Issue with measured input changing with frequency

Hi there,

I’ve recently designed and built a small attenuator for a college assignment.
I’ve tested it using the gear at college and it works, perfectly.
However, when testing it with the DSO nano V2 with the 3.64 firmware, I get rather strange values when using the mV range.
I was trying to read a 300mV AC signal on the 10 - 100mV scale at 1kHz which worked fine, but when the frequency increased above that, the signal levelled out to a DC voltage level.

When I switch to a 200mV reading, it works fine again.
Any particular reason for this?

Dear customer,

According your description, you said when you use 10~100mV to read 300mV.That’s not advocate.
Because when you see NANO signal value is calculate by software compensation that could effect your real value when you reading your signal.

If you view the NANO schematic you can see there are lot of resistance to switch your different range.

Hope that can help you.

Best regards,


Well, for example.

If I read a 300mV squarewave signal with the DSO set on 100mV range, at 1kHz, it will read the signal fine.
However, if I increase the frequency above 1kHz, the signal will begin to flatten to a DC level.

If I switch to the 200mV range and above, it will read the 1kHz signal and any frequency above that correctly.

There is a difference in DC input resistance between the 100mV (1.02M) and 200mV (546k) input range. This may explain a difference in bandwidth between the two, but I did not expect this to be an issue at 1+ kHz (or even 100+ kHz). Input capacitance is not specified for all paths on my schematic and so cannot be fully determined by analysis.

I does kind of render the 10 to 100mV range pretty much useless above 1khz. soon as it see’s 2khz and above, the wave levels out and peak to peak voltage drops off.

shame really as it makes the nano a rather unreliable machine.

I too have seen this with a frequency response sweep
on the 10mV - 100mV ranges.
These ranges use no additional attenuation, so
problem lies in the first voltage divider.
Starting at 2KHz the signal drops off, and then
levels out at 10KHz about 12dB low.
The higher ranges are usably flat.

I checked the capacitors in the front end and
found discrepancies with the schematic.

C101 is 24pF, not 100pF, but
this is ok with the higher ranges attenuations, and
for use with an External X10 probe ( 1M || 13pF)

C2 is 75pF, which is totally wrong for the 2:1 input divider.
I removed C2 and the unit is usably flat now on the most sensitive ranges.
Tech Support, please forward this to the designer.

Al Dutcher

can I ask what schematic your looking at as with the one I’ve found C2 seems like an integral component.

Hi nigelakaboo

The schematic is DSO0201V2.3_SCH
Is this the latest one?
If not, would you point me to one?

Thanx, Al D.

BTW I love this little scope,
especially with the BenF upgrade.
Hope he s working on FFT for it too.

My big scope is a TDS2002C.
Used it to verify the freq response anomaly.

That’s Fine.

It was just that the schematic that I found had slightly different labelling and so wasn’t sure which C2 was the one you were talking about.

ill remove it and try again.

Cheers for that!

Hi tan106

If you view the NANO schematic you can see there are lot of resistance to switch your different range.

The problem is that the caps do not track the resistances.
All of the RC pairs should have the same time constant (t=R*C), so that
the attenuation is flat across frequency.

Al D.