[sdiy] Digi-Mod schematic

[Neil Johnson]

Seb,

Feel free to ignore these comments!!

First pass comments:

- page 1: where are the slave-select signals for the SPI interfaces?
- page 1: do LEDs _really_ need to run from 3V3 rail?

- page 2: do something with the enable input of X1 (don't leave it to  
float)
- page 2: where is the supply decoupling capacitor for X1?

- page 6: if you're going to use the 7805 to drive the LM2937 I  
suggest a low-value resistor (a few ohms) between the two, otherwise  
the active load of the LM2937 can upset the 7805, causing possible  
oscillation.

- page 7: you need to control the CS\ pin, otherwise you won't be  
able to talk to the chip.  Read section 7.2 of the Winbond datasheet:

http://www.winbond.com.tw/NR/rdonlyres/93A5FE5A-EEF2-417B-87A8- 
CB8144F6120D/0/W25X10A_20A_40A_80A.pdf

- page 8: given the critical nature of the 220p capacitor I would put  
the BAT54S on the left-side of C47.  Or, an alternative approach  
would be to put the BAT54S on the input side of R85, and put, say,  
100R on the output of the 4580 inside the FB loop to protect its o/p  
stage when going -ve.  Also simplifies layout as you don't need to  
protect up to +15V rail then (one less track to worry about).

- page 9: ditto above

- general: the 4580 is a bipolar op-amp so you really need to think  
about input bias currents, otherwise you're going to get unwanted  
offsets all over the place.  For example, on page 9 U14B, its -ve  
input sees an impedance of roughly R82, R83 and R114 in parallel,  
while the +ve input sees a direct short to earth.  This will give  
rise to a DC offset on the output due to the input bias current.   
Without R114 I'd put a 47k or 51k to ground on the +ve input, a lower  
value if R114 is fitted.

Repeat the exercise for all other instances of the 4580.

- page 12: the TLV2264 is a CMOS device, so the bias currents aren't  
generally worth bothering about in this type of application, so its  
reasonable to connect the +ve input direct to ground.

- page 12: not sure what you're trying to achieve here with your  
funky dual feedback circuit.  This looks like an output buffer, for  
driving long capacitive lines.  Are you trying to make a LPF?

- page 13: ditto above
.. all the way to ...
- page 23: ditto above


General Comments

1. If you can avoid the external +5V input then you can make your  
module supply-compatible with MOTM and Oakley (amongst others).

2. For the high-current 3V3 rail I would seriously consider a small  
switching converter, and keep the lower-current analogue supplies  
separate.

3. Took me a while to work out what you meant by "V ptp".  This is  
more commonly written "Vpp".

4. Any out-bound supply pins on connectors I'd protect with a low- 
value series resistor, to protect against shorts.


Hope this helps,
Neil
--
http://www.njohnson.co.uk