I am analyzing a circuit which uses an microcontroller to measure analog signals in digital form. I don't understand the function of one connection in the circuit.

See the figure. Here Vref of the MCU is connected to ground via two parallel capacitors. This microcontroller is chinese-made so its datasheet is available, but not very detailed.

The datasheet shows that Vref is internally connected to VDD/VCC which is 3.3 V, though I am not sure, because I think it should be zero, because it compares an input analog signal to detect its +ve and -ve cycle.

According to the datasheet Vref is an external voltage input pin.

The capacitors in question are in the red rectangle.

I need to know these facts:

enter image description here

What would be the function of the capacitors if Vref is internally at VDD/VCC?

What would be the function of the capacitors if Vref is an input pin and it is grounded via these capacitors?

  • 7
    \$\begingroup\$ Perhaps the processor has an internal regulator to generate the VRef, and it needs some capacitance to become stable or filter the output. This is a question for the manufacturer of the processor really though. \$\endgroup\$
    – Colin
    Jul 20, 2018 at 9:43
  • 1
    \$\begingroup\$ To me, reading this question, you are talking about a thing that has no part number and no link to a data sheet. All we know is that there is a Vref pin. Put yourself in my shoes - if you were me what would your action be? \$\endgroup\$
    – Andy aka
    Jul 20, 2018 at 9:51
  • \$\begingroup\$ "if Vref is internally at VDD/VCC" It is possible that the "manufacturer" didn't understand the purpose of this pin when making this bootleg IC, since just an extra cap on power rail doesn't make much sense without connecting it to external power rail. \$\endgroup\$ Jul 21, 2018 at 2:38
  • \$\begingroup\$ That is the problem.. as i have said. the controller is locally made, and doesn't give proper info like internal structure of Vref, where it is connected internally. \$\endgroup\$ Jul 23, 2018 at 11:04

3 Answers 3


Vref usually means the reference voltage output of the built-in ADC (or DAC, or analog comparator). A reference voltage circuit is basically a very low current voltage regulator (a poor power supply, but very stable).

Components like ADCs are "using" voltage from this regulator (drawing current). Capacitance on the output of regulators helps to improve stability and reduce noise.

You can think of those capacitors like regular decoupling capacitors on the output of an LDO.


Many MCU and other digital ICs do have embedded voltage regulators, usually linear LDO type, for various functions. The regulators may feed internal core at lower voltage (saving power or to meet internal CMOS node specifications), can provide standard 3.3V power from 5-V input for, say, USB PHY, or, as in this particular case, the regulator provides some reference voltage for ADC or whatever. The embedded LDOs save product designers cost, eliminating many external components.

However, most LDO regulators need a sizable load capacitor to be stable, usually with certain ESR, usually carefully specified in datasheets. These big capacitors can't be implemented using silicon technology, they are too big for that, so they must be connected as external components. Typically these voltage outputs have not much extra capacity, and are not intended to be loaded with something else, with some exceptions. That's why you frequently see some MCU pins with only capacitors attached, and nothing else.

  • \$\begingroup\$ Right, so ths vref should be internally connected to VCC/VDD or LDO, and capacitors are there for stability. This Vref goes into internal comparators to present reference to oncoming analogue signals.. \$\endgroup\$ Jul 23, 2018 at 11:09

These might be filter capacitors. Usually they are used to improve a digital signal. For example, you could see how capacitors work in the image below:

enter image description here

Without capacitor we would have a wave rather than a "line" so it could help to stabilize voltages.


Not the answer you're looking for? Browse other questions tagged or ask your own question.