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Slow turn-on time of e.g. ADXL103/203?

The ADXL103/203 (1 or 2 axis accelerometer) block diagram is shown below.

The turn-on time is listed as 20ms. A footnote says “Turn-on time is approximately 160 × CX + 4 ms, where CX is in μF” – i.e. part of that 20 milliseconds is due to the time needed to charge that external capacitor CX.

However this doesn't explain the 4 milliseconds. Where does this come from? The AC Amp, Demod IC, Output Amp, all should turn on 1000x faster than that right? Why the 4 milliseconds? I need to understand the ultimate turn-on speed limitations.

Secondly, what's the fastest turn-on time capacitive MEMS accelerometer out there?

  • The XL203 is designed to work over a wide temperature range with low offset tempco.  The bias circuitry for the capacitive sensor has a long time constant.  That time constant is longer at cold temperature as well.  The part is functional more quickly, but may be slightly less accurate until the bias is settled. 

    What start up time do you want?  Have you considered a part like the XL362 that can remain operating continuously at low power?  Bandwidth and noise may constrain the start up time.

  • Thanks tdave. By "the bias circuitry for the capacitive sensor has a long time constant" do you mean (looking at the diagram I posted) the "AC AMP" or the "OUTPUT AMP"?

    I understand the RC time constant on the output (The Rfilt and the CX on the diagram above) is millisecond timescale but want to know how much the amps themselves contribute to the turn-on time.

    I looked at the XL362 and we may use that, but I'm really trying to understand the limits of capacitive sensing here. i.e. in a nutshell: if you must sense differential capacitance as I assume you are here, does it necessarily follow that you must have amps that must have millisecond turn-on times?

    Thanks

  • Regarding the 4ms start up time.  That is a limitation of this design.  The input of the ac amp has to have a DC bias.  The sensor is a pure capacitor.  The bias circuit has up to 4ms settling time.  It is slowest at cold temp.

    Other parts we make have faster startup.  The XL335 may be a bit faster.

    Could you please say what turn-on time you need? Practically speaking, the sensor resonant frequeny is 5kHz and takes time to settle.  There is a filter built into the demod as well. 

    Best Regards

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