From: sleeper75se_at_yahoo.se
Date: 2001-11-29 17:09:17
Hi,
I've been reviewing the design a bit and here's a list of things that
might need improvement.
modEEGdigi01.sch
* IC8 - microcontroller
The datasheet (page 66) recommends that an LC-filter is added to the
analog power input (pin 20) to reduce high frequency noise. So
consider replacing the resistor R28 with an inductor.
* IC10 - serial port
It's possible to save a bit of money if you remove this and drive the
serial port directly from the optocoupler. +/-12V can be supplied by
RTS and DTR for instance. It's not the 'correct' way to do it, but it
works for surprisingly long cables.
PCB
* It's definitely a good thing to add ground planes on the top layer.
It makes it harder to manufacture by hand, but it is far from
impossible if you know the right tricks. Another option is to use
surface mount parts. Again, more difficult to solder by hand but it
improves overall signal quality since you can make critical paths
shorter and don't have to break the ground plane wherever you used to
need a hole.
* VREF starts very far away from the microcontroller and will pick up
a lot of noise. Shielding it with a guard ring connected to a 2.5V
voltage source might be necessary too.
* The guard ring around the crystal is a bit small. Wider traces and
larger distances to surrounding parts is a good thing.
* Avoid routing signals directly under the microcontroller chip. The
chip and the surface of the track will act as a capacitor, which
picks up noise. Even 'slow' signals like /reset can pick up and
transmit noise.
* Try to put C40 closer to the chip.
I haven't had time to give the analog part a proper look yet, but
I'll get back to you about that. Adding a ground plane is of course
the best overall way to lower the noise levels. Surface mount parts
the second best.
/Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:32 BST