ground is often enough to stop the drift. If your monitored circuit can't handle the parallel path through the resistor ( 10 Meg?) THen you either have to insert a buffer amp or in the extreme cases then you can short the input to ground to keep the output from floating. I had to do this in an early app where I was characterizing thin film ferroelectrics and was attmepting to measure the hysteris. I need to monitor the vlotage across a cap that was in series with the thin-film so I could infer what the Q on the smale was. THe extra bleeder resistor would dischange the cap and through off my measurements.

I'm not even sure if I am even close but that my 2 cents.

Ben 

[uranus999]

Dear Ben,

Thank you for your suggestion. I agree and was thinking of short the input of the voltage preamplifier to ground when it is not used. However, I am also thinking on how to control it using the Labview program. At the moment, the Labview program which I attached in the previous mail is just used to control / or should I say switching between the current preamplier and the voltage preamplifier. 

I am really sorry for the questions as I am not really knowledgeble on electrical related issues. I really do appreciate any advice that can be provided.

Thank you in advance.

Regards,

uranus999

[NIquist]

I agree with Ben that it sounds like your diff. pre-amps are floating.  Look at this page and see Fig. 11 & 12 for the prescribed solution.

http://zone.ni.com/devzone/cda/tut/p/id/3344