Protect SINE OUT BNC from SR830 lockin

[Acosta]

Hi,


I am trying to use SR830 lock amplifier at scientific experintal.

I need to apply the signal from SINE OUT BNC output in a thin ceramic sample.

My sample is an plane disk with electrodes in opposite faces.

This sample is isolante material, but sometimes the voltage can damage the sample and cause a short circuit between the electrodes and also a short circuit between pin signal e GND from SINE OUT BNC. I need 5Vrms from Lockin.

My question:
1) How can I protect SR830 not modify or affect my overall result?
2) Can I put an resitor 10Mohm in serie with my sample for to limite electrical current? For exemplo:
[signal from pin BNC]-----{resistor 10M}-----{sample}-----[GND from BNC]

3)This circuit would change my results if the sample remain undamaged?

Thanks a lot,
Acosta.

 

[Baluncore]

Welcome to the forum.

Some more specific information would be useful.
What is the voltage that will damage the sample?
What is the frequency of the signal applied to the sample?
What is the capacitance of your disk sample?

A 10Mohm resistor with the sample capacitance will form a frequency dependent low pass filter.
You could select a lower value resistor appropriate to the sample capacitance and frequency.

 

[Baluncore]

Sine Out amplitude maximum is 5.0 Vrms.
Sine Out has an output impedance of 50 ohm.

This limits output current peak to 142 mA.
A short circuit cannot damage the Sine Out terminal, because it has an internal 50 ohm limiting resistance.

High voltage applied accidentally to the Sine Out BNC terminal could damage the Sine Out amplifier.
If high voltages might be applied, then it should be protected by a voltage clamp such as a zener diode in a rectifier bridge. The best protection circuit in your case will depend on details not yet provided.

 

[Acosta]

Hi Baluncore,
Thanks for your reply.

I´m sorry my delay.

1) What is the voltage that will damage the sample?
R: I do not know exactly, because the samples are new. Some can endure 5Vrms, but others do not.

2) What is the frequency of the signal applied to the sample?
R: I will use 5kHz - 60kHz. But may be i need 100kHz too.

3) What is the capacitance of your disk sample?
R: I do not know. This information is unknown to me, because I don´t have equipment to measure the capacitance.

4) A 10Mohm resistor with the sample capacitance will form a frequency dependent low pass filter. You could select a lower value resistor appropriate to the sample capacitance and frequency.
R: Thanks for reminding me. No I was not considering it. I will use now.

5) This limits output current peak to 142 mA. A short circuit cannot damage the Sine Out terminal, because it has an internal 50 ohm limiting resistance.
R: I did several tests using oscilloscope and changed the resistance to 10k.


Thank you very much by your help.
Helped very much.
Acosta.

 

[f95toli]

I would avoid using the REF OUT output from the SR830 if possible. Ideally you should use a separate function generator to bias the sample and then connect its SYNC OUT to the REF IN on the SR830.
While you can use the SR830 as you describe, it is not really meant to be used that way and I would be careful if I were you, not so much because you will damage it but because I would not quite trust it it. I would at the very least hook an oscilloscope to monitor the signal to make sure the S830 is actually doing what it claims

(for the record, I like the SR830 and have used that model for many years; all of the above applies to all lock-ins, not just ones from Stanford Research)

 

[Acosta]

Hi f95toli,
Thanks for your reply.

Good idea.
I will try to do this in my experiments and I can compare the results with my mount without 10k.

Thanks,
Acosta.