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PC USB scope project.
PC USB scope project.
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Old 24th August 2018, 09:04 PM   #11
nigelwright7557 is offline nigelwright7557  United Kingdom
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I designed a new version which uses a simple buffer front end with a 1 meg input resistor.
The bandwidth is now well over 100KHz so that's fixed a few problems.
It does however mean that the input voltage cant go above 3v3 or below -5 volts.
This means using a *10 probe for high voltages.
This seems to work ok down to the 100mV range where 50Hz hum starts to take over a bit.
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Old 24th August 2018, 10:10 PM   #12
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by nigelwright7557 View Post
This seems to work ok down to the 100mV range where 50Hz hum starts to take over a bit.
50Hz hum was due to a couple of reasons.
1/ Mains light hanging over pcb under test.
2/ Poor scope lead letting in hum.
With nothing connected to the pcb the 50Hz is minimal so it must be coming from other sources.
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Old 10th October 2018, 01:10 AM   #13
nigelwright7557 is offline nigelwright7557  United Kingdom
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I decided to add x10 and x100 modes to the scope pcb.
Sadly the layout had so much capacitance that even the x10 didn't work right.
The capacitance getting across the input circuit was adding 50% to the signal amplitude at 100KHz. Below 1KHz it was ok.
I modelled the circuit on paper and I am getting 15pf capacitance between the input to the scope and the first op amp. This lifts the top end of the input spectrum.
The scope works fine in x1 mode as the capacitance is shorted out in that mode.
x100 was a disaster, it output same as x10 mode !!

So revisiting the pcb layout to clearly separate the input from the op amp input stage.
The mode switch is 0.2 inch spacing on the pads to keep capacitance down but I also put a ground track between the pads so capacitance should be pretty much zero.

I will wait until I have another pcb to be made and will get both done at once, that way I only pay $2 for scope pcb's. JLCPCB have a special on at the moment of $2 for first pcb's if the yare less than 100mm by 100mm.
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Old 17th October 2018, 04:54 PM   #14
nigelwright7557 is offline nigelwright7557  United Kingdom
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Received latest pcb.
x1 mode works great.
x10 mode works fine with 0.1pf capacitor in parallel with 10M input resistor.
x20 mode works ok with any compensation on the input so there must still be a little stray capacitance.
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Old 18th October 2018, 06:14 PM   #15
nigelwright7557 is offline nigelwright7557  United Kingdom
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I have been working on my times 10 mode.
The compensation capacitor needs to 0.25pf !
Only caps available in that size are 0402 !
I cant work with anything that small so going to put 2 or larger 3 caps in series to get the right value.
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Old 19th October 2018, 11:34 PM   #16
nigelwright7557 is offline nigelwright7557  United Kingdom
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Found another way around the small capacitor problem.
I put 10pf in parallel with input 1M resistor to ground to op amp
This means the input compensation capacitor can be 1pf which I can get in through hole.
Did some measurements tonight and it works great.
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Old 21st October 2018, 04:01 AM   #17
nigelwright7557 is offline nigelwright7557  United Kingdom
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The x20 input mode just needs a 0.1pf capacitor across the 19 meg input resistor.
It seems to work right without that capacitor which suggest there is still a little stray capacitance on the input.
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Old 27th October 2018, 12:36 AM   #18
nigelwright7557 is offline nigelwright7557  United Kingdom
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Built up a couple of USB scopes pcb's.
The x1, x10 and x20 modes are working well now.
And managed to do it without silly capacitor values or SMD.

I found I have to be careful with any mains wires floating near the pcb as it picks up 50Hz nicely due to the high input impedance especially in x10 and x20 modes.
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Old 23rd December 2018, 11:57 PM   #19
nigelwright7557 is offline nigelwright7557  United Kingdom
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Started on a new dual channel USB PC oscilloscope.
Its based on my old single channel design but has another input channel.
Channel 0 is as before with lots of functions like shifting waveform, frequency and voltage measurements using pointers and a FFT function.
Channel 1 is very basic with just a simple display of the waveform.ScreenHunter 10 | harrabylad | Flickr
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Old 24th December 2018, 08:23 PM   #20
trobbins is offline trobbins  Australia
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x10 and x100 cheap (ie. ebay) scope probes typically come with a variable cap plus a fixed cap across the 1Meg BNC end.

I've sometimes had to replace the fixed cap with a different value to match the input stage capacitance of the soundcard to then allow probe compensation across the frequency range. Using x10 and x100 probes certainly makes bench work a lot easier for amps.
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