DSO2250 AC-DC coupling

That looks like a layout screw-up. Traces of unequal length between AC/DC settings and even between channels :-(

If it is a trace issue then some nifty kludges might work. The old trick to produce a very small "variable capacitor" is to solder on some enamel twisted pair, then snip - snipety - snip ... until just right. IOW you'd have to determine the setting with the worst roll-off and then bring the capacitance of all the other traces up until the same roll-off occurs there.

Doesn't seem to have anything to do with the analog measurement hardware function.

The same square wavform becomes decompensated as it is relocated from the lower to the upper portions of the display screen.

A square wave that is properly compensated midscreen, shows overshoot when shifted to the bottom, undershoot when shifted to the top.

The leading/trailing egde difference is in the order of 7LSB.

At the same time, the on-board ppk measurement increases proportionally solely due to screen positioning. The overshot waveform measures less ppk. Delta 70mV in a 2V measurement. This in the opposite sense to the presented distortion.

How do they do it?

Why do they do it?

RL

But you wrote that when the relay clicks to the other position the compensation gets messed up. That sure looks like a bug in the analog hardware.

Here we'd need the schematic. To see whether this offset function is still analog. Not sure whether they send a USB command to the box and some DAC or PWM adds an offset. In that case we could see slight changes in a junction capacitance somewhere.

You could probably find out if you'd open the box and see if a DC level in there changes if you move the offset via the GUI.

Out of 8 bits? That's quite a lot.

Sounds more and more like they do the offset before the ADC and the analog circuitry was done by a rookie :-)

It is probably not intentional and they either didn't notice or ignored it.

The apparent change was due to the shift of the waveform on the display. It was a positive pulse waveform that shifted down-screen when AC coupled.

All center-screen compensated waveforms can be caused to overshoot by shifting baseline down or to round-off by shifting baseline up. This with unchanged input or coupling method.

I suppose that screen offsets have to be physically enforced as a DC component introduced before the ADC, as the coding expires at the display limits ( this evidenced by manipulating math functions to overdiven signals )

The unipolarity of effect suggests that the ADC is seeing a unipolar input - perhaps a protection network's capacitance is being modulated by this shift.

I would have thought, however that internal measurements of ppk would be generated from ADC output, and this demonstratedly cannot be the case, if the GUI display is unaltered from the ADC output.

Input protection is specified at 35V levels, and +/-20V levels are expected. The behavior is visible using the unipolar 2Vppk square wave signal provided as calibration, by the device.

Anyone else see this effect on other models of similar devices?

I've only been working with this one for two days.

RL

Ok, then I misunderstood. Now it sounds less like layout and more like sub-optimal analog circuitry.

Now it really sounds like an analog circuit issue in there.

I would be very surprised if they didn't take the cheap route and do it off of the ADC output. Who knows what the software does.

It's time to take a 2nd analog scope and probe around in there. I suspect the analog DC offset changes some junction capacitance and they do not correct for that in SW. Or worse, haven't noticed ...

If this is indeed the case then probing around with an analog scope should bring out the truth rather quickly.

I've tried posting a few images to a.b.s.e., but my own server doesn't seem to pick them up.

RL

I use giganews... your posts are there. ...Jim Thompson

Thanks. RL

Problem is, I bet that >95% of participants in this NG have no access to a.b.s.e. any longer. Better to find some free picture-hosting server, preferably one without nasty scripting or excessive ads but one with mild ads.

Why not? Too CHEAP for $5/month ?:-) ...Jim Thompson

What good does it do to sign up if almost nobody else can access binary groups?

My newsserver costs around $15 a year but the nice thing is that they seem to do a stellar job of hosing off spam. Lately I took the googlemail domain filter out and ... no more flooding with ads for fake watches and handbags.

Unfortunately, after Cuomo's tirades, binary groups are history for engineers. You and I and a lot of others aren't liking it but it's a fact :-(

I've thrown away the warranty, for what it's worth, and given the circuit a quick look.

