Noisy Signal from USB 6211

[mmatoug]

Hello All, 

I'm very new here and need help.

I just got new USB 6211 and used to get signal from lock-in amplifier.

The signal seems to be very strange. To check it, I applied only 5v from the lock-in and the signal is so weird. See the attached file.

it suppose to be 5v output, but as you can see it is noisy at some interval, goes to zero at other intervals, and solid 5v sometimes in a repeatable systematic way!

Can you please guide me through this.

thanks

[cbutcher]

Does the lock-in amplifier take some time to 'lock in', and then after a period of stability, reset so that it can get a new measurement? It's my guess that's what's happening for you.

If you provide some information about the amplifier (model number, or preferably a link to/upload of the manual) then we could look closer.

[mmatoug]

Hello,

the lock-in is the standard one from Stanford research ( SR830 )

however, I believe it is not lock-in issue. Cus i got similar response with 2v battery 

[cbutcher]

Sorry - to confirm: you connected a 2V battery to the 6211 (presumably as an analog input) and the behavior was similar? At what sampling rate - I note the graph you posted a picture of shows THz sampling and the 6211 only supports up to 125kHz. The SR830 only claims to be able to reach around 100kHz and has outputs to Serial and GPIB.

What are you connecting to what, and where did the THz graph come from (or is the scale just completely wrong?) I'm pretty confused...

Edit: I see this in the manual for a possible analog input to 6211:

AUX OUT 1-4 (D/A Outputs) These are auxiliary analog outputs. The range is -10.5V to +10.5V and the resolution is 1 mV. The output impedance is <1Ω and the output cur-rent is limited to 10 mA. These outputs may be programmed from the front panel ([Aux Out])or via the computer interfaces.

X and Y The X and Y lock-in outputs are always available at these connectors. The bandwidth of these outputs is 100 kHz. A full scale input signal will generate ±10V at these outputs. The output impedance is <1Ω and the output current is limited to 10 mA.

[mmatoug]

Thanks for the reply.

please ignore the THz part. Im just using this labview software to view the analog data coming from the lock-in

[cbutcher]

Can you show a screenshot of the 2V battery output? I'm surprised that it's the same, but if so, it makes the amplifier a less likely suspect (as you described).

[mmatoug]

Thanks.

i dont have the 2v picture now, i may update it later.

i was reading other posts and i suspect there might be issue in the connection.

i made the connection between Al0 and AI8. Some seems to suggest that there might be grounding issue. Do i have to connect them with the AI GND?

[cbutcher]

If you're using the Aux Output, Channel 1 or Channel 2 outputs, they should be BNC connectors. If you're splitting the cable to two outputs, then wiring both to the 6211 in a differential configuration, I'd have expected you to be OK...

If you're splitting it but only wiring the core, positive output, then yes - you might have a problem with your ground. If you connect all of the grounds together and then connect that to AI GND, then you can make single-ended measurements of the positive parts of the BNC output, but I don't know for sure that the 830 has shared negative/ground outputs on each terminal, so take care if it's more than one output from amplifier/input to 6211.

[mmatoug]

Yes, i have splitted  the BNC into two cables. One to AI0and the other to AI8.

i think the lock in amlifier is grounded

everything seems correct to me but dont know where is this issue coming from!!

any advise on what else i can do to test the device?

[cbutcher]

I think to get further we need to look at the code you're using to acquire the data in the graph.

If you're sure it isn't a wiring problem, and you're reading a 5V constant output from the BNC Aux Out (1-4) channel on the amplifier (you could check this with a separate standalone DMM perhaps), then either you have some peculiar repeating noise (I note the drops are at intervals of around 60 'ps', but I don't know what the real scale is, so it's hard to draw anything from that) or the code has some strange problem.

Or perhaps the 6211 is broken, but I'd like to continue to doubt that for at least a little while longer.

[DuncanW.]

I also suspect that this issue is related to a ground loop in your system. The output you are experiencing is typical of a system that is improperly wired. While it is possible, I doubt that the 6211 is broken if you just recently purchased it. It is much more likely that these signals are floating at points and causing undefined input.

Take a look at this guide on wiring DAQ systems. Based on quickly looking at the SR830 datasheet, it appears you have the option to select its grounding mode (virtual vs. physical). Make sure that this is configured according to the field wiring guide. For example, if you are using a virtual ground and would like to use a differential configuration in LabVIEW, you will need to add pull-down resistors between your amplifier and the 6211. Otherwise, your system will not work.

Furthermore, it is possible that you have a ground loop between your sensor and your amplifier. This would also cause strange output, but is unrelated to the 6211. My recommendation to you is that you verify that all of your grounds are properly wired. The goal here is to make sure that each of your devices is referencing the same ground, all in parallel. Would you mind explaining your grounding decisions and motivations for them? This is probably where your problem is arising.

Best,

Duncan W.