Hi Ti-engineers.
We have a product used PH sensor, we designed and used with LMP91200.
But now we need a low-cost solution.
Could you please help to check and give me an advice for Opamp and Vref ?
ex: analog output range (0.5-1.8V)
Thanks for yours support.
Hi Alone,
=> not sure, but i used external PH probe.
Most mid to high end pH meter is equipped with external pH probe. Does your pH probe comes with Ag/AgCl reference inside the probe? If the pH probe has the chemical reference, then you can use it as the reference (rather than electronic reference).
0083.PH_probe_design_national_AN-1852.pdf
=> under 2.5-3$ for both vref and op-amp ic ( 1000pcs).
In our pH application note, we suggested to use LM4140A-1.0 voltage reference (1.024Vref), which it has 0.1% accuracy (grade A) with 3ppm/C drift. Yes, this cost more than others. The circuit is shown below.
When the Vref is divided down, the buffer U1 is measured at 1.024/2 = 512mV. As long as the 512mV or the reference is stable, then Vout will be stable as well.
In the Nernst equation, the key to accurate pH measurement is T (degree K = 273.15 + C), op amp U2 & U1 and ultra stable and low noise V+ supply voltage.
Here is what I would do in order to lower the cost and improve the pH measurement performance.
1. Get a precision RTD temperature sensor for the pH measurement solution. Sometimes, the temperature sensor comes with the pH probe, which it will be good, if accuracy and repeatability is good. 0.01C to 0.1C temperature accuracy for the application. These days, RTD sensor is widely available and should not cost much. TI has many RTD and accurate temperature sensing application note. I can send these to you, if you are interested.
2. If you use Ag/ACl chemical reference, then you do not need electric potential reference. The standard electrode refers to Ag/AgCl electrode in saturated KCl solution, which its electrical potential is measured at approx. 198mV.
3. If the pH electrode does not Ag/AgCl chemical reference, then you need to have a fairly good bandgap reference + U1, which zero drift precision op amp buffer + electrical voltage reference. I may be able to find you a suitable electrical voltage references.
4. Is your system battery powered or use regulated LDO? Please make sure that V+ is ultra stable and low noise. Please let me know.
5. You may continue to use LMP91200 or use discrete LMP7721 (OPA928 is our latest and greatest low Ib op amp, but it costs more). If you happy with LMP91200 IC, then continue to use it. LMP7721 and other ultra low Ib op amps may work as well. Since most of pH probe is high impedance device, therefore ultra low Ib op amp is required. If you tell me the impedance ranges of your pH electrode, I can find an alternative op amp, if you want to go with discrete solution.
6. Last thing is the cable length and shielding. If you purchase these from a manufacture, then it should be adequate. Please make sure that you have a good electrode contact with the precision op amp.
Once we have these issues answered, we can simulate the entire pH electrode and check out the electronic noises and its performance.
If you have other questions, please let me know.
Best,
Raymond
Hi Alone,
could you show the part number for both Vref and Opamp that meet my target cost ?
Here is the low cost option. I would select ±0.5% B grade or better in TL431 reference. The reasons for the selection is that the Vref for pH measurement is a just an electrode chemical reference, it can be Ag/AgCl or electronic one. This reference has to be placed very close to the pH measurement electrode. As long as it does not change between calibration and measurements, it should be good. Ultimately, the Vref is dropped out from the pH calculation.
You should spend more investment in temperature measurement. With 0.1C to 0.01C of the temperature accuracy of the measurement pH solution, the accuracy will also improves.
pH Vref with LPV821 07312023.TSC
Here is the search list of low cost voltage references: For instance, TL432LI may be better than TL431, which TL431 is for higher current application. T432LI or TL431LI, 0.5%, B grade package would be better option than TL431 for pH measurement application.
If you have other questions, please let us know.
Best,
Raymond
Hi Alone,
Yes, i also used the part number same that TL431 (AZ431LBNTR-G1) in my design.
AZ431LBNTR-G1 is NOT TI part number, and you should ask for Diode Corp for the support. In addition, I request to get ±0.5% B grade, you buy ±1% part. What do you want me to do?
Best,
Raymond
Hi Alone173,
In the previous reply, I attached the following link in TI E-store. If you login your user information, you should be able to see the inventory availability on various parts, pricing, performance etc.
You should try out the following voltage reference, again 0.5% or better.
TL431LI / TL432LI Programmable Shunt Regulator with Optimized Reference Current datasheet (Rev. A)
https://www.ti.com/lit/ds/symlink/ref34-q1.pdf?ts=1691126150010
I do not know what the budget for your references, if you are able to get Low Temperature Coefficient ~100 ppm/°C, which it would be better choice. The electronic reference in pH measurement application does not require the absolute accuracy, but it requires low temperature drift and consistent voltage reference. You need to understand that you get what you pay for. Your electronic reference selection should be based on the performance/price ratio for the application.
Best,
Raymond
Hi Alone173,
The integrated LMP91200 sensor is supported by the sensing team, please contact the team for further assistant.
Please take a look at the REF35, the specification seems to be good for pH application; and it is on the low cost Vref category.
If this is a discrete pH measurement using LMP7721 or similar PRAMPS op amp, I can be assistant. Please let me know.
Best,
Raymond
Hi Alone173,
Could you give a parameters for opamp requirements?
These are the precision op amp that may be suited for pH application. Do you know the impedance of your pH probe?
Previously, you mentioned that you want us to review the pH design schematic. If the design is a discrete pH op amp design, you may send it to me. I can take a look. LMP7721 is good for the pH application, OPA928 is better, but it is higher in price as well.
Best,
Raymond
