Lock-In amplifier capabilities

Question

Hello,
I am using the Lock-In amplifier start-up kit. I generate an output waveform and get 2 inputs back - reference and measured signal.
I connected the output with a coax wire to the 2 input channels (using BNC-2120) in order to test the program and the results aren't good. When I increase the frequency of the waveform the results even get worse (up to 1000 Hz needed).
I am using PCI MIO-16E-1 (PCI 6070), max reading rate 1.25MSa/s, and I understand that the start-up kit was written to much lower rates. I have another DAQ card PCI-6143 which has A/D converter for each channel (I think) but do not have analog output. I thought about using this one as input only and the first one as output, but I don't know how to do it (need RTSI cable?)
I have 3 questions:
1. What is the best configuration for my purpose, 1 card or 2 different ones? (also available to me PCI-6154)
2. How do I synchronize 2 different cards?
3. How can I find the optimum input and output rates? What are the limitations (min/max requierments) of the Lock-In amplifier? Is it possible to see the censored parts?
I am using LabView8.0
thank you,
Noam.

Answer 1

Hi Noam,
Without the PCI-6154, the best way to proceed will be to use a RTSI to synchronize the simultaneous sampling card with a card providing the analog output. The simultaneous sampling is essential for the lock-in amplifier. Additional reasons why this would be the best solution can be found in this article. There are examples in the Example Finder which show how to share the sample clock and a start trigger between the two cards. Instead of looking under Hardware Input and Output » DAQmx » Synchronization » Multi-Function, check under Hardware Input and Output » DAQmx » Synchronization » Multi-Device. There is also good information about synchronization and related topics such as signal routing across RTSI in this article.
The rates of both the analog output and input will be the same since you are sharing the sample clock. One thing to check is to make sure that the rate is at least twice as high as the highest frequency that you will be testing. It is not a bad idea of start off with a high rate to ensure that the sine wave that you are generating is smooth.
The error that you are seeing is a common overflow error. While you do not have to increase your buffer size to solve this error, you may have to read faster. The buffer size is different than the number of samples you acquire each time you call the DAQmx Read function. The number of samples to read each time can be changed by wiring a different value to the DAQmx Read function. If both of these values are set, then the way to make sure the buffer does not overflow is to call the DAQmx Read function more often. This will require making sure that there is nothing in the loop that is slowing it down. This may require moving all processing functions to a separate loop. There is more information on this error in this article.
Regards,
Kent
Applications Engineer
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