MPXV6115V vacuum sensor offset question

[lotsoffun]

I am using the MPXV6115V vacuum sensor.

When no vacuum is applied I have a reading, after converting the Vout to kPa, of around +1 kPa, which is 10 hPa or 10 mbar (the value we must use).

So this looks as some kind of offset value. So I guess the sensor reads 1 kPa ~ 10 hPa 'too high' 

1. When vacuum is applied and the reading is, let's say 500 hPa vacuum (=50 kPa), I must subtract 10 hPa (=1 kPa) ?

2.Or is the offset part of the error/uncertainty ?

3. Is the reading of this absolute pressure sensor independable from the air pressure in the room, so is reading the same as when I am on sea-level, or when I am in place which has another air-pressure (or is much higher then sea-level).

hope someone can help

[lotsoffun]

I Studied that application note and tried to figure it out, but after a lot of experimenting I am still getting readings I do not expect.

Please think with me.

1. Device is the MPXV6115V
2. We need to measure the vacuum in a piston and we want to compensate for the offset error
3. First I measure the voltage out of the sensor with no vacuum applied and no piston connected, so in free air. This voltage is converted to pressure using the formula in the datasheet.
4. This formula is in kPa. We want the value in hPa so we multiply this value by 10 and this gives + 13,5 hPa.
5. Now we start to vacuumize the piston. It goes down to -500 hPa.
6. In my opinion the piston is 'vacuumized' 500 + 13,5 = 513,5 hPa. Because the 'starting point' at the start of the vacuumize process was 13,5 hPa.
7. Do you think this approach is correct, so the 513,5 hPa is the correct total vacuum ?

The problem is that the value we expect is around 10..15 hPa lower, which is very close to that 13,5 hPa 'starting' value.

[lotsoffun]

Thanks a lot. But it gives me with another question: I do use + 5.0 volt to power the device + the capacitors, but also use a 51K resistor on the output like the 6115 data sheet describes. From another post I understand that:

the difference with the 4115 family is an analog sensor with the output given in voltage, while the 6115 family (the latest family in the NXP market) is an analog sensor with the output given in current, so you need to make sure you add a 51k resistor on the output pin (check Figure 4 of the 6115 datasheet)

In our particular instruiment type 4115 was used before, but it has a long lead time and is not recomended for new designs. So we changed it to the 6115. But you do not mention that 51K resistor. We must we use that 51K resistor (we measure the output voltage), am I correct ?

[diazmarin09]

Hello lotsoffun,

Correct, the MPXA6115 have a current output voltage, so the external 51k pull-down resistor (from Figure 4 of the datasheet) is needed to convert the current into a voltage signal.

I hope this information will help. Please let me know if you have any further question.

Best Regards,

David Diaz

[diazmarin09 · Accepted solution]

Hello lotsoffun,

Please review the information below in regard with the first two questions:

Sources of offset errors are due to device to device offset variation (trim errors), mechanical stresses (mounting stresses), shifts due to temperature and aging. Performing auto-zero will greatly reduce these errors. The amount of error correction is limited by the resolution of the A/D.

So I do recommend to review the application note below in order to correct the sensor's output:

Implementing Auto-Zero for Integrated Pressure Sensors

I do also recommend to use +5.0V to power supply this device and use one 1.0uF and one 0.01uF decoupling capacitors placed close to Vs pin of the pressure sensor, and a 470pF capacitor close to the Vout pin as an output filter capacitor.

A gauge pressure sensor measures the pressure at its port with respect to the local atmospheric pressure, so answering your last question, the reading should be the same at air pressure room and at sea-level.

Note that, when positioning the sensor in your application, it is important that the vent is open to the atmosphere. Besides, NXP warranties its pressure sensors for clear air only and its customer’s responsibility to validate and estimate the behavior of the device in media other than air.             

I hope this information will help. Please let me know if you have any further question.

Best Regards,

David Diaz