# Purpose of variable capacitors C2A, C7A?

When opening my DSO201 V2.4, I found two variable capacitors C2A and C7A with 10 and 30 pF respectively. What is the purpose of these elements?

Greets,
Reini

These are used to compensations for better waveforms.

please can you more explain how to calibrate these caps? (reference frequency, voltage …)

ohh,it seems hard to explain the reason,l 'd like to say my own opinion for your ref:

1. why need to “compensate”
I think it is because of the feature of the amplifer used in it ,TL082 .the frequency response of TL082 drop at about frequency of 80K ,that is ,the response of signal higher than 80k ,eg.2M is litter than response of signal little than 80K(general speaking , it is a curve actully,pls ref.datasheet of tl082 ). So, we need to compensate ,to ensure signal of different frequency enter the next step of DSO be the same V/V value.
2. how to compensate.
with RC circuit .if you draw the bode drawing ,you could see, in this RC circuit model in you hand ,signal with high frequency has a bigger v/v value than signal with low frequency, for example ,the 0.1*1 chanle, the v/v value(called transmission gain ) of low frequency signal is 0.5(just a voltage divider with low frquency signal); and that of high frequency signla is about 0.56,so ,in this way ,compensate the TL082 response.
and,of course ,the compensation may need little change according feature of TL082,that is why we need a vareable capacitor .
1. how to adjust the C2a and C7a
C2A is used to adjust 0.1*1 chanel. that is ,adjust you DSO V/DIV to 0.1v with a X1 proble, then ,with a signal generator ,produce a signal of 300K,p-p 0.5V sinewave, adjust the C2A to ensure the wave in your dso is same as the generator output.
adjust C7A in the same way,just change DSO v/div to 1v .and change to a 300K p-p 3v sinewave. when the wave in your dso is same as generator ,work finished!

Maybe you could describe the location of the adjustable caps on the unit. After looking at the dissassembly pictures provided by others, I don’t see them.

Thanks

``take the back cover of DSO apart, you could see two adjustable caps near CN1(proble connector), that is C2A and C7A``

there is also a easy way to calibrate C2A and C7A,just input a square wave signal instead of sine wave , for C2A,with p-p 0.5 v and any frequency you like,and adjust V/div to 0.1V/div, then ,ajust the C2A to make sure the wave in your DSO is the same as the signal input ;for C7a , ,with p-p 3 v and any frequency ,and adjust V/div to 1V/div,ajust the C7A to make sure the wave in your DSO is the same as the signal input .
in this way,you do not need to sinewave which may diffcult to get , just use square wave signal instead.

Thank you for your good description.
I’ll try this next time I reopen my Nano.
And I’ll shoot a photo of the Caps location.

Thanks for sharing your information with us.

The square wave is the correct input for circuit frequency compensation. This is because perfect square edges of a square wave requires an infinite number of odd harmonics. You adjust for no overshoot and no undershoot of the leading edge. Adjust as close to a square leading edge as possible.

Attached is a screen capture of the pictures found at:

seeedstudio.com/blog/2009/12 … -dso-nano/

Can you remember if the caps are under the number “6” in the picture?

Thanks

It would be good if you and others in this post would specify which circuit board you have. Mine is DSO201V1.3A and it has no adjustable capacitors. Instead it has fixed capacitors as shown in the linked pictures. Perhaps mine is newer or older and they only used fixed factory value caps. Perhaps ESP can explain who does and who doesn’t have trimmers.

These pictures have high resolution and can be made large enough to read the surface mount component labeling. Imgxxx41.jpg shows the caps.

sendspace.com/pro/dl/yn9gaw

I’m having trouble adjusting C2A - I input 2 KHz 0.5V square wave, set nano to 0.1V/div, but the wave is very low passed, and adjusting C2A has no noticeable difference.

I’m wondering about the possibility of replacing the TL082 opamp with a faster one

At the following link are pictures of the V2 Nano circuit board. You can readily find the adjustable capacitors in these pictures.

http://www.sendspace.com/pro/dl/ro9e8e