After the input attenuator, a DC shift is introduced before the DAC, as expected, to provide a unipolar ADC input and to control the screen centering position. There's no sign of alterations in rise-time or overshoo on either side of the buffer feeding the ADC, as the DC shift is modulated.

At the same time, these errors are visible on the PC display.

The protection network is composed of the CB junction of a vhf bipolar transistor to the +3V5 rail (~1pF), and a DO214(SMA) diode body to the common return. This diode is soldered manually and is marked with the signature (RD)found on all of

-a 54V transil,(~120pF)

-an 18V transil (~470pF)

-a conventional 200V rectifier (10-7pF)

-others?

One thing that wasn't noticed before, is that the effect is most pronounced on channel 2 ( the channel used to demonstrate in the images posted on a.b.s.e.). The effect on channel 1 is probably small enough to have been ignored by a bored operator.

The ADCs are identity-ground-off. There's a nasty void in the one used for channel 2, probably caused by metallic grit accidentally included in the grinding operation. Some of this metal is imbedded in the void. Image on a.b.s.e..

RL

Removing both transient protection components from their position in channel 2 makes no difference. The frequency compensation is still dependent on display screen position, without visible alteration at the transient limiting node.

I also note that the actual frequency compensation waveform displayed seems to bear little resemblance to the actual waveform before the ADC, regardless of position, on either channel. In an analog circuit, I might not be overly concerned, as there is the possibility of intentional peaking or tweaking farther downstream that need not be DC-dependent.

The intention of probe calibration (in the x10 setting) is simply to tweak high frequency response of the probe, to adapt to scope input capacitance.

A linear input signal should give a linear display, within the limits of the circuitry involved. Signals that are rolled off dependent on screen position would be difficult to get your mind around, no matter what their actual content.

I'm guessing that there's some kind of ADC error that is time and dc-voltage dependent, in the 1/4 millisecond range. Possibly a S/H section that is not properly 'flushed' during reset.

Perhaps there's an attempt to correct for this in SW, if so, there may be firmware corrections in the pipeline. I somehow doubt that it could be effectively applied at the full bandwidth of the instrument, or to two channels with non-identical characteristics.

I will make inquiries, but I can only expect an offer to accept a returned unit, for servicing or replacement (under warranty...).

RL

Excuse duplicate, if present. Server issues again.

There should be not intentional peaking, at least not right at the ADC. That could potentially lead to a signal becoming "different" with higher amplitude and would make the scope rather useless as a tool.

Does the waveform at the input to the ADC change when moving the offset? It should not ...

I hope not. That would be a major design screw-up on their part.

No problem. If the mfg can't fix this strange compensation error on the DSO2250 I'd insist on getting my money back.

Contact at Hantek states that this is normal, and within specified behaviour.

Is there a quick way of estimating the difference in frequency response, demonstrated by a 100mV difference in rising edge amplitude of a 2Vppk 1KHz square wave? I know the rise-time requirement rule of thumb, but never stopped to consider the other, though probe compensation is one of the first things you run into, with normal scope use. Just never stopped to consider....

RL

Simple spice calculations show that the simple compensation scheme's of 200MHz probes cannot produce overshoot with the time frame illustrated in the pictured 1KHz calibration signal - it should normally be all over and done with in the first 100uSec of risetime.

RL

Every post is a frigging test.....

I do not have access to a.b.s.e., so can't see the pic. 100usec is a whole lotta time.

But it couldn't possibly be the probe anyhow because the probe doesn't know whether an offset is entered or whether the scope is on AC. My DSO does not do that, no matter where I move the waveform to it doesn't change.

Our whole life is one big test :-)

In any case, I'll stick the screenshots up.

The DSO2250 is one of the USB scope interfaces being offered in the last few years. I would expect the input structure to be similar to any of the lighter-weight 8-bit scopes, with LCD or phosphor displays, made since Y2K.

RL

TVS/111126b_under.jpg

Geesh, can you send that POS back? George H.