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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I got a new DSO recently, an Agilent DSOX3014A. This scope is spec'd for 100mhz, and 2GSa/s (half channel, 4 GSa/s interleaved).
I am working on a primarily digital circuit right now, and I keep getting cross talk on the probes despite the fact it is a 12.5 mhz signal (the TCK on a JTAG interface). Why am I getting this? Without measuring TCK: With TCK connected to probe 4:
I am also pretty sure that the signal on TDSNK is noise from the TDSRC channel.
All the probes are grounded with their respective clips.
EDIT: Actually, I do not think it is crosstalk on the probe end. When one probe is left unconnected and another has a 20mhz signal; all is fine. When I connect it to a grounded pin on the PCB though; it gets noisy. This is odd, as I have both a ground plane and power plane, and the majority of signals take perpendicular paths. EDIT2: Checked my board design, and the 20mhz clk is no where near where I had it connected to. The only thing in between the two is the processor it is connected too...
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
You are using x10 probes right?
Otherwise when you add the capacitive load of a x1 scope probe (cable capacitance plus the scope input impedance) to MHz speed logic signals you are going to get a lot of capacitive current flowing. The capacitive loading of a x1 probe on a logic line with rapid rising and falling edges will cause large current spikes to flow at the transitions.
These large currents may well tax your grounding, or the displacement current originating from one probe may partially return from the scope to the equipment-under-test via the shields of the other three scope probes! Both of these effects can look like crosstalk in the scope, but are really artifacts produced when you connect the probes.
You can minimise the first effect by using x10 or x100 probes to load the fast logic signals as lightly as possible during measurement. And you can reduce the second effect dramatically by putting ferrite clamps (or sleeves) over all of your scope leads. These have no effect on the wanted signals, but choke any common-mode current that tries to run down the shield of the scope probe and would otherwise corrupt what you are trying to measure.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I am using 10x probes, and if memory serves, they are 14pf. So if I am understanding you correctly (that is a lot to take in!), the ground return path is not capable of returning through just the single probes shield, so it takes the paths through the other probes? I do not have any ferrite, but the scope does have a large GND terminal at the front. Will tying this to my board ground, and leaving the probe grounds unconnected, help any? I will try that when I get home. Thank you, that was very helpful!
EDIT: My PSU has an earth ground connection, but I have been leaving that unconnected. Should I tie that to common? EDIT2: see #6 and #7. #7 shows what you mentioned about the ferrite. I am going to take a look at #6 first, since I don't have any ferrite on me. EDIT3: It does seem a ground loop was part of the problem. I connected the PSU common to GND, and disconnected the probe grounds. Here is the result: This actually looks correct, considering the driving voltage is 3v3 and the other configuration showed a peak of 4.2v, with a very dirty wave. However, there is still a signal on the other probes.
Thanks for the explanation. Now Im understanding why I burned up a probe back in the day on my analog scope!
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new DSO signal cross talk
