Arduino with External Voltage Reference

I am planning on using a 4.096V voltage reference with an Arduino, and I have a few questions:

Let's say I'll connect a MAX6126AASA41+ (4.096V) voltage reference to an Atmega1284's VREF pin, and I add a analogReference(EXTERNAL) statement on my setup() call.

Is it correct to say that an analogRead() call that returns 1023 will be equivalent to 4.096V on that analog pin? analogRead() = 511 is almost 2.048V, right?

What if, in that scenario, there is 5V on an analog pin? What problems will that cause, if any?

Thanks in advance.

Alex

Is it correct to say that an analogRead() call that returns 1023 will be equivalent to 4.096V on that analog pin?

Yes.

What if, in that scenario, there is 5V on an analog pin? What problems will that cause, if any?

Smoke happens.

If your analog signal is higher then ARef, I don't know if something will fail internally, or if you just max out the reading and get 1023 back for anything higher than Aref.
I think there will be no problem as long as the input does not exceed VCC +0.5V, which is when the input clamp diodes would kick in.

Datasheet states something like all voltage greater or equal Vref is coded like 1023. I doubt smoke will happen or protection diodes will have any work.

Thank you for the reply, folks.

CrossRoads:
I think there will be no problem as long as the input does not exceed VCC +0.5V, which is when the input clamp diodes would kick in.

Smajdalf:
Datasheet states something like all voltage greater or equal Vref is coded like 1023. I doubt smoke will happen or protection diodes will have any work.

That's what I am hoping.

Actually the excellent MAX6126AASA41+ has been kind of unnobitanium around here, and even although I had already placed an order for 3 units on a local retailer who said had them in stock, they called me today and asked me if I would settle for an ISL21009DFB841Z (new old stock). It was either that or nothing. The problem with the ISL21009 they have in stock is that it isn't even the best grade (ISL21009B = 0.5% innitial accuracy), but the worst of them (ISL21009D = 2.0%). I accepted and I'll get 3 of them in 2 days.

Although affordable, they aren't exactly cheap, but at least they are a fraction of a LTZ1000A!

In my case it had to be something in SOIC (or *SOP), so I can use my SOP-to-DIP adapter to try them out on a breadboard before etching the actual PCB, soldering, etc. Apparently most of the new affordable VREFs are only in SOT-23 packages, which are a PITA to solder and breadboard.

The project I am working on is a programmable dummy load, and although I've already built it on DipTrace, I am now sourcing the parts and building on a breadboard to then make it on a homemade PCB.

Whatever reference you get, measure the voltage with a precision voltmeter, and calibrate to that..

regards

Allan

allanhurst:
Whatever reference you get, measure the voltage with a precision voltmeter, and calibrate to that..

regards

Allan

Thanks, Allan. The best DMMs I have are a Fluke 17B+ and a Fluke 117. The 17B+ is better for measuring mV, while the 117 is better for much higher voltages and AC. They are far from being one of those fancy Fluke 87 V, but they are what I could afford, since I bought them both with a 45% discount on a Farnell closeout sale in Brazil (they've ended operations here a year ago).

Could be easier/cheaper/better to use an external ADS1xxx A/D converter with inbuild 4.096volt ref.

Leo..