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pH measurement with TI's INA116

Other Parts Discussed in Thread: INA116

I am trying to implement a pH measuring device with INA116 chip. One of the applications of the chip is pH measurement.

Constrants: pH ranges from +- 430 mV. Max input/output 2 Hz frequency. Gain of 11.5 V/V


PH probe have very high impediance ranging from 100 - 1000 MOhms. So INA116 chip is a good choice.

I have attached a pic of my current ckt. 

Problems I am having right now:

1. Noissy signal. On the output signal, I am getting frequency in thousand to mega Hz.

2. The offset voltage is fluctuating from -800 mV to 300 mV or sometimes stable at -400 mV. Now if I place a .10 uF capacitor between inputs the output goes down to -70 mV. Have a offset is a big problem for me as I need 0 mV offset to start with other wise my pH reading screws up. Is there a way to implement auto-zero input offset circuit.

NOTE: with gain set to 11.53 v/v

3. I don't know what type of (LPF @ 2Hz) filter should I implement to at the input and output to block any noise [ without blocking dc input mV signal] from entering my signal and high frequencies. AC-DC power supply do sometimes introduce 60k Hz frequencies too.

  • Hi Ambar,

    The INA116 is a very good choice for measuring pH due to its low input bias current. Using a PCB with the recommended layout reduces noise on the input signal and helps acheive the low input bias current. To get the best performance, proper use of the guard rings is necessary. The guard rings must be connected together and encircle the input trace as shown in Figure 4. Please follow the Circuit Board Layout and Assembly section in the datasheet when laying out the printed circuit board (PCB).

    To adjust the offset voltage please see the Offest Trimming section and Figure 2 in the datasheet.

    Hope this helps!

    -Tim Claycomb

  • I understand that PCB would be helpful. I am using the INA116 on breadboard to make sure I have all the components of my design with good output. With breadboard I should not have much leakage between pins or breadboard surface. So I think my noise is being introduced as a form of interference from environment such as lights, power supply or other things. 

    All the power wires, input signal wires and output signal wires are shielded. Also I placed the INA116 amp inside the Faraday box. 

    Input signal wire shields are attached to guard pins respectively. 

    Question 1) Is it appropriate to connect Faraday box ground to the same REF terminal ground?

    Question 2)  Figure 3  on Page 8 shows to place a mega ohm resistor connected to each input and ground it. I did this and got more noisy signal at the output. Not sure if this necessary for pH measurement as Figure 7 on Page 9 does not show this in the circuit. 

    Question 3) Figure 1 on Page 7 shows to place 0.1uF capacitor (Disc type) on both power supplies. I did that when testing. On the other hand, I had to short both input pins (3/6)  0.1uF capacitor to get rid of fluctuation in the output signal. Why is the output signal jumping up/down on the oscilloscope without the capacitor, I am not sure? 

    Question 4) Can high tolerance resistors / Capacitors induce more noise in the signal compare to very low tolerance resistors / Capacitors? I would like to make a RC LPF to keep frequencies under 10 Hz.. 

     

  • Hi Ambar,

    We highly recommend using a PCB to test the INA116. This will reduce stray capacitance, leakage current and any other effects caused from the breadboard.

    Question 1) The noise you are seeing may be caused from using a breadboard. Please follow the recommended PCB layout for the INA116 in the datasheet. But if you would like to enclose your circuit you should not connect the Faraday box to ground. Here is a TI Design that shows how to use a paint can as a Faraday box.
    http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slau522&fileType=pdf

    Question 2) Yes you do need the resistors at the input to ground to provide a path for the input bias current. Figure 3 is a simplified schematic. Notice Figure 5 also shows a 1M ohm resistor to ground.

    Question 3) Decoupling capacitors are necessary. Here is a blog post discussing why.
    http://e2e.ti.com/blogs_/b/precisionhub/archive/2013/08/13/the-decoupling-capacitor-is-it-really-necessary.aspx

    If you need to short the input pins together, connect the pins together using a wire and then connect them to ground.

    Question 4) Resistor tolerance can introduce a differential voltage which will be seen as an error at the output. Resistor magnitude is directly related to noise...please see figure.

    -Tim Claycomb

  • Hi Tim,

    Thank you for answering some of my questions. It would be very useful for consumers to have a Pspice model for INA116. The company should provide a component for simulation. 

    Thanks,

    Ambar Rathie

  • hi !

