Effects of AC coupling a couple of USB scopes (picoscope 2202, hantek 6022B)

[tehmeme]

Hi,
First time poster.

I was given a couple of USB scopes that were meant for teaching in the field, but were deemed useless due to their peculiarities and limitations!

Although the scopes perform well within their stated specs (low noise, accurate measurement and good frequency response), the main issue being that they don't have the internal electronics to do AC coupling.

I've played with putting a passive high pass filter on the scope probes (before and after the probe ).
This removes the DC but of course affects the signal at low frequencies.

I'll eventually either give up on them and regard them as toys OR get round to adding more stages, increasing the roll off rate & lowering the cut off frequency, plotting an actual frequency response for it and fiddling with software to compensate.


The questions that arose were:
1- what would this do to the overall input impedance? (as seen by the test point?)
2 - would using active filters be a better idea? (wouldn't this cause other issues related to the amplifiers' own bandwidth?)
3 - more important, how do real oscilloscopes vary the coupling? (particularly AC coupling)

Thanks in advance.

[tehmeme]

I answered my own questions. I was over complicating something very simple. Using the scopes 1M input impedance and a 1uF cap works fine at x1 probe setting. x10 setting is causing some minor issues, but i'll deal with that after I get some shut eye.

Although, I am still interested in how different oscilloscopes vary their input coupling, as the specific circuits seem to be hard to find. (do they just use an input cap? is that just after the input BNC in order that the scopes input impedance isn't affected?)

 

[alm]

You can find service manuals with schematics for many old analog scopes (eg. Tektronix 453B). In general it would indeed be a cap switched with a relay. See this block diagram of the Tektronix 465 input circuit (taken from the service manual). C3 is the DC coupling cap.

[amyk]

I've played with putting a passive high pass filter on the scope probes (before and after the probe ).
This removes the DC but of course affects the signal at low frequencies.

...isn't that the point of AC coupling? It's supposed to affect the signal at low frequencies...

[tehmeme]

I've played with putting a passive high pass filter on the scope probes (before and after the probe ).
This removes the DC but of course affects the signal at low frequencies.

...isn't that the point of AC coupling? It's supposed to affect the signal at low frequencies...

Yes, that is the point. But I obviously didn't communicate my point clearly.
 I meant that the cut-off frequency and roll-off rate were not as I had miscalculated due to me not taking into account the correct circuit configuration ( call it a brain fart).
Hence me later stating :

.... get round to adding more stages, increasing the roll off rate & lowering the cut off frequency, ......

Which is based on an incorrect assumption based on a miscalculation. (again, I'm blaming mental flatulence!) 

You can find service manuals with schematics for many old analog scopes (eg. Tektronix 453B). In general it would indeed be a cap switched with a relay. See this block diagram of the Tektronix 465 input circuit (taken from the service manual). C3 is the DC coupling cap.

Thanks alm, that was helpful and much appreciated.

I'm glad to say after a few hours of sleep it took all of 5 minutes to rig a BNC-cap-BNC AC-coupler this morning. It's working fairly well.
They'll probably be very useful as audio scopes.