Welcome to the Forum

Hi, just checking if this is still on someones 'to do' list?

Better measurement results with the 2 Port Shunt State file provided. Able to see the difference Isolator makes in overcoming the ground loop error at low frequency. But the value is still ~400uOhms. Theoretically, I shouldn't be able to measure a lower resistance value than the real value? So not sure if this 1 milli ohm resistor from the PITK01 kit is actually only 400 uOhms in value.

The B-AMP 12 Amplifier (https://www.picotest.com/products_OL000168.html external power amplifier for the Bode 100) is used to amplify the Bode 100 output signal by 12 dB. It is primarily for PDN, and specifically for ultra-low impedance power rails. The P2102A/P2104A PDN probes work with, or without, the B-AMP, but all PDN measurements are mostly dependent on the cable resistance and the CMRR of the isolator (e.g., J2102B). In fact, in the University paper (https://www.picotest.com/measurements/download/UNIVERSITY_How_to_Measure_Ultra_Low_Impedance_(100uOhm_and_lower)_PDNsVersion17b.pdf ) we showed the impedance error can essentially be reduced to Rshield/CMRR. We also showed that a source power amp is the ONLY way to improve the SNR of the measurement, but has no impact at all on measurement accuracy, only on SNR (noise).

Ben Dannan (https://www.linkedin.com/in/benjamin-dannan/) showed the benefit of the B-AMP12 for measuring COT converters, which are typically noisy. That is where the SR issue comes from. Without VRM noise there is no need at all for a B-AMP12, but since the VRM noise raises the noise floor, the signal needs to be increased to maintain a given SNR. That is what the B-AMP12 does. The Bode 100 inputs can tolerate an absolute max voltage of 5Vrms but the recommended is below 3.3Vrms. The +13dBm source power is 1Vrms so at the output of the B-AMP12 the signal is 4V, not including the DC voltage of the rail being measured. It is VERY easy to damage the front end. Oscilloscopes typically have a maximum rating of 2.5Vrms, so even easier to blow out the front end. Having done it twice, in two different brand scopes, I can tell you that all channels are generally on a single board. Each scope repair cost more than $6K, so be careful.

I hope it is clear that the B-AMP12 is not a replacement for the PDN browser probes. So, comparing them as interchangeable does not make sense. The B-AMP can produce a cleaner plot for noisy power rails or ultra-low impedance (our record is 22uOhm, but Ben Dannan just demonstrated 4uOhms, using B-AMP12). This improvement would be the same with soldered cables or probes and accuracy would not be noticeably affected in either case. In most cases, an oscilloscope offers better noise performance for measuring PDN impedance and so doesn’t require an amplifier. On the E5061B ENA we demonstrated a 22uOhms measurement, without a power amp despite the instrument being limited to +5dBm. So, the only reason we could see for you to add a B-AMP12 is to overcome a noisy output, but beware that you can cause damage with it if you measure with too large a signal (as in impedance above a few Ohms).

Thanks for the feedback, so I guess I have some follow-up questions.

So from my understanding, the Q(Tg) plot is essentially related to the S21 magnitude by some derivative. This means that the tiny, but sharp, blips in the impedance are responsible for the massive Q(Tg) spikes. In general, I noticed that as I increased the number of points in my measurement, the more definition these blips got, the sharper they would get as well, resulting in increasingly large Q(Tg) peaks. I also notice that these blips shift around from measurement-to-measurement, my main suspicion is that they might be caused by changes in the load of the POL adding noise to my measurement, or causing small shifts in the loop gain of the converter (since the bode plot I've measured isn't constant over load). It seems to me that these blips can be generally ignored, but one area where I could see a problem coming up is where I have a less well-behaved DC-DC converter. If I increase the number of points of my measurement, I think I might be worried about these sharp "fake" peaks blowing out the true, lesser peak. Could there maybe be an issue where I might want to set an upper limit on the number of samples I take to reduce the impact of the small blips?