03-22-2013 09:57 PM
I have to sample a 1MHz square wave to determine the signal's high voltage. So the signal I need to sample is only 0.5 us long. And I don't need to sample repeat pulses, just one.
The 4070 DMM can sample at 1.8 MHz, which is a lenth of 0.556 us between samples.
Question 1: if I trigger the 4070 off of this .5 us wide pulse, will it be able to accurately determine the voltage? I assume that the DMM uses an integration to determine the voltage, so is that integration time less than .5us?
Question 1a: If it cannot do this, knowing that the second half of the pulse train is 0 volts, can I set the measurement to, say, 1MHz and then multiply that result by a correction factor (in this case, probably 2.0, but I understand that I might have to determine this experimentally).
[background: My customer did not communicate their full needs. Knowing what I know now, I would have ordered a PXI scope. But instead, we have a PXI 4070 DMM. And of course, their budget has shrunk. So it's either getting the 4070 to work, or it's buying a separate, low cost usb scope, which I hear os rather difficult to fit into the PXI chassis.]
03-25-2013 01:27 PM
I strongly recommend not using a 4070 for this application. To specifically answer your questions, yes, the 4070 integrates over an aperture, and yes, you can take a complete sample every 555ns at full rate (1.8MS/s) in digitizer mode, but this isn't fast enough to capture just the peak portion (500ns) of your square wave. Depending on when the DMM triggers, your amplitude readings will vary and always be (indeterminately) less than the actual peak amplitude (unless your square wave duty cycle is greater than 55.5% AND you happen to start acquiring immediately on the rising edge).
Also, note that trigger latency is up to 2us (2x more uncertaintly than your signal's period), and maximum trigger rate is only 6kHz, whereas a good digitizer might have 4ps (500,000 better accuracy) and a maximum trigger rate of 1MHs (167x better rearm time).
In short, our mutual customer's signal is certainly too fast for the 4070 DMM. Tell them to buy a digitizer.
03-25-2013 02:58 PM
Unfortunately the 4070 only has a Bandwidth of 300kHz. This means that the signal will be extremely attenuated before it reaches the ADC. This article explains in more detail the difference between Bandwidth and Sample Rate.
I apologize for the inconvenience.