DSO2250 AC-DC coupling

Not after voiding the warranty, I shouldn't think. With the box being previously opened to temporarily remove sppressors (no effect), I'm left with the alternative. Putting together a rough schematic to see if some simple adjustment to part type, supply biasing or other might correct things sufficiently to serve.

There appear to be jfets used as buffers/limiters, feeding a DPDT video switch - so not as conventional as hoped. Both could be misapplied to introduce nonlinearity. Patience and research needed here.

RL

If this happens just by scooting the offset then it looks like a screwed-up design :-(

Since you wrote that the warranty is possibly toast I guess you can only go in there with another scope to find out where they messed up and correct that, or live with it.

It's not always hopeless. I once found and fixed a design bug in a shortwave transceiver that was know to "always have this oscillation on one band". It ended up needing a rather minor change after which it became a very nice piece of equipment.

Anyone with info on 8bit+ high speed ADC, in 24pin SO outline, partial part number ~VM134B?

RL

Chances are, it could be a "disguise part number", one assigned only to this company. When you hold a regular other scope to the input, is the waveform always ok there and not distorting when moving the offset?

No evidence of distorted rise settling time on (assumed) input pin, but visible output code changes within the longer rise-time interval.

I'm looking for supply and reference information that might explain the problem, but this is occuring with a 1.7Vpk 700mVppk signal, which shouldn't stress something running on a 3V supply.

Looks like a long search for pin-compatible replacement. Hoping for something simple like ADC08200....

RL

still loosing posts through primus, so don't be suprised at a resulting non-sequential post order.

Dang. That would have been too easy but Murphy wasn't on our side.

^^^^^^^^^^^^^^^^^

This I don't quite understand.

A 700mVpp signal should not, a 1.7Vpp signal might.

But are you sure it's defective? A slope can't really be explained by a defective ADC. A reference sag or something, that would explain it. I'd poke all around that ADC with a 2nd scope, trying to find a pin that moves inexplicably. There has got to be a reference, for example. Even if that's internal it will have some bypass caps on the outside.

Also, unless you did already, check all the solder connections of that ADC and its bypass caps with a magnifier. What surprised me though was that the mfg said this is "within spec". Almost like saying "Oh, that clunking sound from the rear axle? That's normal".

That's ok, everyone will know what goes where :-)

This is the amplitude of the biased signal going into the ADC.

I've found input signal dependant load regulation on a 1.25V reference node.

Looks like the reason there's no apparent droop on the other chanel is simply because the regulation is as fast as the signal.....(wtf)

The 'distorted chanel showed regulation with a time constant directly related to the visible distortion.

Grossly decoupling the node, simply reduces the visible effect lower in frequency - getting the same distortion now with 10Hz calibration signals.

This is sort of good news. It means there's a reason and therefor a possibility of a cure without resorting to major component changes. If the reference is sourced internal to the ADC, there are limits to what can be done without the possibility of damage due to power sequencing,

RL

Of course the scope is junk without the fix, so the downside is....?

Cheers

Phil Hobbs

When you're down, any direction is up.

RL

Look at the bright side: Now you have found the root cause. That is usually 90% of the way to the solution. Since one channel is good and one bad, chances are that it boils down to a simple mis-stuffed component. Maybe a 0.1uF cap where a 100pF should have been.

It was the same with a Japanese transceiver. Took me many hours to find the root cause, I was a teenage kid back then so know-how was limited to what the public library had. Once I found the root cause the whole matter became totally boring for me because the fun was out of it :-)

I've made inquiries at Hantek, re the reference load regulation issue and will let the thread know if there's anything more constructive offered from that direction.

The unpopulated decoupling capacitor positions on the board are - from memory - C66(ch1) and C104(ch2). There may have been a very good reason why these parts weren't populated. The node impedance to ground for each reference, in an unpowered circuit, is only ~140R. There is also an unpopulated position in parallel with C66 that could easily have held an electrolytic-type body size.

The effect on frequency response might be a low-frequency to DC peaking of ADC output. The response would exhibit a dip dependant on average DC amplitude. I'm not sure I've got my mind wrapped around that quite yet......

RL

There were 3 decoupling cap positions that were unpopulated. Two were on channel one. Because channel one exhibited a fast droop, I'm guessing that there was an unidentified decoupling cap still hanging on the reference of channel two.....

Channel one drooped too fast to be noticeable in the 1KHz sq wave signal.

When the reference droop is within the period of the displayed signal, the effect would be noticeable. I wonder what that does to the bode plot. I was speculating a DC-xKHz peaking, but if the reference droop is average-responding, that might get messy, as you can PWM right past the full bandwidth...

RL

It would be good if Hantek or another owner could send you a photo of the board of a know good unit. That should identify it.

It's all kind of weird, this 2250 scope. But anyhow, you could probably pour yourself a cold one or maybe a nice big coffee and ohm out each reference node. See which parts and especially caps are connected to it.

I've been in touch with Geoff Graham at

I'm about to be hijacked from the bench for three weeks. One of those things a small portable was intended to handle.

RL

Maybe he's willing to take a few close-up photos.

Yes, for the road such gear is amazing. I just bought a USB spectrum analyzer. Size of about three cigarette packs yet goes from almost DC to