LMP7721: Looking for alternative for isolated loop powered ph measurement

Hi Onkar,

Yes, LMC6062 can work, depending on the level of impedance of the probe. LMP7721 is our lowest input bias current device today, with a 5 pA max. spec. at 125C, where LMC6062 has typical input bias current of 10 pA (generally it is easier to compare bias current at 125C, since lower at 125C means lower at room temp. as well).  Some PH probes do not require IB levels of the LMP7721, generally the most sensitive PH probes are not used for very low power applications, it is dependent of the series impedance of the probe itself, that can be on the order of GOhm. 

I recommend looking into one of our new devices called OPA391, it has the same bias current as LMC6062 at 2.5 V supply, but lower offset (45 uV) and power (24 uA) along with wider bandwidth (1 MHz).

Hope this helps,
Mike

Hi Michael,

Thanks for the reply.

OPA391 seems good alternative. I will try to use this one.

As considering impedance of PH probe, unfortunately client do not have exact value of it, but I assume it should be in GOhms.
How can I measure output impedance of probe ? Any idea?

Thanks,
Onkar

Hi Onkar,

I can't say I've measured the impedance of a Ph probe itself but have measured the impedance of many other things; typically, you would want to change the probe output by a known current and measure the change in voltage, then RO will be change in voltage/change in current.  How accurately you can do this depends on what access you have to certain instruments or equipment, the easiest would be an SMU capable of sourcing/sinking current in the pA range, in which case you could load the cell with, say, 10 pA and 20 pA, measure the voltage in both cases, and divide the delta voltage by 10 pA.  Looks like an instrument such as Keithley 6220 is capable of current sourcing/sinking down to 100 fA.  The instrument that makes the voltage measurement would also need very high input impedance (i.e. Agilent 3458 has a > 10 G mode).

A simpler approach would be to load the probe with a known high-value resistance (i.e. 100 MOhm resistor), connect the other end of the resistor to a voltage source, and measure the probe output voltage.  Use two different voltages with the voltage source and measure the probe output voltage. Then, the current is known since you know the voltage across the resistor, and the Ro can be calculated again with delta V(of the probe) / delta I.

Regards,
MIke

Hi Michael,

That's very complex process probably skip it. Resistor method could be easy to do.
But thanks for the detailed Information.

Regards,
Onkar

Hi Onkar,

the source impedance of a typical PH probe ranges from about 1M to over 1G. It depends on many factors like temperature, storing conditions, aging, etc.

Be very careful, even very small seeming measuring currents or load currents can easily destroy the PH probe.

Kai

Hi Onkar,

pH electrode vs. [H+] (hydrogen ions) concentration is a differential voltage measurement, where Es - Ex differential output is being measured via op amp (represented in mV on the vertical axis in the following plot). pH(S) is a typical standard neutral aqueous solution, where hydrogen ions' concentration [H+]= 0M, and typically calibrated solution is pH=7. Most important variables in the Nernst equation are Temperature and (Es - Ex) measurement, which needs to be accurately measurement. pH reference buffer solutions (used for calibration and linearity checks) are fairly stable and does not change over time.  

In my opinion, there is no point to measure the impedance of a known and working pH electrode in your application. The reasons are the pH  impedance are going to change from time and time, and you may only be able to obtain an approximate figure. The impedance is based on the ionic impedance, it will change over temperature, solution concentration, frequency, applied measurement potential by measurement device etc. (a difficult measurement as suggested by Kai). The impedance at DC does not have much meaning in your circuit, because it is going to be high in MΩ -GΩ range (H+ exchange rate is going to be in sub-Hz or mHz range). 

As you and Mike pointed out, the application does require a very low I_bias op amp, preferably is in fA or no less than pA range. The differential current is very low and the pH potential measurement should not "overload" pH electrode in the measurement process. If pH electrode is overloaded during potential measurement, it will alter your results and (Ex -Ex) vs. pH plot will not be a linear relationship. 

If you have additional questions, please let us know. 

Best,

Raymond