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  • Author Author: idanre1
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Arduino oscilloscope - DIY entry device

idanre1

 

The big problem with being a maker newbie, is that you have to have equipment.

When I started, I bought soldering kit and a multimerter - things that don't cost allot and are very needful to start your first steps.

But I believe the most important test equipment one need is oscilloscope, but it also very expensive.

So there is a conflict...

Many times you need to debug a circuits because things can go wrong (we all know that...) and you need a scope to figure out what's the problem.

Consider you still in the noob phase, you don't want to spend hundreds of $$$'s just for buying a simple scope

 

If you could just have a simple cheap one that is good enough for very simple circuits you wouldn't think twice

 

Now, let's define what is a simple circuit.

For me it has two major parameters

1. Low bandwidth

2. Low voltage, let's say 0-5Volt and if you insist 0-15Volt

 

I have searched the internet and found many of them, some are better then others, some are complicated to build than others.

I put my eyes on Girinoscope www.instructables.com/id/Girino-Fast-Arduino-Oscilloscope/

This scope is using aruino uno (I built my with arduino nano) and reaching 150Ksps with 5V voltage swing.

The problems with the tutorial are:

  1. Its explaining allot of the software and a general knowledge on how a scope trigger works, but:
    1. The info not that well explained to a noob that just want to build it.
    2. There isn't even a simple connections diagram.
    3. You need to deep dive the Atmel documentation to really understand what the tutorial explains.
  2. The provided code needs to have patches to work in max BW (Currently 150kbps).
  3. The GUI provided by 3rd party only visualizing the sample but can't save it for future use.
    1. GUI link: https://github.com/Chatanga/Girinoscope

The Software

The instructables describes how to configure arduino's CSR and you get a pretty good feeling what is happening, but only if you have some knowledge in VLSI and logic design (Muxes, Flipflops, PADs, etc...)

I have tweaked the SW a little bit to support higher sampling rates (code improvements to save MIPS)

I also put the GUI in the same github for ease of access, Also with script to run it from arm CPU (RPI, db410c)

https://github.com/idanre1/Girinoscope2_0

  • main path contains the arduino code
  • capture_exe - perl program to acquire samples that can be saved to a disk (not just watching on the screen)
    • excel can be used to manipulate data and show graphs
  • scope
    • contains the GUI
  • db_src
    • source code for test suites in db410c.
    • RPI can be also used with minor changes.

 

The Hardware

All you need is an arduino + some resisters and capacitors.

For simple testing purposes I used 555 timer with output swing from 0-5v, calculated the expected frequency and tried to match expectations with result samples.

 

{gallery} My Gallery Title

Full circuit: Arduino ADC connected to LM358N as input probe. ne555 as the test circuit. ne555 is supplied with 7v rail to rail so the output will be about 5v.

DB410c as tester: I use the 1.8v digital GPIOs on the dragonboard to the arduino ADC. I sometimes using level shifer txs0108e to provide arduino with 5v from DB410c

Girinoscope schematic: From girinoscope intructables it is very hard to tell what to connect where. You need to deep dive the arduino code to understand what to do. This simple schematic can help allot

555 Timer: I use this circuit to see if the calculated frequency of the 555 timer will match what I sample using arduino.

GUI: Acquiring sample from the girinoscope GUI

 

The probe - Try 1 (not that good, please review try 2)

If I want to cross more than 5 volts I actually need to create a probe.

Special thanks to and for helping me navigating the world of opamps (there are thousand of them, and you gotta choose )

I have tried a simple simulation with the MCP6024

Basically I made a 1/4 voltage divider for being able to insert arduino more than 5V.

This is the simulation results:

WAY BETTER THAN LM358N! (See photo gallery for old opamp result)

Vin is square wave 20V@15KHz

You can see output is 4.98V (fair enough for basic scope)

Only thing that worries me is rise and fall time on the max acquired BW (15KHz)

 

Questions:

  1. Maybe I need a higher BW op-amp?
    1. Does adding some passive component to the feedback loop will help?
  2. Do I need to add to Vout of the opamp some reverse protection diode to save arduino from human fault?
  3. Why V_rise is slower than V_fall?

