How to trigger Scope from Maths channel

[doggy]

This seems a simple question but I cannot figure it out. AD3 with Waveforms.

I have a differential signal with a fair bit of common mode noise. The signal of interest is only every 5secs and lasts about 50msecs. I am using C1 and C2 with a simple Maths channel to display it.

Triggering off C1 or C2 does not work because of the noise. I cannot see how to trigger off the Maths channel, which is what I really want to do. I'me sure there must be a simple way and I'd appreciate if someone more experienced could please point me in the right direction.

 

Thanks,
David

Sydney, Australia

 

Edited February 23 by doggy
added tags

[attila]

Hi @doggy

How does your signal look like ? could you attach a screenshot ?
Have you tried triggering options like: type, hysteresis... or filter channels ?

[doggy]

I can't show you a screenshot because I cannot trigger it!

It is a RS485 signal so balanced differential. There is a common mode 2.5VDC on each of the A & B lines.

It is ASCII data at 115200baud and just a short burst of about 48 characters but only once every 5seconds.

Peak to peal voltage is +-2.5Volts

I can see and trigger it on an analogue scope but it is non storage so not possible to see well (which is why I want to use the Discovery 3) 

I cannot see any way to trigger but triggering via the maths channel (Ch1 minus ch2) would be easy (if it existed) and how I triggered my analogue scope, just set in differential mode.

I had a look at the logic analyser and it does not seem to handle a differential signal.

[attila]

Hi @doggy

Math channels are implemented in the application, so they are not available as trigger source.
The Analog Discovery scope inputs are differential, so you could you use these like: C1 = '1+' - '1-'

[doggy]

I am already using the BNC module with the scope inputs in differential mode- i.e. maths channel = C1-C2.

I need to trigger from the differential signal. I understand you saying you cannot trigger from the Maths channel.

Triggering off a single input is no good because it can/will then trigger off Common Mode noise.

17 hours ago, attila said:

so you could you use these like: C1 = '1+' - '1-'

I do not understand what you say here. Can I somehow set a trigger= C1-C2?

This is SO SIMPLE on an analogue scope- even my old NEC 100MHz one from about 30+ years ago. Is there some way to trigger off Channel1 rising simultaneous with Channel2 falling (or the inverse)? I suspect that would achieve what I want.

Edited February 27 by doggy

[attila]

Hi @doggy

The Analog Discovery oscilloscope inputs are differential. This means the read value is the difference between the positive and negative inputs. For channel 1 the positive input is labeled 1+ and the negative 1-, for channel 2: 2+ and 2-
The BNC adapter and connectors are single ended. This means the negative input is tied to ground.
Triggering is available on oscilloscope input channels, internal and external triggers, and for newer device also on hardware filters, AWG output...
Trigger on Math channel is not available since this is performed only in the software/application.
If you want to trigger on differential signal use differential input.

[doggy]

Thanks.

Okay.

I've ditched the BNC module and using 1+ and 1- directly and it all works fine, thank you.

No problem with signal levels for my application, 'though one would have to be careful with higher voltages where the BNC module and probes would give divide by 10.

Regards,
David