With the MMA8451 Self Test output change for the Z axis.

[johncampbell]

 The nominal value is  +1680. (Table 2 page 6 of the Rev10 datasheet)

There is no minimum and maximum value specified in the datasheet .

What is the expected tolerance on the +1680 value?

I am particularly interested in the variation at low temperature. -40c

regards John Campbell

[david_diaz]

Hello John,

Please keep me updated about your project.

Let me know if you have any further question. I am glad to help.

Regards,

David

[johncampbell]

David,

Thanks for the fast response.

We are using the test mode to confirm the 8451 is operating while testing over the full temperature range.

We have one device that the step change is around 850 at -40C.

From your response I will assume that this value is not unexpected.

Finally do you have any test reports at other than +25C?

best regards johnc

[david_diaz]

Hello John,

I am afraid that the last report is all the information that I have. As I mentioned, there is no a specific value because the self-test output is not trimmed and its purpose is to just check if the transducer is moving and the signal chain is functioning.

So, if you are experiencing step changes at 25 degrees using the Self-Test mode, I assume the device works correctly due to it is guaranteed to work from -40 to 85 degrees.  

Please let me know if you have any further question. I am glad to help.

Regards,

David

[johncampbell]

David,

Thanks for replying.

The unit is a shock and tilt alarm for an automotive customer.

It’s working OK in normal mode alarm mode over the full temperature range.

We have a self test/ production end of line test mode where we test all functionality of the hardware and compare normal and test mode accelerometer values.

We are also using this mode during our Product Validation Testing as the temperature chambers all have mechanical noise that affect the sensor readings.

Just as a go , no go test before and after other tests

We think several units have a smaller step than others at the minimum temperature.

I had the test window at +1000 in test mode. Some work OK, others we think are less than this value when you get below -35C.

We will keep investigating

Thanks again for your response.

Regards johnc

[david_diaz]

Hello John,

Thank you for writing.

Please note that there is no a specific value because the self-test output is not trimmed and its purpose is to just check if the transducer is moving and the signal chain is functioning.

In this case, a self-test data report is attached as a reference for you.

I hope this information helps.

David