AREF is making me lose my hair

[Constantin]

I have a dumb, basic question:

When analogReference(EXTERNAL); is set in setup(), a external reference can be used to apply a desired voltage to the ADC AREF, right? I also tried using ADC0_SC2 = 0 but that didn't help either.

If so, why would my teensy force the applied voltage up from the 2.5V reference level to ~2.84VDC? When I remove the teensy 3, the reference voltage is beautiful and stable at 2.4980-2.502, re-socket the teensy and the voltage climbs back up to 2.84V despite using either of the above commands. What am I doing wrong?

 

[PaulStoffregen]

Teensy 3.0 has a 470 ohm resistor feeding current into the reference. You need to use a "shunt" type reference (which is the most common type) that can sink the current (approx 1.7 mA). A LM336 would be one possibility. There are many others on the market.

You didn't mention the specific chip you're using or other technical details, so this general advice is the best answer I can give.

 

[Constantin]

I use a LM4132a series reference, which served me well with the Atmel series of AREFs. It's not a shunt type. Argh!

In other words, I expected the Teensy to accept whatever external voltage was being applied vs. having to sink current. Thus, assuming I am desiring something similar to the LM4132 in terms of accuracy (0.05%) package and so on, I'd have to use a LM4030 instead? How much current is one allowed to use for other devices (as having a common reference seems like a good idea)? Or is one better off using a series reference like the 4132A for all devices other than the teensy?

Thank you and apologies for not bringing up more details up front.

 

[Constantin]

FWIW, another (perhaps obvious) solution for those of us in the same conundrum is to remove the interfering 470 Ohm resistor. This 0402 chip is located at the corner of the freescale chip, right by the RTC oscillator through-holes. I simply used one of my tools to mechanically remove part of the resistor, then used flux and a hot iron to clean up the pads. I now have consistent 2.5V happiness on the AREF pin.

Without the 470 Ohm resistor in place, there is no electrical connection between AVCC and AREF anymore, so any external reference voltage source can be used. Obviously, a shunt style solution is preferable (as it requires no modification of the Teensy 3) but this works in a pinch.

 

[JBeale]

By the way, if you are reasonably comfortable with a soldering iron, it is easy to remove an 0402 part using a single iron. Just first apply a large enough blob of solder to the tip of the iron so that you can wet both ends of the 0402 part at the same time. A few seconds of application to the part and a sideways swipe of the iron pushes the part away, and you are left with clean pads. The part is generally left sticking to the soldering iron tip. In my experience, mechanical removal risks more damage to PCB pads and/or traces.