 

The probe - Try 2

  • Red circuit is is Differential amplifier
  • Orange Circuit is voltage offset
    • Since the opamp is 5V we need to offset for half meaning 2.5v
    • By adding 2.5V to to red circuit V+ we get -2.5v:2.5v input only by supplying 5v and GND to the opamp supply.
      • That I understood but could find proper analysis why??? (TODO - find a link to article)
  • Power supply
    • C2,C3 are for noise cancellation of the power supply (power supply decoupling)
  • U2,U3 are only 1:10 multiplier to work on low voltages.
  • C1 - power supply filtering
    • Remove some of the noise coming from the 5V and give a cleaner pseudo ground to work with.
  • R11
    • Isolates any capacitance on the output a little from the op-amp
  • R14
    • Input pit of GND that is connected to GND of the circut
    • Resistor is there to prevent high current from flowing when grounds are uneaven.

 

Probe schematic

Simulation result

I put square wave of 20v out of 25v.

You can see the output is from 0.5v-4.5v.

T_fall is about 4u_sec which is pretty reasonable. less than 1/7 of the square wave low voltage time...

 

Pad schematic from atmel datasheet:

 

 

I think the next step is to order this opamp

This would probably not happen during this competition period (shipping takes a while where I live)

 

Summary

Hope every noob will find this article:

  1. with full details how to built it
  2. Step by step explainable
  3. Have enough analysis details for understanding without inner assumptions not said.

 

I want to have a special thanks to for helping me allot trying to build the probe.

 

Proof of work

The following video shows db410c (similar to RPI) generating square wave using one of its GPIOs in 1.8v

Then a level shifter which is design to between-chips communication is used to amplify the signal to 5v www.ti.com/lit/ds/symlink/txs0108e.pdf

The signal is captured by girinoscope and The specified item was not found. for a reference.

I demonstrates trigger modes and sampling of the square wave by both scopes using girinoscope GUI.

 

The following video shows the probe detailed in this thread with 1:10 attenuation

Currently the opamp is the only one I have got LM358n which is manufactured around 30 years old, I have extracted it from an old circuit I have.

The opamp output is very noizy, I hope a newer opamp will do a better job (I put this video for proof of work of the probe)

Again caputes of both scopes are provided for a reference.

 

 

 

Regards

Idan

Anonymous
  • in reply to idanre1

    I'm still not doing very well.

     

    I was trying to say you have two basic approaches.

     

    One is to say what the things you want to measure are like, which will give you a spec for what you want the input resistance to be and you can then try and meet that with your design.

     

    As an example, the output resistance of a logic gate might be 20 Ohms and the output of an op-amp might be 100 Ohms, so if you want a measuring resistance of 250 times that (you're working 8-bit?), then you need 25k at the input, to avoid error.

     

    The alternative is to look at what all the other people  who have designed oscilloscopes have done, as they've already trodden the same path as you and what they've arrived at is probably (after half a century, or more) pretty much the optimum way to do it.

     

    Unless you were using active probes of some sort, which is unlikely, the probes on those scopes would have been passive and either x10 only or a switchable x1/x10. The x10 probe gives you a dc resistance at the tip of 10 MOhm and the 1x probe a dc resistance of 1 MOhm.

     

    Note that the probe tip has some capacitance to, so the impedance falls with frequency. You only get that 10Mohm at dc and low frequencies. The x10 probe has much less capacitance than the x1, which is why you mostly use it for general work in preference to the x1 because its high frequency performance is better.

     

    Passive scope probes aren't perfect and will load the circuit being measured (as you noted), but most people accept the limitations and they're fine to work with as long as you understand the limitations.

     

    So, to get back to your design. One approach would be to buy a x10 probe and feed that to the amplifier input with a 1MOhm to ground [include a 10pF to ground, too, to bring the probe compensation into range]. That gets you a 10:1 reduction and a 10MOhm input resistance. But it's a bit pricey for something that's meant to be cheap and cheerful.

