AN217 データシートの表示（PDF） - Silicon Laboratories

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AN217

C8051F35X DELTA-SIGMA ADC USER’SGUIDE

Silicon Laboratories 

AN217

SENSOR

EMI Induced

Noise will appear

in measurement

Ground Noise

Vnoise

ADC

C8051F35x

Figure 5. Single-Ended Measurements and Ground Circuits: Poor Performance

SENSOR

Twisted Wire Pairs

Shielded

(Ground to One Side)

ADC

C8051F35x

Figure 6. Differential Measurements Reject Common-Mode Noise

Figure 5 shows a sensor connection for a single-ended measurement, in which the sensor and ADC systems do

not necessarily share a common, low-impedance ground connection. This allows more noise due to ground

variations and EMI to appear in the ADC measurement.

Figure 6 illustrates a method which rejects more noise. The ADC rejects common-mode noise, that is, noise that

appears equally on both AIN+ and AIN– in a differential measurement. In effect, EMI-induced and ground noise is

better cancelled when a differential measurement is made. The idea is to have all unwanted signals the same on

both AIN+ and AIN–. Twisted sensor wire pairs helps to keep the noise voltage the same on the AIN+ and AIN–

pins. Shielding is used to reduce EMI-induced noise on the wires. When shielding is used, it should be electrically

connected to (chassis) ground and at only one place in the system (to prevent ground current flow). Take care to

match the input impedances at AIN+ and AIN– to preserve common mode rejection.

12

Rev. 0.2

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