Post History
I see that in evaluation design it is mentioned component values will be updated after evaluation, so is it the case that these values are more or less arbitrary and tuned with a VNA during proto t...
#2: Post edited
by
Olin Lathrop
·
2025-04-16T11:22:48Z (3 days ago)
Fixed typo
- <blockquote>I see that in evaluation design it is mentioned component values will be updated after evaluation, so is it the case that these values are more or less arbitrary and tuned with a VNA during proto testing?</blockquote>
- That was my first thought when I read that disclaimer.
- Your original analysis suggested that you only needed to add a 600 fF capacitor. That's <i>tiny</i>, and easily swamped by the parasitic capacitance of the PCB traces, connectors, etc.
- Your analysis also used a specific impedance for the STM pin which apparently didn't come from a datasheet but someone's measurements. Was that from a single unit? Did they measure many units and then average? What should you expect for unit to unit variation?
- When fractions of a pF matter, then calculating everything up front when using ordinary PCBs is impossible. You start someplace plausible, then tweak by experimentation.
On a separate topic, the purpose of the C-L-C pi filter may not only be impedance matching. Attenuating out of band frequencies may be important for regulator compliance. Also, the asymmetrical capacitance should not be surprising when different impedances are connected to each end of the pi filter.
- <blockquote>I see that in evaluation design it is mentioned component values will be updated after evaluation, so is it the case that these values are more or less arbitrary and tuned with a VNA during proto testing?</blockquote>
- That was my first thought when I read that disclaimer.
- Your original analysis suggested that you only needed to add a 600 fF capacitor. That's <i>tiny</i>, and easily swamped by the parasitic capacitance of the PCB traces, connectors, etc.
- Your analysis also used a specific impedance for the STM pin which apparently didn't come from a datasheet but someone's measurements. Was that from a single unit? Did they measure many units and then average? What should you expect for unit to unit variation?
- When fractions of a pF matter, then calculating everything up front when using ordinary PCBs is impossible. You start someplace plausible, then tweak by experimentation.
- On a separate topic, the purpose of the C-L-C pi filter may not only be impedance matching. Attenuating out of band frequencies may be important for regulatory compliance. Also, the asymmetrical capacitance should not be surprising when different impedances are connected to each end of the pi filter.
#1: Initial revision
by
Olin Lathrop
·
2025-04-14T11:36:54Z (5 days ago)
<blockquote>I see that in evaluation design it is mentioned component values will be updated after evaluation, so is it the case that these values are more or less arbitrary and tuned with a VNA during proto testing?</blockquote> That was my first thought when I read that disclaimer. Your original analysis suggested that you only needed to add a 600 fF capacitor. That's <i>tiny</i>, and easily swamped by the parasitic capacitance of the PCB traces, connectors, etc. Your analysis also used a specific impedance for the STM pin which apparently didn't come from a datasheet but someone's measurements. Was that from a single unit? Did they measure many units and then average? What should you expect for unit to unit variation? When fractions of a pF matter, then calculating everything up front when using ordinary PCBs is impossible. You start someplace plausible, then tweak by experimentation. On a separate topic, the purpose of the C-L-C pi filter may not only be impedance matching. Attenuating out of band frequencies may be important for regulator compliance. Also, the asymmetrical capacitance should not be surprising when different impedances are connected to each end of the pi filter.