With your follower circuit simulation, try it with a sinewave going in.

Then you can think about whether the circuit configuration you've chosen can meet your requirements and maybe consider other possible circuits.

(As a general note, you can't depend on the model being accurate once you move away from the normal operating conditions like operating within the supplies.)

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

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

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?

Here's a revised circuit. I tried to replace the existing one, but just end up with the red ribbon telling me I'm probably logged out (even though I'm obviously not).

I've increased the input resistors to 1 MOhm (I forgot to modify the previous one before posting it).

Looking at what it does if you throw a very fast edge at it, the differential amplifier rings a bit. One way to deal with that is the capacitor I've added (C2). You might need to experiment a bit if it is a problem - that value works in the simulation, but won't be right for a real circuit (a real circuit might be fine without the cap at all; might need a larger value; you might need to adapt the internal compensation in some other way; or you might need to stop the fast edge reaching the op-amp at all).

For a traditional oscilloscope, the rise time is tr =  0.35/f. For your 18kHz bandwidth, that gives a risetime of something like 19uS. So your slew time of a couple of microseconds shouldn't be an issue.

Finally, I ought to say that this is just a circuit suggestion to experiment with. I haven't tried it and there's no warranty. (That's for the benefit of anyone who has just stumbled upon this blog and thinks they're looking at an expert piece of analogue design.)

Hi Jon

I was searching the internet how to do a attenuation with opamp and didn't find, Only after you provided the differential amp structure I was able to get to the point.

Thanks for pointing that out...

Since this is a noob scope I want to make a in-depth analysis on the circut. So a novice which basic knowledge will know what he/she building.

Please correct my analysis if it's wrong.

• Red circuit is is Differential amplifier
  • Vout=(R3/R1)*(V+-V-)
  • How to reach that formula: http://www.electronics-tutorials.ws/opamp/opamp_5.html
  • C2 is for remove ringing. Real circuit have capacity in the wires, might need to be adjusted.
• 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
• U2,U3 are only 1:10 multiplier to work on low voltages.
  • http://www.electronics-tutorials.ws/opamp/opamp_3.html
• Why you need C1?
• Why do you need R11?
  • Have you calculated impedance matching to the PAD of arduino (Schematic is provided below)
• Why do you need R14 to GND?

Probe schematic

Pad schematic from atmel datasheet:

More questions (Hope you have the time for all of my questions)

Where did you get that from? can you provide a link with in-depth or something?

For a traditional oscilloscope, the rise time is tr =  0.35/f.

Trigger

If you will take a look on the original circuit you will see AIN0/AIN1.

I am using arduino ISR of a comparator to catch IRQ when Vin is answers to one of the triggers:

Toggle, Rising edge, Falling edge

Basically arduino always buffer 1K readings in internal memory and upon trigger capturing 0.25/.075 per-post trigger and sends it to PC upon IRQ.

Indeed I am using 8bit for reaching 15KHz using arduino MIPS budget

In-depth on how its done you can find in the instructables link I have provided at the top of the blog

That's it for now.

Regards

Idan

This is what I was trying to guide you towards originally (I was hoping your university course might have introduced you to some simple op-amp circuits).

I didn't design using the formula, but it's fine. The output is simply the difference between the input voltages multiplied by the ratio of the resistors. For the formula to be true, you probably need to add that R1=R2 and R3=R4.

If you want to understand what it's doing, put voltmeters on both input pins in the simulation (the inverting and the non-inverting inputs) and see where they sit. [I tend to think of op-amp circuits in terms of levers and pivots - possibly that's how it was originally taught to me - and follow a sort of visual approach to mental design rather than working through formulae.]

The orange circuit is just a pseudo ground sitting at half the supply. If you had two voltage rails, plus and minus relative to ground, you'd just use ground as your reference, but here we need to produce a ground of our own. That's quite common in single rail circuits. By fooling the differential amplifier into thinking that's where ground is, we get the swing both ways around 2.5V at the output, giving 0-5V out for -25V to +25V in.

C2 and C3 are power supply decoupling. They make no difference to the simulation, but you'd want them in there for a real circuit (make the 10u an electrolytic and get it the right way round, the 100n is best as a ceramic and put it as close to the op-amp supply pins as you can get it). You might need to review this if your power comes from something like an Arduino, as it could be quite noisy [remember that this is a starting point for a prototype, not a finished, polished circuit].

C1? That gives a bit of filtering - it will remove some of the noise coming from the 5V and give a cleaner pseudo ground to work with. The value I chose was a bit arbitrary (100n might have been enough) - I didn't sit down and calculate anything. If you are going to use 1k resistors, you may want to increase the capacitance a little (it's an RC filter, so as you decrease the resistors, you need to increase the capacitance to keep the same degree of filtering).

