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A/D with the FEZ Mini


#1

Hi folks.
Just trying to get my head around some things here. I’ve normally used chips/modules that ran directly on 5V and so if the have an analogue output it would be between 0 and 5v(approx)

On many of them the analogue out volts would be from 0 to 4.7v - but the FEZ mini is a 3.3v device.

Can the A/D compensate for the higher voltage and if so what value would I be reading via the A/D i.e. 4.7V > 3.3v so reading 255 for anything above 3.3? or will be 4.7V = 240

Many thanks
Dave


#2

you can’t messure past 3.3V. I don’t shure, but you can hurt you mini if you pass 5v to analog.

Luckily there is simple solution for you: put a resistor between mini and your analog output, and a second resistor (exactly double in resistance) wich is connected to mini analog pin and ground. It should make your analog output 0-3.3V from 5V.

100k and 200k should do the job


#3

Ok… and thanks for the info.

My main issue is that I have a few circuits that use voltage dividers and I really don;t want to remake them again

Take Care
Dave


#4

BTW Marc, FEZ is 10 bits, not 8. So maxed out is 1 024, not 255.


#5

Ah… Many thanks for that MarkH… Will make working some things out easier.

Just out of curiosity - how fast is a standard A/D reading?. For some processors it can be quiet a load.

Take Care
Dave


#6

According to http://www.tinyclr.com/forum/14/943/ :

Panda uses the same USBizi as mini.


#7

all FEZes are very similar/identical in that area


#8

I meassured 49.5us per analog read without scaling.

Also, take into account the input impedance of the ADC, I’m not familiar with nxp Specs but input impedance in microcontrollers are in the range of 20 to 100kohm so using 100k and 200k to make a voltage divider would be wrong.