Getting a stable reference voltage from the Teensy 3.1

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AKA

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Hi, I have looked around but have not found a best-practice way to generate a supply-independent reference voltage from the Teensy3.1

I'm interfacing with an LT1990 diff amp (datasheet: http://cds.linear.com/docs/en/datasheet/1990fb.pdf) and basically implementing the "Telecom Supply Current Monitor" example circuit on the datasheet's last page, and hoping to avoid the whole LT6650 subcircuit.

My ideal voltage reference is 1.25V, as I will be driving the LT1990 with regulated 5v

Is there any way to, say, access the bandgap reference on the MK20DX256VLH7?

If not, is making a trimmable voltage divider the best way to get a stable reference below my supply voltage?

Thanks in advance for any help or insight you can provide!

AKA

(PS, First post! Been using the Teensy3.1 for a year now, and it's just a delight - nice work!!)
 
You can reference the internal bandgap voltage by setting the reference to internal. However, I doubt you could get that to go to the AREF pin, even if you removed the 470 Ohm resistor that goes from the analog power supply to AREF.

I'm not quite sure why the removal of the LT6650 microcircuit is so desirable. You have a verified solution to start with. Plus, you could always run the output into AREF as well (though this would likely require the removal of the 470 Ohm resistor next to the RTC pins).
 
Thanks for the quick reply!

I was hoping to avoid the LT6650 for cost/complexity reasons, but you're right about the (dubious) wisdom of abandoning a verified solution.

I've ordered the LT6650 but in the meantime I am curious enough to try a simple trimpot voltage divider between my 5v reference and the LT1990 circuit...

Thanks again,

AKA
 
You are correct! Go for pin 17 or the capacitor attached to it to measure the internal AREF. But that output is not available to users by default (i.e. no pad or hole to hook up to). Attaching a wire to the cap or the pin can be done, but you need a steady hand!

But if you roll your own Teensy then the output may become an adequate substitute for the LT6650. However, I would still use the LT6650 and run its output into the AREF pin instead, removing the 470 Ohm resistor that goes from the 3.3V Analog voltage supply bus to AREF along the way.

EDIT: Just looked up the pin... may not be impossible to jumper off of due to its location on the edge of the chip... so while not as challenging as a pin in the center, it still requires a steady hand.
 
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Thanks for the additional tips! I'm going to go with the external circuit, but it's good to know that there's a pin onboard the Teensy 3.1 that I can get to if I need to (and haven't had too much coffee).

Thanks for the quick help!

AKA
 
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