[SI-LIST] Re: Measuring Power supply noise
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 20 Mar 2006 11:05:36 -0500
> a) By using a 50Ù probe, did I create a voltage divider?
Adding a probe of any kind (except non-contact optical, perhaps) always
affects the circuit. There is always some loading (voltage divider), and
other things that affect what you see.
> To check the sanity of my measurement, Instead of my 50Ù probe, I used a
> standard 1MÙ probe and carried out the measurement.
Check the sanity check experiment by connecting both probes at the same
time.
You could also try experimenting with a passive probe by inserting a small
series resistor between the pad and your coax cable. For example, 450 ohms
gives you a pretty good 10:1 probe with 10 times the input impedance, if
you're careful with the layout. http://www.emcesd.com/1ghzprob.htm and
other places have tips on this.
Use the scope (e.g., bandwidth controls) to see what kind of noise you are
getting.
Be careful about the maximum input voltage at the scope. Some are rated for
less than 2V before damage may occur.
Regards,
Andy
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