View Full Version : switching inputs

06-15-2009, 09:51 AM
I am using a phidgetts 8/8/8 board and have attach to it a QT113 Charge Transfer Touch Sensor (attached is its datasheet, I used the exact setup on the datasheet except to fit the phidgetts 8/8/8)that I bought from parallax. I got it running for a one plate system. But I need to have a 6 plate system that the phidgetts board reads off of one analog inputs but I need to read each plate separate. I was thinking off a switching module board that will turn to one read switch to the next read and on and on does anyone have any idea on how to do that. Thank you for your time
Joshua Treadway

06-15-2009, 10:05 AM
What you are describing is an analog multiplexer. Specifically, you'll want an 8 to 1 since you need >=6 channels. That means you need 8=2^3 addresses, or 3 address lines (which means it will require 3 digital outputs, and 1 analog input to your phidgets). A number of companies sell them in small chips. Maxim IC probably has a dozen varieties that will meet your spec.

But, are you planning to use up the other 7 channels of analog on your phidgets? The 8/8/8 has 8 analog channels, why not just use 6 of them for your sensors?


06-15-2009, 10:10 AM
Yes I'm planning on using the other 7 inputs for other sensors I wanted to make it as clean as possible
Thanks for the reply

06-15-2009, 10:16 AM
The thing you'll have to worry about is timing. I would assume that the Phidgets board probably already has an 8:1 multiplexer on it's analog channels (the actual analog to digital conversion hardware is fairly pricy on each chip, adding a multiplexer rather than 7 more real channels is the typical solution found on most microcontrollers). Now, you're bumping that up to 15:1 in effect.

The issue is, that the analog system is gonna do a sample/hold/convert on each channel, I'd imagine that they just walk down the line, sample/hold/convert CH1, then 2, etc... each time, it takes a little time for the analog voltage to settle down on the output of the multiplexer. You're gonna have that same amount of time on your multiplexer. If the update to your digital outputs is not in sync with the analog sample/hold/convert, you're gonna run into issues where you don't know which channel you are really sampling... speed will also be majorly affected (I think the phidgets has about a 65Hz update rate back to your PC, if you have to manually change the 3-bit address each time, that gives you only 65 samples, split amongst 6 channels, for a whopping 10hz update rate..)


06-15-2009, 10:50 AM
thank you for the help i found what i need