AN217

Rev. 0.2

11

3. Hardware Design

Designing a low-noise, high-precision measurement system requires not only proper ADC configuration, but

careful circuit board design and interface considerations as well. For an in-depth discussion concerning proper

mixed-signal circuit board design, see application note, “AN203: Printed Circuit Design Notes.” AN203 covers

many topics concerning proper PCB design such as power supply, decoupling, ground circuit, component

placement, and routing of traces (layout guidelines). Also included is a checklist that can be used to ensure all

relevant items have been considered for the application.

Here is a list of notes concerning the use of the C8051F35x on a circuit board:

Use ground planes: Typically a minimum of 4-layers should be used. Connect components’ ground pins

directly to the plane. Use copper plane fill between traces on the signal layers, and connect the fill to ground

using vias (do not leave “floating” copper). Separate analog and digital ground planes can be used; connect

them in one place. Design a few locations for connection so that the best connection can be tested, usually near

the ADC or near the power supply to the board. See AN203 for details and recommended layer assignments for

ground, power, and signal planes.

Avoid ground loops: Unless careful isolation has been designed, all circuit boards and systems must share a

common, low-impedance ground connection so they have the same ground potential. This is especially true of

the sensor if not on the same board as the ADC. Do not connect a sensor’s ground or negative side wire to a

different ground plane than the C8051F35x’s. Connect both sensor wires to AIN+/AIN–, and make another

common ground connection between the boards (chassis ground) when applicable.

Place components carefully in order to control ground currents: All ground currents from each component

must return to the power supply. Place digital, noisy signals and their traces away from analog signals and the

analog input circuits connected to the ADC. Avoid ground current return flow in the area of the analog signal

path (e.g., digital components placed in such a way that return currents must flow by the anti-alias filters at AIN+

and AIN– pins will degrade performance). See AN203.

Keep analog traces away from digital traces: Component placement and orientation will help the designer

route digital traces away from the analog signals. Analog traces (including the voltage reference) must be kept

as short as possible.

Sensor input: The delta-sigma will reject noise (though not at the modulator sample frequency). However, care

should be taken to use a simple anti-alias filter and a high-quality amplifier (when applicable). Keep wires as

short as possible. Input impedance at AIN+ and AIN– should be matched to preserve common-mode rejection.

Reject common-mode and ground noise: Make differential measurements and use twisted wire pairs

(shielded if possible). EMI and ground noise that appears equally on both differential inputs to the ADC will not

appear in the measurment.