When creating a Simulated DAQmx Device, what device series should I select for the Stanford Research 830 Lock-in amplifier?

[Researcher101]

I am trying to simulate a SR830 Lock-in amplifier input for my VI. I am unsure what series DAQ device to select. I am somewhat new to LabView and this is my first simulated device. Guidance is appreciated.

[Dennis_Knutson]

You don't use a simulated DAQ device at all with that instrument. There is no way to even pass a simulated signal to an instrument. An instrument requires a physical connection to an actual signal. This has nothing to do with LabVIEW.

[Roohcifer]

Hey Researcher101,

As you may have surmised by this point, the simulated devices do not operate with real equipment.  They just generate an arbitrary sinusoidal  signal that you can read in using the DAQmx API.  They typically will not be able to replicate any advanced timing or triggering, and you will not physically be able to integrate them with your lock in amplifier.  They can sometimes be helpful for checking supported DAQmx properties, basic timing capabilities, basic code architecture compatibility with the device behavior, etc. The DAQ device you would choose in reality would be based on the amplitude of the signal your lock-in amp was returning, the sampling rate you needed, the resolution and accuracy you want, the number of channels you require, what kind of synchronization you need, whether or not you mind if you channels are multiplexed or simultaneous, any signal conditioning you need, any other measurements that need to be made, what you want to do with that data in software (i.e., FFTs, time-domain analysis), etc.

If your lock-in amplifier has an ADC on board, samples your signal, and can return the sampled signal data via a serial/GPIB command, then you do not need a DAQ device at all.  If, on the other hand, your amp only provides signal conditioning or you want to sample that data yourself and have the capability to do so (i.e., access to a line containing the conditioned signal), then we may be better equipped to recommend a device.

Regards,

[dmsilev]

Hi David,

I've got a fair amount of familiarity with that instrument, so can give some additional detail on capabilities. Both of your suggested approaches are viable with the SR830, and which is preferred will depend on what Researcher101's application is. The SR830 does digitize the signal (and most of the signal processing is done after that step, using a DSP), and can communicate with a computer via RS232 or GPIB. The instrument can respond to immediate requests for readings, sending back whatever the current value is. Alternatively, if reading quickly or at precisely regular intervals is an issue, it can sample at fixed rate into an internal buffer which then can be read out by the computer. The buffer isn't huge (16K points) and the sampling rate tops out at 512 Hz (and is only available at 512, 256, 128, etc.), so if that's not suitable, analog output is needed (your second suggestion). 

The 830 has four coax outputs for monitoring the signal. Two of them (on the rear panel) are fixed to providing the in-phase and out-of-phase ('X' and 'Y') components of the signal relative to the reference. These are updated at 250 kHz, with about 100 kHz of useful bandwidth (though if the time constant is set to anything except the 10 usec minimum value, that number will drop fast). Output signal magnitude is +/- 10 V, and it's scaled from the input based on the sensitivity setting of the LIA (so, depending on application, one might need a serial or GPIB connection to the instrument to set or read the sensitivity). There are two additional output coaxes on the front panel. They can also provide X and Y (at the same 100 kHz bandwidth) or be bound to various other quantities (e.g. the phase angle theta). When outputing calculated quantities, the bandwidth is much lower (a few hundred Hz); amplitude is still +/- 10 V.  To summarize, DAQ requirements would be +/- 10 V, and sampling rates of at most a couple hundred kHz on two channels and possibly sub-kHz sampling on a second pair of channels. 

[Roohcifer]

Thanks for that info!  

Researcher101 - based on the above information, would you like to communicate via serial/GPIB, or are you looking for a DAQ device?  If you're looking for a DAQ device, what bandwidth would you be interested in?  Would you like any additional signal conditioning like low-pass filtering or AC coupling?  What resolution do you need?  Will you be taking FFTs or doing time-domain analysis?

I ask all of this because there are slight changes and caveats for synchronization, coding, supported sample rates, and other important factors depending on the DAQ device you choose -- specifically whether it's a Multifunction DAQ device or a DSA device.  From what I can gather, the 830 has a 102.4kHz bandwidth, low noise floor, and other characteristics that seem to mirror our DSA devices, like the 446x series, which come in the PCI form factor.  That said, if you're only going to be using the lock-in amplifier for signal conditioning, you could just use an X series device or not use the lock in at all and only use a 446x device, as it has very comperable specs to the lock-in:

http://www.ni.com/pdf/manuals/373770j.pdf

If you have other spec questions, I would recommend calling in at 877-387-0015 and requesting to speak with a sales engineer.  They would be able to help scope out a good card for your application.

Regards,