# What does "heavily decoupled voltage source" mean in this context?

Regarding the following paragraph:

To measure the amount of input-referred noise, the input of the ADC is either grounded or connected to a heavily decoupled voltage source, and a large number of output samples are collected and plotted as a histogram (referred to as a grounded input histogram if the input is nominally at zero volts).

What is meant by "heavily decoupled voltage source"? Could you provide a circuit to illustrate it?

• A very stiff voltage source, capable of sinking and sourcing great currents at high speed without apparent change in voltage level. Apr 24, 2019 at 8:00
• like an analog DC input with an RC filter? Apr 24, 2019 at 8:03
• I would say no. Unless the C part is very large, your R is not helping here. I'll try to do a schematic of an answer. Apr 24, 2019 at 8:20
• It means to place a capacitor directly at the input to the ADC. Any ground noise is to be avoided. Turn off the MCU during the ADC conversion. And insert a resistor in the voltage path, to implement a 16Hz low-pass-filter; thus 100uF cap needs 100 ohm resistor to whatever is sourcing that voltage. Apr 24, 2019 at 16:48

The "heavily decoupled" is a subjective qualifier, which basically translates to saying the source should be trusted to have a constant output. The method described for noise calculation probably assumes the voltage source does not contribute to any additional noise.

A "weakly decoupled" source, in this context, means a source which is prone to noisy outputs. This would negatively affect the noise assessment of the ADC.

The term "decoupled" is related to isolating the output from the mains power supply, or any possible adverse features from mains (e.g. voltage spikes), or any external interference overall. This is associated with the use of some common design methods, such as decoupling capacitors and overall board design.

For a mental exercise, pretend this is the output of a voltage source, with an inadequately picked decoupling capacitor. This could be a "weakly decoupled" source:

simulate this circuit – Schematic created using CircuitLab

While this would be a "heavily decoupled" one, in comparison:

simulate this circuit

This is just a silly example, decoupling concerns in real power supplies go beyond just output capacitors.

• Any practical example or diagram? Apr 24, 2019 at 8:14
• Any (respectable) voltage source schematic will include decoupling capacitors between output and gnd. This is an example, but decoupling also includes avoiding parasitic effects of the final design, which would include trace separations and metal planes. However I fear this will go too beyond the scope of your question. Apr 24, 2019 at 8:17
• Could you provide a circuit to illustrate it? part of the question Apr 24, 2019 at 8:18
• is this what you mean i.stack.imgur.com/IwMoP.png ? Apr 24, 2019 at 8:19

Something like this. You want your voltage to be a flat line regardless of any high frequency current consumption down the supply line. This is decpuoling.

simulate this circuit – Schematic created using CircuitLab