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Grounding Problem / Circuit Design

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Dear Community,

in advance I'd like to thank you, I hope this is the right place to ask this kind of question.

 

I'd like to design a very small pre-preamplifier - a headstage because it's mounted on the head of an behaving insect, and a subsequent analog frontend for signal conditioning adn additional gain. Unfortunately I have major problems regarding GROUNDING.

 

Please see the attached PDF:

1) the headstage should be powered with an galvanically seperated power source e.g. a battery. as I need a dual power supply i would just use two batteries connect them in series and take the middle as 0V which is usually connected to GND right? so in this case I would Connect the 0V of the headstage powersupply to the same connection as REF?

2) the shield of the cable from the headstage to the preamplifier should be connected to ground also the chassis of the preamplifier. In this case I would use the ground from the power net to which i have access at the preamp power supply which is made up with usual voltage regulators/rectifier (i know taht there are parts missing 😉 )

the 0V of the preamp powersupply should be connected to the ground of the filter stages etc. which finally end up in AI GND at my NI A/D card ? right?

3) Now I have to admitt that i haven't made the differental amplifier stage myself and this actually makes me a problem as here is one Opamp used to drive the input of the differential amp and this opamp is than connected to BOTH grounds: the virtual ground at the headstage and to AI GND via the opamp. I have unfortunately no idea what this opamp is doing???

 

So it would be great If someone could tell me if my grounding considerations are right especially with (AI GND) and what the OpAmp at the Differential is doing and if I should use Single-ended ground referenced of NRSE.

 

ManyManyMany THANKS to all of you,

Yours,

Prefrontal

 

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Hello Prefrontal,
to 1) you're right. You have to GND with REF.
Ask the other, I would have to know which NI A / D card you use.

Can you attach the layout created in Multisim also? So I can simulate the circuit.

 

What is stored in my picture with red, you should have to connect. If you give me the Multisim and your exact card, come all other connections and settings.

 

Best regards

NI Application Engineer

 

Marko Bojanic


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Dear Mr. Bojanic,

thank you for kind answer.

In the meanwhile I've got some help elsewhere which deals with the headstage design. I am using a PCI-6040e NI card and I think as now the headstage and the preamplifier are galvanically separated and don't share a common ground anymore which was partly the case in my former design, I shouldn't have any trouble with GND anymore.

The Preamplifier/Filterstage can now be connected SE ended to AI GND, right?

 

I might have a minor question in addition? If I'd like to use the +5V of the card (which is referenced to DGND) to build up a small powersupply which powers the filter stage directly so I can get rid of any additional power supply: What is the maximal current I can draw from the +5V outputs? And as these outputs are referenced to DGND, can I reference the filter stages also to DGND instead of AI GND. It is written in the manual that AI GND and DGND are connected internally, right?

 

Unfortunately I have now Multisim file yet as I didn't calculate the values for the cascaded Sallen-Key Filters yet!

 

Thank you!

Regards,

Prefrontal

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Hello Prefrontal,

 

The Preamplifier/Filterstage can now be connected SE ended to AI GND, right?

Right, you can do like this.

 

What is the maximal current I can draw from the +5V outputs? 

It is 1A. look on this page -> http://www.ni.com/pdf/manuals/370722c.pdf  on page 1.

 

It is written in the manual that AI GND and DGND are connected internally, right?

Right. There should be no problems.

 

Best regards

NI Application Engineer

 

Marko Bojanic



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Accepted by topic author Prefrontal

THANK YOU!

 

Regards,

Prefrontal

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