LMP91200: Have problem with LMP91200

Hi Josh.

Thanks for your respond.

i have removed some component as you suggest, but this issue still happen.

I only using 1 PH sensor : 

I have changed to use triax cable, but it not solve.

I have test my PH sensor with other device (PH kit of DFrobot) it is ok, that means Signal R57 to MCU is ok and PH sensor is OK

What should i do in the next step?

How can i calibrate PH and calculate PH value?

Thanks and best Regards.

Hi Josh.

Thanks for your respond.

i'm not understand about "shouldn't your guard and VCM traces be at same potential - looks like you may have not connected them", because i have traced VCM and Guard in difference layer, can you explain more a bout it?

In my case, i'm not modified cable, i only change BNC adapter to SMA.

Really, i try to understand how to get correctly PH value, but i don't know about how to calculate it,

in my schematic i used VDD=5VDC, Vref =3V3DC

i saw that on the datasheet LM91200  have a formula : 

E = Eo + (2.3 RT/nF) log aH+

where:

• E is the measured potential from the sensing electrode -> ADC value i will read directly form LM91200-> that right?

• Eo is related to the potential of the reference electrode -> It is Vref (3.3V)-? that right?

• (2.3 RT/nF) is the Nernst factor,

• log aH+ is the pH, (aH+ = activity of Hydrogen ions). -> That means log aH+ is the pH value (0-14) i need to find?

Beside that i saw: The output of a pH electrode ranges from 415 mV to −415 mV as the pH changes from 0 to 14 at 25°C, that means every PH probe have same this spec?

Thanks And Best Regards.

Hi Josh.

thanks you so much for your explain.

-i saw that un the datasheet: we have a formula:

E=Eo + (2.3RT/nF)log aH+.

but in your replay, i saw slope = -2.3RT/nF and -59.18mV at 25oC, but follow the datasheet i saw it is 59.16 at 25oC. Do you have any misconception in this point.

- can you explain more detail about paraneter in the formula? as i know:

    E = value from pin VOUT of LM91200 that is right?

    Eo is reference electrode voltage. i don't understand this parameter, how can i calculate this parameter or where i can get it?.

- finally do we have any difference between use Vref =2.5V and Vref=3.5V

it is good if you can share with me your example about calculate pH value reality feild using LM91200.

Thanks and Best Regards.

Hi Gordon Varney.

Thanks for your respond.

Because i'm using STM32 to read ADC from LMP91200, VREF STM32 i set 3.3V, it is same VREF LMP91200, ADC resolution i used 12 bit =4096.

I have tested again om my PCB (Schematic i have posted last time), 

i'm using formula as bellow:

volt = (value)*3.3/4096;
pHvalue = ((volt - 1.65)*1000)/59.18;

+ Value = VOUT (Pin VOUt of LMP91200)

+ 1,65 is Eo, because VREF = 3.3V

+ 59.18 is  Nernst factor at 25oC

But pH value i readed is 4 although i used pH liquid is 7 to test.

I'm don't know where is my mistake in this formula?

If i connected pin VCOM of LMP91200 to pin ADC of MCU then Eo = VCOM? and that can reduce more noise ?

(2.3 RT/nF)is the Nernst factor, can you explain to me more detail about "n" in this factor, i saw that n=1 at temperature = 25oC, but if temperature # 25oC how can i find "n"?

Thanks and Best Regards.

Hi Gordon Varney.

Thanks for your advice.

I have used your formula: ph = 10^((1.700-1.650) / 0.05918):

In my PCB, when i test with pH=7, i measured VOUT=1.8V that meanz pH value i readed about >200.

i have checked again but these value not change VOCM =1.65V and VOUT =1.8V.

Can you tell me: Where is my wrong in my schematic or PCB layout?

Thanks and best regards

Hoang,

  I am running the numbers. I should have an answer soon.

Hoang,

  I would like for you to remove D15 diodes and remeasure the VOUT with a ph7 standard.

The ph probe is designed to deliver 0mV at ph7. reference to the set zero. That means that you should have 3.3V REF = 1.65V zero = 1.65V ph7.

The formula I gave you previous was the invert of the E formula. That formula will not provide you with the ph level. I have calculated the correct method to get the ph and added the temperature correction. However this formula doesn't work  with your ph VOUT of 1.8V, unless an offset error is added to the formula. This may very well be correct for calibration. I will continue to verify the formula if you can please provide the measured ph7 voltage with the diodes removed.

Hi. Gordon Varney

Until now. i'm not solve my problem.

can you give me more advise to struggle with them?

Thanks and best regards

Hi. Gordon Varney

Thanks for your help.

As your formula:

pH = (((2 - 1.65) + Er)/0.05917) + 7.

2 =  measured voltage at VOUT pin

1.65 = measured voltage at VOCM  pin

0.05917 = Nernst factor temperature corrected at 27oC

7 = Reference ph.

so What is Er in that formula?

Thanks and best regards.

Hoang,

  My apologies, Er it the Error. If you measure VOUT at ph7 then subtract Eo, then you have an error correction. Remember ph7 should be 0mV, so if the measured voltage is 1.8V at ph7, and the zero is 1.65V, then you have an error of 150mV. This is your Er (error). It will allow you to zero ph7 by removing the error.

Hi. Gordon Varney

Thanks for your help.

I seem face with other problem, because when i calibrate with pH = 7 as you formula, after that i  have changed pH = 4, but measured voltage in VOUT pin have been not change. that mean VOUT always  = 1.8V although i changed pH =7 or pH =4.

Do you think my schematic or PCB have problem?

Thanks and best regards.

Hoang,

  I have looked over your schematic and layout. I don't see anything wrong.

Check your board for cleanliness, make sure there is no flux residue on the board. These probes operate in the uA range and any contaminates will cause problems. Make sure after cleaning that you bake the boards to drive out moisture. even condensation will effect the probe.

Also can you try another probe in case this one has a problem?