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Products => Test Equipment => Topic started by: spiff72 on October 15, 2021, 04:41:43 pm

Title: How much current can the AWG output on the Rigol MSO5074?
Post by: spiff72 on October 15, 2021, 04:41:43 pm
Hello all,

I haven't been able to find this info through google searches (or I am not skilled enough with my google-fu to find the right search terminology)...

Simple question - if one wanted to output a sine wave of a certain amplitude, and drive current through a resistive load, what is the upper limit of current that it can source?  I think the output voltage is limited to 5v (not sure offhand if this is 5Vpp or RMS), so I am curious how large of a resistor should be used to keep the current under the max allowable given my selected (not necessarily 5v) amplitude.

Thanks!
Title: Re: How much current can the AWG output on the Rigol MSO5074?
Post by: RoGeorge on October 15, 2021, 05:12:53 pm
The output of a 50 ohms AWG is in fact an ideal voltage source series with a 50 ohm resistor.

That means if your AWG is set for 1Vrms/50ohms it will deliver 1Vrms on a 50 ohms external resistor.  Therefore the AWG internal source with no load is generating 2Vrms.

When you short circuit the output to measure the max I out (don't worry about shorting the output, any AWG is designed to withstand a permanent short circuit) you'll find I = 2Vrms / 50ohms, so you'll measure 40mArms with an external DMM (set the DMM to measure AC current).
Title: Re: How much current can the AWG output on the Rigol MSO5074?
Post by: egonotto on October 15, 2021, 05:19:30 pm
Hello,

the Arbitrary Waveform Generator has max 5 Vpp and 50 Ohm.
If your resistor has R Ohms than the max current  I is

I = 5/(R+50) [App]

If R = 50 than I = 0.05 App

If R = 0 than I = 0.1 App

If your output voltage is 20mVpp <= U Vpp <= 5 Vpp than

I = U/(R+50) [App]

Best regards
egonotto
 
Title: Re: How much current can the AWG output on the Rigol MSO5074?
Post by: spiff72 on October 15, 2021, 06:04:44 pm
Thanks to both of you for the responses!

I noticed that the AWG has a setting to switch between 50 ohm and High Z.  Should this be set to 50 ohms for the above statements to be true?

What I would like to try is to replicate the process shown this video - evaluating a cheap current probe (which hasn't arrived yet) using the bode plot function of the Rigol:
https://www.youtube.com/watch?v=Aw-O-7UpZMQ (https://www.youtube.com/watch?v=Aw-O-7UpZMQ)

I was just trying to understand the setup...

Thanks again!
Title: Re: How much current can the AWG output on the Rigol MSO5074?
Post by: RoGeorge on October 15, 2021, 06:39:49 pm
Quote from: spiff72 on October 15, 2021, 06:04:44 pm
I noticed that the AWG has a setting to switch between 50 ohm and High Z.  Should this be set to 50 ohms for the above statements to be true?

Usually high Z only changes the numbers displayed on the AWG, it does not change the internal resistance of the generator.  For example if you set 1V/50ohms and measure the output voltage with no load, you'll find the output has in fact 2V, not 1 as you set it.  Only if you add a 50 ohms external load you'll see 1V as expected.

That is why the high Z setting.  When you set 1V/High Z you'll measure 1V output with no load (and only 0.5V output if you attach a 50 ohms load).

I didn't watch the video you linked.

The circuit inside a generator has a series 50 ohms at all times, HighZ or not, like explained in this video:

https://www.youtube.com/watch?v=e446nQ9cXdc (https://www.youtube.com/watch?v=e446nQ9cXdc)
Title: Re: How much current can the AWG output on the Rigol MSO5074?
Post by: spiff72 on October 15, 2021, 09:59:50 pm
Thank you!  That video helped immensely (showing me the voltage divider circuit).  It has now clicked for me!