On 2/27/22 12:07 AM, moensted1@... wrote:
Hi all - many inputs and much to consider - but thanks! I'll address your feedbacks in sequence below:
@Jim : Thanks for commenting above and also your setting up/linking to your ADC interference measurements. However, I'm actually after something else in what I'd like to measure ...
As it is I have for some time now been trying to simulate (in LTSpice) a myriad of different combinations of capacitors in order to setup a low impedance - and importantly - straight impedance PSU supply network for ADCs or DACs. As you may/likely know MLCCs typically have relatively narrow low impedance spikes and then the MLCC impedance rise on both sides of this "spike". Combining more/many capacitors leads to anti/parallel resonances which may end up to unevenly attenuate - or actually amplify - the PSU network noise at these anti/parallel resonances unless capacitor capacitance values are close to each other (and "same type capacitor").
Additionally, according to an IC designer, many ICs include "hidden" PSU line capacitances inserted by the IC designers to mitigate internal IC issues with the combination of inductance etc. on these IC PSU lines.
What I'd like to establish is a better coherence between these LTSpice PSU supply network simulations and the real world impedance response of a DAC/ADC PSU supply network. That is: injecting a signal into the PSU supply network (e.g. the CH0 of the VNA) - while the DAC is powered on but not "active" (i.e. no clock or data signals) - and then measure the impedance characteristics of the PSU supply network with the VNAs CH1.
So you'd hook up CH1 at the power pin on the IC?? - That might work. You might need a buffer amplifier to turn it into a High Z probe, so the 50 ohm impedance of the VNA doesn't load down the network.
I recently did a trial where I relatively carefully adapted capacitances etc. in such a DAC PSU network and then measured the noise on the network with an FFT (maximum 200 MHz - picoscope oscilloscope).
That's a High Z probe, effectively.
I then compared this noise with the noise on the same DAC with the ubiquitous 100 nF decoupling capacitor in place instead (sampling frequency 384 kHz ~ 24.576 MHz clock). The PSU network noise on average dropped 10-15 dBs with the adapted network. I suppose it is likely that the noise spikes I measured coincide with the impedance peaks on the PSU supply network but I can't be sure of this and would like to be able to measure the impedance/vs frequency in a way that I can also see how it interacts with the DAC's internal PSU network.
Anyway, this is why I would like to use a VNA - and then also for characterizing passive components. I hope it is clearer now ...? I am just unsure whether the square wave output of the VNA will mess up things (provoke component non-linearities, DAC odd responses) - or it may work ... ???
As long as you are measuring a linear system (which your LCR "decoupling network is) at single frequencies, the harmonics of the square wave shouldn't make any difference. If you're measuring it hooked up to the load (your DAC), it's possible that some circuitry inside the DAC might have some IMD, but on the other hand, the square wave is on the *input* of the decoupling network, which is generally a low pass.
