FXTH87EH11DT1 Pressure Reading Discrepancy

[kumaichi]

Hello, I'm using a FXTH87EH11DT1 and the sample CW application contained in the starter package.  With the device sitting on a desk with 584 feet of elevation, I get a reading of 132 kPa.  If I place the device in a pressure container and apply 100 kPa, the FXTH87E reads 297 kPa.  I'm using the transfer equation listed in the datasheet (FXTH87EK116):

Transfer Function: P kPa = ( 2.80 kPa/LSB * PCODE ) + 87.20 kPa

kPa = (2.80 * 0x10) + 87.20

kPa = 44.8 + 87.20

kPa = 132

I have verified the value being sent by the FXTH87E is what is being received (0x10), while sitting on a desk.  Is this the incorrect transfer function or maybe my calculations are incorrect?

Any insight would be greatly appreciated.

Kindest regards

[TomasVaverka · Accepted solution]

Hello Craig,

The transfer function you are using is incorrect.

Please use the following transfer function valid for the FXTH87EH11DT1:

P [kPa] = (1.69 kPa/LSB * PCODE) + 69.7 kPa

With PCODE = 0x10 = 16 => P = 96.74 kPa.

BR, Tomas

[kumaichi]

Hello Tomas,

Thank you for your reply.  May I ask where you found that transfer function?  I need to calculate the Z and X accelerometer values next and want to make sure I'm using the correct functions.

Kindest regards

Craig

[TomasVaverka]

Hello Craig,

The transfer functions are in the datasheet which does not seem to be available on the website. I do not know why, let me check and come back to you.

For Z-axis (-215 to +305g range):

Offset Step 6 Transfer Function: A g's = ( 0.144 g/LSB * ACODE ) – 36.5 g

For X-axis (-80 to +90g range):

Offset Step 7 Transfer Function: A g's = ( 0.056 g/LSB * ACODE ) – 14.2 g

BR, Tomas

[kumaichi]

Thank you, Tomas.  That would be a great help.

Kindest regards,

Craig

[TomasVaverka]

Hello Craig,

I was informed that the FXTH87EHx datasheet is shared only via email under NDA, so please contact your NXP Sales representative or distributor. Also please note that the FXTH family is on End Of Life plan so it is not recommended for new designs. NXP recommends using the NTM88 family instead.

BR, Tomas

[kumaichi]

Hello Tomas,

Yes, thank you!  The go forward is to use the NTM88, these were a bunch of legacy units I was learning on.  Thanks for your help and quick responses.

Kindest regards,

Craig