ADXL001

Rev. A | Page 13 of 16

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

CLOCK FREQUENCY SUPPLY RESPONSE

In any clocked system, power supply noise near the clock

frequency may have consequences at other frequencies. An

internal clock typically controls the sensor excitation and the

signal demodulator for micromachined accelerometers.

If the power supply contains high frequency spikes, they may be

demodulated and interpreted as acceleration signals. A signal

appears at the difference between the noise frequency and the

demodulator frequency. If the power supply noise is 100 Hz

away from the demodulator clock, there is an output term at

100 Hz. If the power supply clock is at exactly the same frequency

as the accelerometer clock, the term appears as an offset. If the

difference frequency is outside the signal bandwidth, the output

filter attenuates it. However, both the power supply clock and

the accelerometer clock may vary with time or temperature,

which can cause the interference signal to appear in the output

filter bandwidth.

The ADXL001 addresses this issue in two ways. First, the high

clock frequency, 125 kHz for the output stage, eases the task of

choosing a power supply clock frequency such that the difference

between it and the accelerometer clock remains well outside the

filter bandwidth. Second, the ADXL001 has a fully differential

signal path, including a pair of electrically isolated, mechanically

coupled sensors. The differential sensors eliminate most of the

power supply noise before it reaches the demodulator. Good

high frequency supply bypassing, such as a ceramic capacitor

close to the supply pins, also minimizes the amount of interference.

The clock frequency supply response (CFSR) is the ratio of the

response at the output to the noise on the power supply near the

accelerometer clock frequency or its harmonics. A CFSR of 0.9 V/V

means that the signal at the output is half the amplitude of the

supply noise. This is analogous to the power supply rejection

ratio (PSRR), except that the stimulus and the response are at

different frequencies.

POWER SUPPLY DECOUPLING

decouples the accelerometer from noise on the power supply.

However, in some cases, particularly where noise is present at

the 1 MHz internal clock frequency (or any harmonic thereof),

noise on the supply can cause interference on the ADXL001

output. If additional decoupling is needed, a 50 Ω (or smaller)

resistor or ferrite bead can be inserted in the supply line.

Additionally, a larger bulk bypass capacitor (in the 1 μF to

ELECTROMAGNETIC INTERFERENCE

The ADXL001 can be used in areas and applications with high

amounts of EMI or with components susceptible to EMI emissions.

The fully differential circuitry of the ADXL001 is designed to be

robust to such interference. For improved EMI performance,

especially in automotive applications, a 1000 pF output capacitor is