AD2 - Network Analyzer - Weird behaviour with LowPass Filter at higher Frequencies

[Black]

Hello,

today I used the Network Analyzer of my Analog Discovery 2 for the first time. I wanted to analyze my filter behaviour of my designed circuit. But something seemed odd, because I had only used low pass filters and at a certain point of a "higher" frequency the network analysis started to look kind of odd (I appended a screenshot). I had done a simulation beforehand where a clean looking lowpass behaviour was determined. So I worried what happened and I couldn't get an answer to that.

To understand the network analyzer better,  I build a simple lowpass out of a 470 Ohms Resistor and a 300nF Capacitor, similar to the various tutorials out there (e.g. here). I can't get a clean curve like at the examples (I appended a screenshot). At a certain frequency point the network analyzer starts to see a dominant high pass behaviour. Even with various other values for resistor and capacitor these kind of behaviour stays.

I can't understand why these behaviour happens. Is something wrong with my Analog Discovery 2 / WaveForms or am I thinking wrong (about my filter)?

 

Thank you very much,

Black

 

 

[attila]

Hi @Black

I see now what causes the confusion...

The resistors and capacitors are not ideal elements. In reality these are complex circuits.

Like here in a 470 Ohm 470nF low pass filter above 1MHz we start to see as inductive circuit, the C/L parameters of the resistor, R/L parameters of the capacitor...

 

[attila]

Hi @Black

Have you connected the oscilloscope negative inputs (1-, 2-) to ground?

[Black]

Thank you for your fast reply!

I have connected both oscilloscope negative inputs 1- and 2- to ground of the circuit and AD2.

[Black]

Anyone having an idea, why this happens at a low pass filter?

Is there any setting i have to do? Is the AD2 possibly misscalibrated?

 

Thank you very much!

 

Black

[Black]

Hi @attila

thank you for your help!

Your answer sounds very plausible. I didn't think about the non ideal behaviour of these components.

I think, I was so confused about these effects, because they happen relatively early in my designed circuit...

 

Thank you very much for your help!