When using ADXL355 in EXT_SYNC = 10, EXT_CLK = 0 — External Synchronization With Interpolation modeADXL355

Question

When using the ADXL355 in EXT_SYNC = 10, EXT_CLK = 0 &mdash; External Synchronization With Interpolation mode, is it possible to set the following: Name: Filter, Address: 0x28, Data: 0x02 (1000 Hz and 250 Hz settings)? (If EXT_SYNC = 01, ODR is fixed at 4KHz, but if EXT_SYNC = 10, EXT_CLK = 0, is it correct that ODR can be set to a frequency other than 4KHz?) 
 
 Currently, the ADXL355 has the following settings: &bull; Name: Range, Address: 0x2C, Data: 0x82 (4G setting) &bull; Name: Filter, Address: 0x28, Data: 0x02 (1000 Hz and 250 Hz settings) &bull; Name: Sync, Address: 0x2B, Data: 0x02 (EXT_CLK=0, external sync, interpolation filter) 
 We are seeing an output that looks like aliasing noise around 600Hz, and we would like to confirm whether the filter is working. We apologize for the inconvenience, but we would appreciate your assistance.

MarioSM · Accepted Answer

Hello UBOTI let's address each question by parts: 
 UBOTI said: When using the ADXL355 in EXT_SYNC = 10, EXT_CLK = 0 — External Synchronization With Interpolation mode, is it possible to set the following: Name: Filter, Address: 0x28, Data: 0x02 (1000 Hz and 250 Hz settings)? 
 Yes, it is possible. You can still configure the ODR to any desired available setting at the 0x28 reg. while using any of the external sync. modes. With no external sync or interpolation (EXT_SYNC = 00, EXT_CLK = 0) but specifically no interpolation filter the bits 0-3 of the reg 0x28 will define the output data rate and LPF cutoff (or samples per second -- SPS) at which the sensor delivers info taking as reference its internal oscillator. On the "External Synchronization and External Clock, No Interpolation Filter " (EXT_SYNC = 01, EXT_CLK = 1) the reg 0x28 will also define the SPS, but this rate will be synchronized with the last SYNC signal. (as soon as you match the external clock conditions) While, when using any of the EXT_SYNC = 10 (interpolation enabled) the selected ODR at the reg. 0x28 will translate on how much extra resolution does the interpolation filter allows for the external SYNC signal. (64x selected ODR at 0x28 reg.) 
 UBOTI said: (If EXT_SYNC = 01, ODR is fixed at 4KHz, but if EXT_SYNC = 10, EXT_CLK = 0, is it correct that ODR can be set to a frequency other than 4KHz?) 
 There is no forced internal ODR for any of the EXT_SYNC = 01 and EXT_SYNC = 10 configurations, you can freely use any of the available settings at reg 0x28. 
 UBOTI said: I have an additional question: If the ODR is set to 1KHz or less (e.g., 250Hz) at address: 0x28, what happens if an external synchronization signal is input at 1KHz? 
 Estimated 1: Output at 1KHz. (Address: 0x28 is only valid for LFP settings) Estimated 2: Data output at 250Hz. 
 As stated in the ADXL354/ADXL355 (Rev.D) (page 33): (applies besides EXT_CLK =0) Your SPS would match the 1Khz (1Khz < 4Khz max SPS) external SYNC. (the interpolator allows such rate; 64x250 Hz = 16K Hz > 1k Hz) Keep also in mind that for all the 0x28 reg configs you are also fixing the cutoff frequency of the LPF. Thus, even if you get the 1KHz SPS your passband will still be the 62.5 Hz that matches with the 250 Hz ODR config. hope this helps, thanks for posting! Mario SM)

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When using ADXL355 in EXT_SYNC = 10, EXT_CLK = 0 — External Synchronization With Interpolation modeADXL355

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