ADXL366 datasheet(20/71 Pages) AD

20 / 71 page

Data Sheet

ADXL366

THEORY OF OPERATION

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Rev. 0 | 20 of 71

EXTERNAL ADC

In addition to a built-in accelerometer and temperature sensor, the

ADXL366 incorporates an additional 14-bit ADC input for digitiza-

tion of any external analog signal. This additional ADC input is

sampled at the same ODR as the acceleration and temperature

data.

Use of the ADC adds approximately 50nA to the total current con-

sumption when operating at 100Hz ODR. The ADXL366 enables

the user to power down the ADC to save power when it is not

required.

The external ADC specification can be found in Table 1.

Analog Inputs

The ADXL366 ADC can convert analog inputs ranging from 0V

(GND) to VREG_OUT, and the input range of the external ADC is

limited to a maximum of 1V by the internally regulated voltage.

Figure 43 shows the typical ADC output vs. input voltage. Notice

that the ADC data format is signed and the input signal is single

ended with respective to VREG_OUT/2; therefore, the code equal to

0LSB is approximately 0.53V.

Figure 43. External ADC Output vs. Voltage Input

Figure 44 shows an equivalent circuit of the input structure of the

ADXL366. The two diodes, D1 and D2, provide ESD protection for

ADC_IN.

During the acquisition phase, the impedance of ADC_IN can be

modeled by the series connection of input resistance (RIN) and

input capacitance (CIN). RIN is typically 20kΩ and is a lumped

component made up of some serial resistors and the on resistance

of the switches. CIN is typically 650fF and is mainly the ADC

sampling capacitor.

To calculate the acquisition time (tACQ) required use the following

formula:

tACQ = 10 × ((RSOURCE + RIN) CIN)

(1)

where RSOURCE is the source impedance.

For 14-bit settling, tACQ must be less than 15µs. tACQ) sets an upper

limit on RSOURCE of approximately 2MΩ. RIN and CIN make a 1-pole,

low-pass filter that reduces undesirable aliasing effects and limits

the noise.

Figure 44. Equivalent Analog Input Circuit

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