This page describes the design of a four channel signal conditioner targeted at use with the Sparkfun logomatic. It is built from mostly surface mount components on a PCB.

The details of the circuitry and operation of each channel are here.

The power supply section is pretty simple so I don't think any discussion is required.

The design of the PCB took a little work. I have designed a couple of other PCB's but for them I used the captive design tools of ExpressPCB. This time around I am using the gEDA tools.

I think it is now ready for a design review. So I am posting images (front, back) of the card layout and the design files. If you have the gEDA tools you can examine it in more detail and even play with changes.

I know that my PCB design skills are not the best so pointing out flaws or improvements is appreciated.

I had originally hoped to make the size the same as the V1 logomatic but that didn't happen. It could probably be accomplished by moving more parts to the back side or using smaller parts. But I prefer to use size 1206 resistors and 0805 is about as small as I care for in capacitors.

Cost

After conducting a less than exhaustive search I have pretty much settled on using the board service from Sparkfun. Costs are $2.50 per square inch plus a $10 setup fee and shipping. This buys a double sided board with solder mask and silk screen.

Component costs for stuffing the board are approximately: (Source is Digi-Key unless otherwise noted.)

QtyDescriptionSourcePart #Price each
1 2.5V reference REF192ESZ-ND 5.65
1 5V regulator 576-1152-ND 1.80
4 opamp MCP607-I/SN-ND 1.33
4 instrumentation amp INA122UA-ND 4.79

The remainder are passive resistors and capacitors. The price of these will depend on how many you buy. Resistors run about a nickle each although I did see a 0.01% metal foil at over $8 each. If you buy a full reel of 5,000 parts the price can drop to less than a penny each.

Some parts come in various grades. The REF192 is available in three grades with the cheapest being $3.15. It has a tolerance of +/-0.4% and 25ppm/C temperature coefficient. For a couple of extra bucks you get +/- 0.08% tolerance and 5ppm/C. Since only one is used this seems cheap to me so that is the part in the list.

The INA122 stands out as a relatively expensive part. If you don't need all of them you could cut costs by leaving one off.