AN2834 datasheet(31/45 Pages) STMICROELECTRONICS

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AN2834

How to get the best ADC accuracy

It is also recommended to connect the analog and digital grounds in a star network. This

means that you must connect the analog and digital grounds at only one point. This

prevents the introduction of noise in the analog power supply circuit due to digital signal

switching. This also prevents current surges from affecting the analog circuit.

Using separate PCB layers for the supply and ground



Two-layer PCBs

For two-layer PCBs, it is recommended to provide a maximum ground plane area. The

power supply (VDD, VDDA) should run through thick tracks. The two layers can have

their ground shorted together via multiple connections in the overlap region if the two

layers feature the same ground signals. The unused PCB area can be used as the

ground plane.

The other convention is to connect the unused PCB area on one layer to the positive

supply (VDD) and the unused area on the other layer, to ground. The advantage is a

reduced inductance for power and ground signals. The maximum ground area provided

for ground on the PCB results in a good shielding effect and reduces the

electromagnetic induction susceptibility of the circuit.



Multilayer PCBs

Wherever possible, try to use multilayer PCBs and use separate layers on the PCB for

power and ground. The VDD and VSS pins of the various ICs can be directly connected

to the power planes, thus reducing the length of track needed to connect the supply

and ground. Long tracks have a high inductive effect. The analog ground can be

connected at one point to this ground plane. If so, it should be close to the power

supply.

A full ground plane provides good shielding and reduces the electromagnetic induction

susceptibility of the circuit.



Single-layer PCBs

Single-layer PCBs are used to save cost. They can be used only in simple applications

when the number of connections is very limited. It is recommended to fill the unused

area with ground. Jumpers can be used to connect different parts of the PCB.

3.2.13

Component placement and routing

Place the components and route the signal traces on the PCB so as to shield analog inputs.

Components like resistors and capacitors must be connected with very short leads. You can

use surface-mounted device (SMD) resistors and capacitors. You can place SMD capacitors

close to the microcontroller for decoupling purposes.

Use wide tracks for power, otherwise the series resistance of the tracks would cause a

voltage drop. Indeed, narrow power tracks have a non-negligible finite resistance, so that

high load currents through them would cause a voltage drop across them.

Quartz crystals must be surrounded by ground tracks/plane. The other side of the two-layer

PCB below the crystal should preferably be covered by the ground plane. Most crystals

have a metallic body that should be grounded. You should also place the crystal close to the

microcontroller. You can use a surface-mounted crystal.

AN2834 Datasheet(PDF) 31 Page - STMicroelectronics

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