Sum or Derivation? Revealing change on DC inputs.

[mastermind88]

I have an analog accelerometer that has 3 analog output, that give an 1.65 V DC output on zero-g and increase up to 3.3 V when subject to an acceleration.
I have to connect these output on a single analog input of a comparator on a PIC24FGA010, to sense the movement of my device.
How can i sum these output, sensing only the changing of one or more of the three outputs?


thanks

 

[WimRFP]

Hello,

I think you need a window comparator. I suppose you want to detect both positive and negative acceleration, so the voltage can also drop below 1.65V and then you want a trigger also.

You need three of this devices and put together via a wired OR or equivalent circuit.

There are other solutions with diodes. This can save some costs, but the analysis is more elaborate. A window comparator requires 2 opamps and some setting resistors.

You cannot just sum the outputs with an opamp summer as due to accelaration one axis may give less output and another may give more. Theoretically you could move your device without being detected.

What is the minimum output voltage deviation from 1.65V that you want to detect?

 

[mastermind88]

is at least +- 220mV

 

[WimRFP]

I assume that you already investigated the analog muliplexer option in combiantion with changing the comparator threshold (requries more SW resource in case of fast varying accelerator output).

Given the swing, I would go for the window comparator option (with LMV824 or equivalent opamp). When current consumption and costs are really an issue, it can be done with a transistor/diode solution.

You should check the drift of the 1.65V. When the required voltage does not change very slowly, you can derive the thresholds for the opamps from the 1.65V via an RC netwok that (slowly) tracks the output