Decoupling LIS302DL – There I fixed it!

December 15, 2009

in Tinkering

As you probably know, I really like I2C sensors and especially the LIS302DL. But adding this accelerometer to your circuit is not as straight forward as I thought. But in the end I got it quite right.
Decoupling LIS302DL
It all started out with a 0.1µF ceramic capacitor. For good measure I put a ferrite bead in front. But this lead to terrible readouts. So I threw in a 10µF 0805 ceramic capacitor and a 4.7µF tantalum capacitor. It does not look nice, nor does it look terribly solid, but it works fine.
So read on, what my analysis of the situation is.
My original layout had just a small ferrite bead and a way too small capacitor. And this on a board hosting a ATmega168 and four alphanumeric displays, drawing from 0 to 300mA, from a step up voltage controller. Of course I had about 400µF decoupling capacitors on board, but the direct power supply of the LIS302DL had still about 0.1 Volts of noise on the power supply line. And the readouts were extremely noisy (>10%).

Not good.

Throwing in some capacitance was a good idea and reduced the noise by something like factor 2. But still it is very noisy. The LIS302DL datasheet suggest using at least a 10µF and a 0.1µF capacitor of different types (e.g. ceramic and tantalum) – I think it is just some minimum requirements. Proper voltage supply with as less noise as possible is a must.

So my next try will be:

  • Redoing the complete layout so that the power supply of the LEDs is uses separate VCC and ground paths than the digital circuit.
  • Redoing the same for both chips, so that I got some star like grounding and supply system. Or some carefully laid out power planes.
  • Adding bigger ferrite beads in the supply line of the LEDs and both chips, so that each component has its own supply line with ferrite beads and capacitors, to reduce cross talk.
  • Perhaps adding some LDO regulator to drive the digital circuitry from 3V instead of 3.3V to reduce the noise of the LEDs or power supply.

I alwayys thought good power supply design is just for audio stuff or other high quality applications, but I learned (the hard way) that you also need it for just some basic tinkering.

So off to the next design.

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