Data Sheet

ADXL358

APPLICATIONS INFORMATION

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Rev. 0 | 13 of 16

POWER SUPPLY DESCRIPTION

The ADXL358 has four different power supply domains: VSUPPLY,

V1P8ANA, V1P8DIG, and VDDIO. The internal analog and digital circui-

try operates at 1.8 V nominal.

VSUPPLY

VSUPPLY is 2.25 V to 3.6 V, which is the input range to the two LDO

regulators that generate the nominal 1.8 V outputs for V1P8ANA and

V1P8DIG. Connect VSUPPLY to VSS to disable the LDO regulators,

which allows driving V1P8ANA and V1P8DIG from an external source.

V1P8ANA

All sensor and analog signal processing circuitry operates in this

domain. Offset and sensitivity of the analog output ADXL358 are

ratiometric to this supply voltage. When using external ADCs, use

V1P8ANA as the reference voltage. V1P8ANA can be an input or an

output as defined by the state of the VSUPPLY voltage.

V1P8DIG

V1P8DIG is the supply voltage for the internal logic circuitry. A

separate LDO regulator decouples the digital supply noise from the

analog signal path. V1P8ANA can be an input or an output as defined

by the state of the VSUPPLY voltage. If driven externally, V1P8DIG

must be the same voltage as the V1P8ANA voltage.

VDDIO

The VDDIO value determines the logic high levels for the self test

pins, ST1 and ST2, as well as the STBY pin.

The LDO regulators are operational when VSUPPLY is between 2.25

V and 3.6 V. V1P8ANA and V1P8DIG are the regulator outputs in

this mode. Alternatively, when tying VSUPPLY to VSS, V1P8ANA and

V1P8DIG are supply voltage inputs with a 1.62 V to 1.98 V range.

OVERRANGE PROTECTION

To avoid electrostatic capture of the proof mass when the acceler-

ometer is subject to input acceleration beyond its full-scale range,

all sensor drive clocks turn off for 0.5 ms. In the ±10 g range

setting, the overrange protection activates for input signals beyond

approximately ±40 g (±25%), and for the ±20 g and ±40 g range

settings, the threshold corresponds to about ±80 g (±25%). When

overrange protection occurs, the XOUT, YOUT, and ZOUT pins on the

ADXL358 begin to drive to midscale.

SELF TEST

The ADXL358 incorporates a self test feature that effectively tests

the mechanical and electronic system. Enabling self test stimulates

the sensor electrostatically to produce an output corresponding to

the test signal applied as well as the mechanical force exerted.

Only the z-axis response is specified to validate device functionality.

In the ADXL358, drive the ST1 pin to VDDIO to invoke self test

mode. Then, by driving the ST2 pin to VDDIO, the ADXL358 applies

an electrostatic force to the mechanical sensor and induces a

change in output in response to the force. The self test delta (or

response) is the difference in output voltage in the z‑axis when ST2

is high vs. ST2 is low, while ST1 is asserted. After the self test

measurement is complete, bring both pins low to resume normal

operation.

FILTER

The ADXL358 uses an analog, low-pass, antialiasing filter to reduce

out of band noise and to limit bandwidth.

The analog, low-pass antialiasing filter in the ADXL358 provides a

fixed −3 dB bandwidth of approximately 1.5 kHz, the frequency at

which the voltage output response is attenuated by approximately

30%. The shape of the filter response in the frequency domain is

that of a sinc filter. While the analog antialiasing filter attenuates the

output response around and over its cutoff frequency, the MEMS

sensor has a resonance at 5.5 kHz and mechanically amplifies

the output response at around 2 kHz and over. These competing

trends are apparent in the overall transfer function of the ADXL358,

as shown in Figure 3 to Figure 5. Therefore, the overall −3 dB

bandwidth of the ADXL358 is 2.4 kHz, and the overall bandwidth

with ±4 dB flatness is about 4.4 kHz.

The ADXL358 x-axis, y-axis, and z-axis analog outputs include an

amplifier followed by a series 32 kΩ resistor and output to the XOUT,

the YOUT, and the ZOUT pins, respectively.