Recently received DSO2250-USB shows AC compensation loss due to GUI
screen position. The same effect of screen position occurs for
calibrated waveforms in both AC and DC coupled modes.

The two screenshots show the effect of shifting an AC calibration
waveform from the lower to the upper part of the GUI display.(CH2)

No other changes, but the loss of compensation is demonstrated.

Note that the internal PPKvolts measurement feature shows the reverse
effect. An increase in measured ppk is indicated for the waveform with
no overshoot/severe undershoot.

The compensation loss results in `7LSB distortion in the display, and
a 50mV difference in a 2vppk measurement.

Anyone know how this fault is achieved and how it might be corrected?

RL

On Sun, 27 Nov 2011 12:57:18 -0500 legg wrote in
Message id: :


Recently received DSO2250-USB shows AC compensation loss due to GUI
screen position. The same effect of screen position occurs for
calibrated waveforms in both AC and DC coupled modes.

The two screenshots show the effect of shifting an AC calibration
waveform from the lower to the upper part of the GUI display.(CH2)

No other changes, but the loss of compensation is demonstrated.

Note that the internal PPKvolts measurement feature shows the reverse
effect. An increase in measured ppk is indicated for the waveform with
no overshoot/severe undershoot.

The compensation loss results in `7LSB distortion in the display, and
a 50mV difference in a 2vppk measurement.

Anyone know how this fault is achieved and how it might be corrected?

Have you checked to see whether there's a software update for the scope?
http://www.hantek.com/english/down_list.asp?unid=14

(Awful site!)

If you're running the latest, maybe try rolling back the version?

If that doesn't fix it and it's still under warranty, I'd get my money
back. Pretty serious flaw. I think I'll stick with my Agilent scope...

On Mon, 28 Nov 2011 22:15:37 -0500, legg wrote:

On Sun, 27 Nov 2011 09:31:43 -0800, Joerg
wrote:

snip

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.


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

[...]


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?

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


Why do they do it?

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

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.


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


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.


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.


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.


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 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..



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.

On Sun, 27 Nov 2011 12:57:18 -0500, legg wrote:

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....

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

Someday this post may actually make it through the frigging primus
server. This is about the 4th attempt to post.