PCB capacitance

On a sunny day (Mon, 09 Dec 2013 21:37:04 -0800) it happened John Larkin wrote in :

Try again with a pink one.

Just for grins,try a 10Meg resistor..you may be in for a small surprise.

I've put the gear away, so it would be inconvenient. What do you think would happen?

I was half asleep; also use a 1pF and a 10pf capacitor (0603 natch). 10meg R should make little change, 1pF and 10F should add a small amount,theoretically (Cadd plus your 0.085pF) in parallel to "original"

I'm doing a 3-terminal capacitance measurement between those pads, so any cap that I put in parallel with that resistor will add linearly to what I'm measuring. Pad-to-ground capacitance isn't as important in this circuit as pad-pad.

The Boonton is great. I ran coaxes out about 2 feet, and transitioned to homemade probes, hardline coax with a bit of center conductor poking out. Boonton ground is connected to the PCB ground plane. I almost touch the probes to the pair of pads, zero the Boonton, then touch down. The low range is 1 pF full scale.

Often true--in TIAs and bootstraps, for example.

And once it's zeroed, it sits there all afternoon reading 0.000 pF. (I have the digital version.)

Beautiful.

Cheers

Phil Hobbs

I had a meter like that once. ;-)

Bless you. It's folks that surplus good kit that have made my lab what it is. ;)

Cheers

Phil Hobbs

Errr... he might mean that the fuse was blown.

Does it have a mirror behind the digits?

Of course. "I had a xxxx like that once" is an ancient gag line.

Cheers

Phil Hobbs

No, which is a good thing--otherwise I might fall in love with my reflection. ;)

Cheers

Phil Hobbs

Cool!

Whoosh... right over my head. Thanks.

Yeah, they're widely available on eBay for $100-$200. I have two 72BD digital ones and a 72A analogue one. Total price: $456 plus $67 shipping (three separate orders). Really great gizmos.

Cheers

Phil Hobbs