MPXV7002GP accuracy at the same pressure

[tonibenitez]

Hi. I am designing a pressure control system with 10 MPXV7002GP sensors. In the design of the PCB where the sensors are placed I have followed the instructions on the datasheet (3 On-Chip Temperature Compensation, Calibration and Signal Conditioning).  I am converting the analog signal with a controller with 10bits ADC so I have  ( 0- 5V  --> 0-1023 bits )

Following the datasheet, I have applied this criteria to scale:

0.5 V  --> -2000 Pa  --> 102.3 bits

2.5 V -->         0 Pa  --> 511.5 bits

4.5 V -->   2000 Pa  -->  920.7 bits

And finally I have implemented the auto zero, taking atmospheric pressures every second during 60 seconds to find the average. Then I substract this average in every reading.

I know that with 10bits ADC I can only note changes in pressure of 5 Pa.

I am testing the sensors at atmospheric pressure. The steps are:

1- Start and apply auto zero

2- Every 30 seconds take the atmosferic pressure.

3- During 1 hour I save the lowest and highest pressures.

4- Save the difference between the lowest and highest pressures.

My question are:

Is it normal that at atmospheric pressure I have differences between 10 Pa to 30 Pa between the lowest and highest pressures?

Normally the pressures are never 0 Pa but  -1, 4, -6.....jumping every 30 seconds normally by 5 Pa.

Some real example:

PRESSURE DIFFERENCE	10:45 12:15 Diff P	P min	P max	14:05 17:36 Diff P	P min	P max	17:45 19:03 Diff P	P min	P max
SENSOR P00	10	-6	4	10	-6	4	25	-6	19
SENSOR P01	15	-6	9	15	-7	8	15	-1	14
SENSOR P02	10	-5	5	20	-10	10	24	-5	19
SENSOR P03	10	-4	6	19	-9	10	24	-4	20
SENSOR P04	15	-5	10	15	-7	8	34	-8	26
SENSOR P05	10	-4	6	15	-11	4	24	-6	18
SENSOR P06	10	-5	5	14	-7	7	14	-1	13
SENSOR P07	10	-4	6	15	-11	4	25	-6	19
SENSOR P08	20	-16	4	39	-12	27	14	-4	10
SENSOR P09	14	-5	9	10	-6	4	29	-6	23

Is this normal?

Have I followed the correct steps?

Thanks

[TomasVaverka]

Hi Toni,

Such a drift is most likely related to the noise that is inherent to piezo-resistive pressure sensors.

Could you please post here your complete schematic? Are you using the recommended capacitors as shown in Figure 3 of the MPXV7002 datasheet? I would also recommend taking a closer look at our AN1646 which provides some additional guidelines to filter out the noise.

I hope it helps.

Best regards,

Tomas

PS: If this answer helps to solve your question, please mark it as "Correct". Thank you.

[tonibenitez]

Hi Tomas

Yes. I followed the instructions in the MPXV 7002 datasheet. As I commented. I have 10 MPXV7002GP sensors. The Schematic is this:

The 10 sensors are connected to a master PCB via connectors. The lines are 0.5mm and I haven't a GRD plane in the sensors' PCB. There are a common ground plane for everything in the master PCB.

I have taked a look at AN1646 and now I have doubts if it would have been better to applied for filtering the schema in the figure 2 instead of the one I applied.

What do you think?

If it helps I am using as controller an Arduino YUN.

 

In this article also speaks about filtering via software and I want to try it, since I have a built prototype and it is easiest than modified the design. But I want to clarify how I must to apply it in my case. The steps I do are:

AUTO ZERO

First I apply auto zero. Save the result so I can subtract it in every reading.

READING 

I am reading data every 30 second. At 0 seconds and 30 seconds. Until now I simply read the data recollected at those times. But now following the AN1646 instructions I suppose I have to do:

1. If I want an average of 60 samples, In every interval of 30 seconds I have to read every 500 ms and the do the average.
2. This average would be the reading with the filter applied for every interval of 30 seconds.

Am I right?

Another last question. In the AN1646 in the section  EFFECTS OF NOISE IN SENSOR SYSTEM It says this:

‘The transducer bridge produces a very small differential voltage in the millivolt range. The on-chip differential amplifier amplifies, level shifts and translates this voltage to a single ended output of typically 0.2 volts to 4.7 volts.’

I have applied an interval of 500 to 4500 mV as working interval for the MPXV7002GP.

Is that correct or I must change this interval?

Any help would be appreciated.

Thanks.