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FEZ mini Analogue IO


#1

Dear all, thank you for taking the time to read this post.

I have been working with the FEZ (Domino and mini) for some time and have a couple of questions that I need some help on.

The first is to the GHI team:
is it possible for me to purchase a FEZ mini that has not been built with the male pins on the back? I have a project that requires it to be flat and have been painstakingly removing the pins for it to work as I need a flatter profile for the device.

The second question is a little bit more technical:
I have two power sources that need to be read into the FEZ mini. Both power sources are continuous and variable in voltage but always below the 3.3v input cap. The question is, can the FEZ mini read both input loads simultaneously? ie can it take two loads in at the same time and read them both accurately with no noise?

I ask as, when I have the system running and each load is read independently, the signal data is read correctly as expected. But when both signals are used at the same time, it seems to become very noisy! is this as expected and should I attach an op-amp onto the input voltages and control their flow in an alternating fashion?

Thanks for any information you can offer.

TC


#2

About header, if you need volume (hundreds/thousands) then you can contact GHI for custom boards or special needs.

About analog, no you can’t read both at same time but the time in between is too small to note in most cases. Also, there will always be noise in any system. It is the designer job to minimize the noise and there are many methods to do that. I have personally designed a 16bit ADC board in the past where the design was done in one day but then spent 2 weeks or more on making changes to reduce noise!!


#3

Thank you Gus for your reply. So I need to alternate the load between pins, making a reading at each iteration? Could you offer any recommendations as to the best way to do this?

Many thanks again

TC


#4

Hi Gus, a quick question based on your reply above. What is the result of reading in two loads at the same time? What causes it not to work?

Many thanks in advance.

TC


#5

How would you read both at the same time?

ADC1.Read();
ADC2.Read();

right?

you can’t do
ADC1+ADC2.Read();


#6

Hi, ah, I see… We are taking about two different issues I think. What I meant by read was the physical part of the board. Such that both pins An1 and An3 have wires that continuously feed them with varying voltages at the same time. When either one wire is connected (on it’s own) and read, it gives the expected data but when both are connected and read the voltages seem to bleed into each other.


#7

Still not possible, the chip internally has only one ADC and all pins are connected through a MUX.

You can connect 2 ADC chips externally and read them that way.


#8

That is a shame but thank you kindly for your assistance.

TC


#9

There is a settling time for the ADC (should be in the LPC manual). If you switch the channel and wait (or take X number of readings in a row) the value should stabilize as long as you don’t have too much cross-talk on the board.


#10

I had a similar issue with a multiplexor that was supposed to have a 10 nanosecond response time, but until I added a Thread.Sleep(1) between reads I got bleedthrough noise. If your app can stand a 1 millisecond delay it might help.