R11? I just threw that in. It would probably be fine without it. It just isolates any capacitance on the output a little from the op-amp. If someone were to use the circuit with something other than Arduino, it might help a bit. It would also help if you wanted a length of screened cable between the output and the Arduino.

R14? Connecting the grounds together gives a reference point to work with. The resistor is in case of accidents - ie if your Arduino wasn't floating and its ground was tied to mains ground, and your circuit had mains ground too, and you then clipped the ground to something in the circuit under test that wasn't ground, you might have a lot of current flowing in that connection, whereas with the resistor it's limited. Value is a bit high and you could reduce it a bit. Again, this is just ideas for your prototype, not a fully worked circuit. The actual measurement is between the plus input and the minus input, the ground just pulls both sides together if one or the other has a floating supply.

Rise time came from

Handbook of Linear Integrated Electronics for Research
by T.D.S. Hamilton

It's based on an assumption that the input of the vertical amplifier looks like an RC circuit, which is reasonable for practical amplifiers. Things are a bit different now, with digital 'scopes, but as far as I can see the manufacturers arrange their waveform reconstruction to give the equivalent result - that's certainly the case with my Tek scope where the risetime is the same as you would have seen with an old analogue scope of the same bandwidth.

Don't worry about the time for the questions - it's all for the greater good, isn't it. [And remember I'm essentially a digital designer, so you'd probably do much better talking to someone who does this kind of thing for a living.]

This is what I was trying to guide you towards originally (I was hoping your university course might have introduced you to some simple op-amp circuits).

I didn't design using the formula, but it's fine. The output is simply the difference between the input voltages multiplied by the ratio of the resistors. For the formula to be true, you probably need to add that R1=R2 and R3=R4.

If you want to understand what it's doing, put voltmeters on both input pins in the simulation (the inverting and the non-inverting inputs) and see where they sit. [I tend to think of op-amp circuits in terms of levers and pivots - possibly that's how it was originally taught to me - and follow a sort of visual approach to mental design rather than working through formulae.]

The orange circuit is just a pseudo ground sitting at half the supply. If you had two voltage rails, plus and minus relative to ground, you'd just use ground as your reference, but here we need to produce a ground of our own. That's quite common in single rail circuits. By fooling the differential amplifier into thinking that's where ground is, we get the swing both ways around 2.5V at the output, giving 0-5V out for -25V to +25V in.

C2 and C3 are power supply decoupling. They make no difference to the simulation, but you'd want them in there for a real circuit (make the 10u an electrolytic and get it the right way round, the 100n is best as a ceramic and put it as close to the op-amp supply pins as you can get it). You might need to review this if your power comes from something like an Arduino, as it could be quite noisy [remember that this is a starting point for a prototype, not a finished, polished circuit].

C1? That gives a bit of filtering - it will remove some of the noise coming from the 5V and give a cleaner pseudo ground to work with. The value I chose was a bit arbitrary (100n might have been enough) - I didn't sit down and calculate anything. If you are going to use 1k resistors, you may want to increase the capacitance a little (it's an RC filter, so as you decrease the resistors, you need to increase the capacitance to keep the same degree of filtering).

R11? I just threw that in. It would probably be fine without it. It just isolates any capacitance on the output a little from the op-amp. If someone were to use the circuit with something other than Arduino, it might help a bit. It would also help if you wanted a length of screened cable between the output and the Arduino.

R14? Connecting the grounds together gives a reference point to work with. The resistor is in case of accidents - ie if your Arduino wasn't floating and its ground was tied to mains ground, and your circuit had mains ground too, and you then clipped the ground to something in the circuit under test that wasn't ground, you might have a lot of current flowing in that connection, whereas with the resistor it's limited. Value is a bit high and you could reduce it a bit. Again, this is just ideas for your prototype, not a fully worked circuit. The actual measurement is between the plus input and the minus input, the ground just pulls both sides together if one or the other has a floating supply.

Rise time came from

Handbook of Linear Integrated Electronics for Research
by T.D.S. Hamilton

It's based on an assumption that the input of the vertical amplifier looks like an RC circuit, which is reasonable for practical amplifiers. Things are a bit different now, with digital 'scopes, but as far as I can see the manufacturers arrange their waveform reconstruction to give the equivalent result - that's certainly the case with my Tek scope where the risetime is the same as you would have seen with an old analogue scope of the same bandwidth.

Don't worry about the time for the questions - it's all for the greater good, isn't it. [And remember I'm essentially a digital designer, so you'd probably do much better talking to someone who does this kind of thing for a living.]

For risetime and bandwidth, a fuller treatment can be found in:

An Analog Electronics Companion
by Scott Hamilton

there's a huge amount of good stuff in this book and it's very well worth reading.

For op-amp circuits

IC Op-Amp Cookbook
by Walter G. Jung

is still useful after all these years (my copy is the 1979 reprint).