AD2 Network Analyzer calibration?

[Walter76]

Hi all, I am facing a problem with a naïve NA sweep...

W1 (or W2) and both scope inputs connected together via MTE (BNC adapter has same behavior).

CH1 (stimulus) drops above 2MHz, but what is mostly annoying is CH2 with a -3dB attenuation from DC to about 1kHz.

I would expect both curves being completely flat... Do I miss something?

BTW, what means the yellow BW limitation alert on upper frequency?

Finally, is there a calibration procedure to compensate for cables and parasitics?

 

Thank you!

[attila]

Hi @Walter76

It looks like it is a filter on C2.
Could you attach a photo with your setup?
You could also test it with Wavegen and Scope using a square signal.

The open and short compensation is available in the Impedance Analyzer.

The NA capture should look like this, with -3dB around 12MHz.

 

See the following about the device bandwidth :

https://digilent.com/reference/test-and-measurement/analog-discovery-2/hardware-design-guide#scope_spectral_characteristics

https://digilent.com/reference/test-and-measurement/analog-discovery-2/hardware-design-guide#awg_spectral_characteristics

[Walter76]

Hi Attila, thank you for the reply.

The connection is very stupid: 

1+ (orange) with W1 (yellow) and 2+ (blue)

1- (white-orange) with W1-GND (black) and 2- (white-blue).

In the past it was working fine, just a few days ago I noticed this strange behavior... With the same connection, and setting W1 for 1kHz 1Vpp, I clearly see CH2 is lower than CH1, while they are the same at 100kHz.

I cannot explain this behavior at HW level... What could be damaged this way?

I only worked with max 5V signals, therefore I would exclude any AD2 abuse, unless sever ESD occurred (I have grounded mat and wrist...).

BTW I noticed that if I simply swap the terminals of WaveGen1 (W1 to 1- and 2-, its GND to 1+ and 2+), the curves are as they should be... Verified with scope too.

Walter

 

[attila]

Hi @Walter76

It looks like some AC coupling going on 2+ Since 2- is fine we can rule out wiring problem.
It may be a cold soldered resistor on the 2+ input:
https://digilent.com/reference/test-and-measurement/analog-discovery-2/hardware-design-guide#scope_input_divider_and_gain_selection

How does 2+ behave at high range, having Scope Channel 2 Range above 500mV/div or in Network Analyzer Amplitude above 2.5V ?

[Walter76]

Hi Attila, thanks for the tip: I found the 820k resistor on CH2+ actually being 976k...

Replaced it, AD2 fixed!

Thank you!

Walter

[attila]

Hi @Walter76

Thank you for the update.

This is the first time I see such a phenomenon... speculating... it could be a faulty, cracked and/or corroded resistor