     

    An alternative would be to use a differential amplifier like this. Try simulating it and looking at how it works - it's worth doing, even if you choose not to use it here, because it's such a useful and flexible building block and you'll almost certainly use it elsewhere when designing. That gives you a 1M input resistance, 10:1 attenuation, operation with your 5V op-amp, and it has a differential input, so the 'ground' lead doesn't have to connect to ground in the circuit being measured (you would want a third 'ground' lead, in addition to the 'plus' and 'minus' probe leads, as a reference). It also allows for an offset, so you can have negative as well as positive voltages at the input. It would be marginal for precision analogue work (actually, a bit useless, but then would you use an Arduino scope for something demanding like that, anyway?), but be fine for general stuff.

     

     

    Since you have 4 op-amps in the package, I've thrown in two other ranges - you just select whichever range you want with the mux ahead of the ADC. You might also want to read the generated ground level to know where it is, though in practice it should give a pretty accurate halfway figure and you probably won't need to adjust the readings anyhow. (Given that the Arduino is operating ratiometrically, the whole thing is so iffy that I don't think you want to fuss over detail like that). Anyway, that's just a suggested starting point and you can use it, adapt it, or throw it away (though I would be curious to see what it does in practice).

     

    Next up, I suppose, will be the triggering, or do you do that in software?

  • in reply to jc2048

    Hi Jon

    Not complaining on the crypt. I do like thinking. And you are doing great job mentoring me. Thanks allot BTW!

     

    In the degree I had (found the lab course pdf)

    HP34401a. Agilent e3631a. Agilent 33120a. And agilent 54621a.

     

    Why I need big Rin, it's kind of a general reason in measure equipment I remember we learned on:

    The voltmeter is connected parallel to a component of the circuit. This means the resistance (or impedance) of that part of the circuit is going to change, which in general changes the potential drop across the component. This effect will be larger if the voltmeter has lower resistance (to the point of completely ignoring the component if the voltmeter resistance goes to zero). Thus, in order to get an accurate measure of what the potential would be without the voltmeter, it needs to have a high resistance

     

    Reference https://www.physicsforums.com/threads/whats-the-point-of-a-voltmeter-having-a-high-internal-resistance.766003/

     

    About voltage swing:

    I can settle on 0-15v for quicker and cheaper design (no neg values for now).

    The scope should be entry level in complexity and price.

     

    The sampling sw is found in my github. Currently there is no filtering on sw, just acquisition.

    This too can be added but I am not an expert on filters.I will mark in my wish list to find something cooked in the internet which is reasonably good. Good point BTW.

     

    Idan

  • in reply to jc2048

    Just remembered - there's also the offset to consider.

     

    You want -20 to 20 volts at the input to map to 0V to 5V at the A/D. You can't do that with just a follower.

  • in reply to idanre1

    Sorry if it was a little too cryptic (which I guess it was) - I was trying to get you to think about it, rather than just feeding you answers on a plate. It might help if we could interest some of the others - I'm not very good at explaining things like this.

     

    If you are going to have a signal range at the input of -20V to 20 V with a follower, then you need the op-amp supplies to be at least that too. You are going to struggle to find anything that will do that. Instrumentation op-amps usually work up to about 30V or so, which means -15V to +15 if you have it dual rail.

     

    So let's take a step back. Why do you need an input resistance of a teraohm (or whatever the resistance equivalent of the input leakage might be - it isn't really a resistance, because it probably doesn't obey Ohm's law, but let's pretend it is)? What kind of things are you going to be probing with your oscilloscope that need that?

     

    Another tack would be to ask you whether you used an oscilloscope at university and, if so, what were the properties of the probe on it like?

     

    If the Arduino can only sample at 150kHz or so (ie every 7uS), then your software waveform reconstruction filter should remove any evidence of those slews (if it's doing its job properly).

  • in reply to jc2048

    Hi Jon

    Took me a while to get your point 

    I know I can put the voltage divider before the opamp but that will miss the point of having big Rin

     

    Maybe I need a different opamp

    Thanks for making that point

     

    Regarding the sine wave... The scope limit is about 15kHz because of the MIPS budget of Arduino.

    I do want to be able to sample that square wave signal. I guess this opamp slew rate  is not good enough.

     

    The other ones in the photo has 9.5V/uS instead of 7.

    But then again I need to have resistors before the opamp.

     

    I  will try to digg some more for a different opamp

     

    Regards

    Idan