Is a 1 nf capacitor really needed?

P!-Ro

I would like to use a microphone with my propeller, so I looked at the schematic for it on the propeller demo board.

http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PropDemoDschem.pdf

It shows using two 1 nf capacitors and I have no idea where to get them, so I was wondering if I could go without it or find a substitute.

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Pi Guy

Mike Green

The analog to digital conversion method (sigma-delta) requires one capacitor and the circuit works better with two. The capacitors and resistors have to be mounted close to the Propeller chip itself with very short leads. The application note on the use of the counters (AN001 - www.parallax.com/tabid/442/Default.aspx) discusses this and there have been several threads also discussing this method.

Mouser has several 1000pF 50V ceramic capacitors that should work (1000pF = 1nF). Other distributors would also have them.

Post Edited (Mike Green) : 11/2/2008 12:22:19 AM GMT

evanh

The A/D method used is quite neat. I've never tried to fully understand sigma-delta circuits myself and this one is by far the simplest I've ever seen but it does seem effective.

It's an extended form of that used by the old analogue joysticks, where the time taken to charge the capacitor is measured. And from that a reading is derived. It's actually somewhat better for two reasons - namely hardware timing is used and the inverting charge/discharge pulse width ratio is the measurement taken rather than the single cycle sawtooth method for your typical joystick.

So, yes the resistors and capacitors are vital. And as Mike said the layout of the board is also important to keep the relation between the capacitor voltage and the input high/low detection threshold stable.

evanh

To make that short - It converts a voltage to PWM and gives you an integrated ratio of ON(high) time to sample time.

Beau Schwabe

FYI -·· A 1nF capacitor might be more commonly recognized as a 0.001uF capacitor or one with numbers that read 102.


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

You'll pay a premium, but you can get them at Radio Shack if you're in a hurry. The part number is 272-126. These are thru-hole types. For surface mount, either DigiKey or Mouser will have what you want.

-Phil

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'Just a few PropSTICK Kit bare PCBs left!

P!-Ro

If I get them anywhere, it will be online, Phil. Our radio shack is so crappy one day I couldn't find any pnp transistors or a ir transistor. Let alone any of the wire conectors I wanted.

I found some 100pf transistors from my PE kit and some other unknown capacitors (I couldn't find them on the parallax website) that say 10j on one side and 5a on the back. Can I use these in place of 1nf if I use multiple ones?

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Pi Guy

Rayman

I've seen it work without any capacitors at all... So, I think you 10j ones will probably work. I'd guess "10j" means 10pF in a "j" type ceramic...

Beau Schwabe

While the Sigma Delta can work without capacitors, you get a small amount of parasitic capacitance off of the I/O pins themselves. Use of larger external capacitors help attenuate noise produced from the Sigma Delta's feedback (output) pin and stabilize the voltage seen by the Sigma Delta Input pin. The 10pF is probably not going to be enough. However you don't want the capacitors to be too big or they will act as a low-pass filter and you will loose the sensitivity of the ADC to higher frequencies.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

P!-Ro

It can be more than 10 pf. Those were just the kind I didn't know what were. I also have a couple 100 pf, so if necessary I could join them together for better results.

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Pi Guy

Rayman

1nF, like Parallax used on the demo board, is probably a good starting point. It probably also depends on the value of your feedback resistor, commonly 100k...