Teensy 3.6, AREF, 470Ohm and VREFH

[storm]

Hello,
im a big Teensy fan, here my first question.
This schematic of the Teensy 3.6 implies that i should measure no resistance between the AREF pin and VREFH. VREFH i think i can get at one of those two 100nF capacitors, point 2 in my uploaded picture.
Also i should read 470Ohm between point 1 and 2. Point 1 is the output of the LP38691.

Actually i measure 470Ohm (component A) between VREFH (point 2) and the AREF pin, and 0Ohm between point 2 and 1. So either im wrong in measuring VREFH at point 2, or the schematic is wrong in this point (i asume it to be the first case).

My goal is to usa a REF1930 as external reference for the internal Teensy ADC, and i thougt i have to remove the 470Ohm resitor to disconect VREFH from the supply voltage.

Second question, could i lower the ADC noise by increasing the capacitance infront VREFH by a factor of for example 10? Is the DAC getting his voltage reference directly from the internal reference with no way to improve its noise performace?


Greetings,

storm

 

[UhClem]

The internal voltage reference output is on VREF_OUT and the data sheet says that the capacitor should be 100nF +/-25%. I wouldn't exceed that because it might overload the reference buffer amp.

VREFH is for an input for an external reference so hanging a bigger cap off of that will do nothing for the internal reference.

The REF1930 can sink or source up to 20mA so the load from the 470 Ohm resistor will not bother it.

 

[storm]

I figured my mistake. The two capacitors at point 2 are not the 100nF, but should be two of the four on VDD. I thought otherwise, because they are directly next to those two 33Ohm resistors for the micro-USB lines, like the 100nF capacitors in the schematic. Two two 100nF capacitors are the one above and to the left of resistor A. I now removed the 470Ohm, works great with REF1930 reference.

AREF is going directly into VREFH. Isnt it beter to remove the 470Ohm Resistor, otherwise the 3.3V supply and AREF go into VREFH at the same time? Yes VREFH is the external reference input for the ADC, but also for the DAC?