5133

[yeonho]

Hi,

I recently bought a USB-5133 digitizer to record analog voltage signals from a photodidode. The output of the detector has 50 ohm impedence, but I learned the CH0 and CH1 of the input channels have 1M ohm impedence. How can I deal with this mismatch?

Thank you!

[EEsRULEtheWORLD]

Hi, Yeonho.

You can change the input impedance of the USB-5133 by wiring a constant 50 to the Input Impedance property of an niScope Property Node (Vertical:Input Impedance Property), as shown in the image below:

If your channel impedance is not properly matched, you will most likely see incorrect voltage readings.  For example, if your signal source has an impedance of 50 ohms, and your scope is configured for an input impedance of 1 MegaOhm (default), you'll see a signal that is about twice as large as the actual signal.  You can use the niScope Input Impedance Property to properly match your channel impedance.

Let me know if you need any more information.  I hope you're having a great day!

Message Edited by MichiganLostToAppalachianStateHAHA on 07-28-2009 04:06 PM

[Jim_S]

Hello Yeonho,

Just a clarification, the USB-5133 only has an output impedance of 1Mohm. The above method will not change the input impedance of the USB-5133 since it is fixed. Please see the manual for the device on page 2 to see more about the impedance and in the help under NI-5132/5133 Input Impedance.

If you use this code with these cards and it doesn't produce an error, please post of this forum so we can consult this issue.

[RCrisp]

The output impedance of your photodiode (50 Ohms) does not necessarily have to match the input impedance of your digitizer in all cases.  It may be that the 1Mohm input impedance of the 5133 is ok for you.

Matching the 50 ohm output impedance of your photodiode to a 50 Ohm input oscilloscope will be best for signal integrity concerns (high slew rates often present in square waves and very high frequencies).  There will ideally be no signal reflections and you do not have to worry about standing wave effects.  Your output will also be 1/2 of that generated by the photodiode (half the signal is dropped across the output impedance of your photodiode and the other half is measured by the oscilloscope) compared to what you would measured with a 1M input impedance.

If you connect the 50 ohm source to a 1M scope, the measured amplitude will be twice what you would measure with a 50 ohm scope (no voltage divider effect).  Depending on the frequency content of your source and the connection between your source and the oscilloscope input, the measured response may change slightly due to signal reflections.  Try to use a 50-ohm coaxial cable with as short a length as possible to minimize the effects from impedance mismath.  The 50MHz analog bandwidth of the 5133 helps in this case, as it will be less sensitive to the high-slew rate signals that will be the main source of signal reflections.

It would be best if you had another 50 Ohm scope to compare the results of the 5133 with.  If the only difference is a factor of two in the amplitude then you're in the clear.

Ryan