In 2g mode, what is the offset values added to the measured accelerations?

[marcberger]

In the datasheet, these offset values are specified for the 4g mode but not for the 2g mode.

After a test, I don't find any correlation between the offset values given for 4g mode and those measured in 2g mode.

So, my question: In 2g mode, what is the offset values added to the measured accelerations?

Many thanks in advance for your replies.

Best regards,

Marc Berger

[TomasVaverka]

Hi Marc,

2g mode selftest output is 2x of 4g mode. 8g mode selftest output is a half of 4g mode. I have measured the following Z-axis output values at different sensitivities:

            ST=0     ST=1

+/-2g:    4108      8169

+/-4g:    2050      4081

+/-8g:    1027      2040

As you can see, the Z-axis is trimmed to deflect ~1g (positive).

Let me know if you have any other questions.

Regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct" or “Helpful”. Thank you.

[marcberger]

Hi Tomas,

First of all, thanks a lot for your quick answer!

Unfortunately, I did not fully understand…

When you say that the 2g mode selftest output is 2x of 4g mode, do you mean that the added offset is typically:

· 362 for the X-axis (instead of 181 as indicated in the MMA8451Q data sheet for the 4g mode)

· 510 for the Y-axis (instead of 255 as indicated in the MMA8451Q data sheet for the 4g mode)

· 3360 for the Y-axis (instead of 1680 as indicated in the MMA8451Q data sheet for the 4g mode)

But your measures is not inline with these data.

Indeed, in ST=0 for the 2g mode, the measured acceleration is 4108 and in ST=1, the value is 8169 ==> The difference is equal to 8169 – 4108 = 4061.

It means a difference higher than +20% ((4061 – 3360) / 3360) between the measured value and the expected one!

Is this difference still within tolerance? If yes, what is the minimum/maximum values for the offset added during the selftest?

Another question: why the selftest control should be ignored on the other axis?

As you can see, I did not fully understand… But, anyway, many thanks in advance for your reply.

Best regards,

Marc

De : Tomas Vaverka

Envoyé : lundi 9 mars 2015 13:25

À : BERGER Marc

Objet : Re: - In 2g mode, what is the offset values added to the measured accelerations?

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In 2g mode, what is the offset values added to the measured accelerations?

reply from Tomas Vaverka<https://community.freescale.com/people/TomasVaverka?et=watches.email.thread> in Sensors - View the full discussion<https://community.freescale.com/message/488094?et=watches.email.thread#488094>

[TomasVaverka · Accepted solution]

Hi Marc,

The reason why the Z axis self-test output does not correspond to the typical value specified in the datasheet is that the self-test is not factory trimmed. The internal spec is 50 to 1000 counts for XY and 50 to 6000 for Z axis. The purpose of the self-test for this product is to just check if the transducer is moving and the signal chain is functioning. Generally, the test is considered successful if the minimum magnitude of change (50 counts for all axes at ±4g mode) is achieved. I attached the characterization data for the reference.

Regards,

Tomas

[marcberger]

Hi Tomas,

Many thanks for your reply!

Everything is clear, now! ☺

So, I presume that the minimum counts in 2g mode is 100, right?

Sorry, but I didn’t find any attached file giving the characterization data?

Could you send to me this file?

I have an additional question: do you know the MTBF of the 3D-accelerometer MMA8451Q?

Again many thanks for your help, it was very useful!

Best regards,

Marc

De : Tomas Vaverka

Envoyé : mercredi 11 mars 2015 10:21

À : BERGER Marc

Objet : Re: - Re: In 2g mode, what is the offset values added to the measured accelerations?

<https://community.freescale.com/>

Re: In 2g mode, what is the offset values added to the measured accelerations?

reply from Tomas Vaverka<https://community.freescale.com/people/TomasVaverka?et=watches.email.thread> in Sensors - View the full discussion<https://community.freescale.com/message/489776?et=watches.email.thread#489776>

[TomasVaverka]

You are very welcome, Marc.

Your assumption is correct, it is 100 counts in ±2g mode. The previously attached file should be visible to everyone, but I will send it to you in another direct message. Regarding the MTBF, please create a SR.

Regards,

Tomas