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Topic: analogreference and resolution question (STILL NEED HELP) (Read 1 time) previous topic - next topic

jcarabet

Jul 27, 2011, 04:54 am Last Edit: Jul 28, 2011, 12:24 am by jcarabet Reason: 1
Hi everyone,

Im new to arduino and I wanna know something about analogread and reference. I have an input from a shunt that measures amps of a circuit.  It outputs a value between 0 and 50 mV.  I wanna use the analogreference and the aref pin to lower my upper limit to 50 mV, So then I can read in raw voltage between 0 and 50 mV with 1023 steps because its 10 bit?

Is there a way to set aref to 50 mV using a 5 volt power supply?

Also, do I need a resistor in the analog input? safety? or a diode? im not sure.

Thanks

MarkT

No, you cannot use Aref below 1.1V - you need to amplify your signal before conversion.
[ I won't respond to messages, use the forum please ]

Grumpy_Mike

There is an internal 1.1V reference voltage you could use. However you can apply a lower external voltage than this but there is little point because then you get into the noise of the system and the values returned in the least significant bits are meaningless. This is true of all A/D converters when you use them with lower than the recommended reference voltage.

jcarabet

Ok, so how would I amplify the signal coming out of the shunt? And just for practice, how do I lower the 5V pin to around 100 mV for the aref pin.

Thanks

Grumpy_Mike

Use a non inverting operational amplifier.

Not sure where you get 100mV from. The 1.1V reference is built in and switched to by software.

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