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Observer raccoon777
Observer
6,080 Views
Registered: ‎02-12-2016

BASYS 2 Power supply measurements with an USB oscilloscope

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I have an USB osciloscope of 25 MHz of Bandwidth and Record length of 5000 samples, what I want to do is to take measurements of the voltage with a shunt resistor in serie with the power supply of the board BASYS2, if my code inside the ROM is working at 25MHz, 50MHz, 100MHz, it could be possible to take correct measurements with this oscilloscope, or it will be necesary an osciloscope with more bandwidth?, I hope not.
I understand the power supply have fluctuations when the code is running but I want to know if these fluctuations are related with the frecuency at my code is working, I need to justify in case it could be possible, thanks in advance!

pentype-USB-oscilloscope.jpg

 

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Teacher muzaffer
Teacher
11,278 Views
Registered: ‎03-31-2012

Re: BASYS 2 Power supply measurements with an USB oscilloscope

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you have two issues: the behavior of the regulator and the behavior of your sink. Because of the regulator you will have noise at 150 KHz and its multiples but the actual current waveform will change based on the load. The behavior you want to observe is the behavior of the processor, not that of the regulator. If the processor is running at 100 MHz, it's possible you will miss some of the cycle to cycle current sink behavior but as I have mentioned you won't be able to measure that unless you probe at the chip pins anyway.
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Teacher muzaffer
Teacher
6,048 Views
Registered: ‎03-31-2012

Re: BASYS 2 Power supply measurements with an USB oscilloscope

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depending on where the shunt is the power line is being filtered quite significantly anyway. There are a series of R-C components between the regulator and the sink. Your shunt is probably near the regulator. So you are most probably OK with a 25 MHz bandwidth scope (and around 100 MHz sampling rate?)
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Observer raccoon777
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Registered: ‎02-12-2016

Re: BASYS 2 Power supply measurements with an USB oscilloscope

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I'm using the LM2596 regulator,  that is exactly what I want to know if my 25MHz bandwidth scope could get correct measurements at 100 MHz or 50 MHz  or 25MHz BASYS2 working frecuencies, c'ause as we know the Nyquist-Shannon theorem say that you need at least the double of sampling frecuency to take correct samples an avoid the Aliasing problem, but the regulator maybe work at lower frecuency than the BASYS 2.

In this video you can see how I'm taking voltage measurements over the shunt resistor in serie with the regulator , you can notice how is my system (The shunt, the board, and the regulator)  and how is the scope earth referenced as well.

How can I justify the use of this scope with BW= 25MHz, maybe it could be better to use the DCM to decrease the BASYS2 working frecuencies  and asure the correct measurements!, that would be catastrophic for me but please let me know if 25MHz bandwith could be enoung to take correct measurements, really thanks! . .

 

 

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Observer raccoon777
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6,027 Views
Registered: ‎02-12-2016

Re: BASYS 2 Power supply measurements with an USB oscilloscope

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LM2596 SIMPLE SWITCHER® Power Converter 150-kHz 3-A Step-Down Voltage Regulator <== the regulator caracteristic say something about 150KHz, it could be the maximun working frecuency for the regulator, I also found in the datasheet this grafic:

 

LM2596_Regulador_frec_versus_TEmperatura.jpg

that means the regulator can't work under frequencies greater than 150 KHz!, am I right??????????¡! . .

 

 

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Teacher muzaffer
Teacher
11,279 Views
Registered: ‎03-31-2012

Re: BASYS 2 Power supply measurements with an USB oscilloscope

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you have two issues: the behavior of the regulator and the behavior of your sink. Because of the regulator you will have noise at 150 KHz and its multiples but the actual current waveform will change based on the load. The behavior you want to observe is the behavior of the processor, not that of the regulator. If the processor is running at 100 MHz, it's possible you will miss some of the cycle to cycle current sink behavior but as I have mentioned you won't be able to measure that unless you probe at the chip pins anyway.
- Please mark the Answer as "Accept as solution" if information provided is helpful.
Give Kudos to a post which you think is helpful and reply oriented.

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Observer raccoon777
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6,015 Views
Registered: ‎02-12-2016

Re: BASYS 2 Power supply measurements with an USB oscilloscope

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As you can notice at the video the scope probe is on the shunt resistor in serie with the regulator, I need to take measures about the variations in the (Power supply-regulator) voltage, my task has nothing to do with any pin in the BASYS2 apart from the battery conector pins:

 

Battery_conector_pin.jpg

Of course the shunt resistor is in serie with the negative battery conector pin.
In spite of that I accepted your answer as a solution if you notice something wrong in my arguments please let me know! because I need to finish my whole tasks in around 2 weeks, thanks! . .

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Teacher muzaffer
Teacher
6,008 Views
Registered: ‎03-31-2012

Re: BASYS 2 Power supply measurements with an USB oscilloscope

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you are trying to measure the current going into the board, which is, by law ;-), equal to the current coming out of the board so whether the resistor is in series with the positive end of the supply or the negative end of the supply doesn't make much difference (for this purpose; by putting it between the board and the reference point of the supply, you are raising the reference point a little which can be problematic if there are other blocks which share the same reference)
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