     

    I am also looking for a PH design , were you able to get yours working ?

    regards

  • Hey!

    Could you please share the schematics of your pH meter? I am particularly interested in how the pH probe is connected to the INA116.

    Best regards!

  • Ambar,

    I must disagree with Tim regarding your question #2. I do not recommend that resistors be used on the inputs.

    A pH probe behaves as a very weak battery or cell with pH of the liquid determining its output voltage. The probe itself serves as the source for input bias current of the INA116--a very small current, indeed. The purpose of using an amplifier such as the INA116 is to minimize the discharging of the pH cell. Adding a resistor will surely load the probe heavily and dramatically change its output voltage.

    Regards--  Bruce

  • Dear mister Bruce Trump,

    Would you please be so kind to show a schematics for a *proper* way of attaching a pH electrode to the INA116 instrumental amplifier?

    I am rather confused about this, as I have also doubted the input pin resistors that were shown in the datasheed. Basically connecting a 10 or 100 MOhm resistor on each input pin to the ground is nearly equivalent to attaching a single resistor of twice the value between both input pins. This provides a new current path with much lower resistance than that of the opamp, so it will introduce enormous measurement errors.

    I have read a number of documents on this topic, and most of them suggest there be some resistors on the input pins to the ground to provide input bias current path. But what is in the correct wiring in case of pH meters? Obviously I cannot use two resistors (one on each input pin). Can I omit all resistors here or must I use a resistor on a single input pin? What do I attach this resistor to - ground or a reference voltage? I have a single supply circuit.

    Kind regards!

  • Marko,

    To repeat a previous my posting in this thread, you should not use an input biasing resistor with a pH probe. The probe itself provides a path for the tiny input bias current of the INA116. Examples showing a need of a resistor for input bias current apply to other cases such as capacitive or transformer coupling where there is no other path for input bias current.

    The output of a typical pH probe ranges from approximately -0.5V to +0.5V. This bipolar output signal is most easily acquired with circuitry powered from a bipolar power supply. This allows input and output signals to be referenced to ground. It is possible to operate the amplifier from a single power supply but the additional pseudo-ground circuitry required is often more awkward than simply providing bipolar (+/-) power supply voltages. Thus, the perceived "simplicity" of a single supply circuit turn out to be not so simple.

    If you must use a single power supply, the diagram below could be one possible solution. The input and output voltage is referenced to a 6.8V zener diode pseudo ground voltage. Thus the pH probe voltage and output voltage float above ground potential by 6.8V. The output voltage of the amplifier must be measured with respect to the 6.8V pseudo-ground reference as shown.

    Note that the pH probe is modeled here as a voltage source (could be + or - depending on pH) in series with a very high internal source resistance. This source resistance could be hundreds of M-ohms, depending on the probe and the solution being measured. It is this very high source resistance that demands an amplifier with extremely low input bias current.

    Regards--  Bruce

  • Dear mister Bruce Trump,

    Thank you for the detailed reply, however I am still confused about some things. I am an amateur/hobbyist, so certain things are not that obvious to me. In particular it confuses me where you mention that the resistors should not be used. I understand why they should not be used on both input pins, so I wanted to place one on just a single input pin, but by the looks of your reply this shouldn't be done either. When you said that the resistor should not be used, did you mean that there be a direct connection to the input pin or no connection at all? I see that you connected a reference voltage directly to one of the input pins of the opamp, so I'm assuming this is okay.

    I take it that connecting one of the opamp inputs to the ground via a resistor on dual-supply case is equivalent to connecting the same input pin to the virtual ground via a resistor on single-supply case, provided that the VGND/VREF is properly buffered. With this in mind and your suggestions above, I came up with circuit of my own.

    I am assuming the INA116 will not output higher voltage than 12V due to the power supply constraint, so I used a simple voltage divider (potentiometer or 2 resistors) to bring down the voltage to an acceptable level for the ADC. The opamp has its own reference voltage at about 6V, and the ADC uses 2.50V as the middle point of its supply voltage to get the maximum range. C1 along with Rd act like a low-pass RC filter. Do you have any comments/suggestions/ideas for this circuit? My only concern is that the 6V ref near the INA116 is connected directly to the outer housing of the BNC connector. An accidental short-circuit could easily damage the device. This is why I originally wanted to use a resistor between the input pin, and the VGND/VREF.

    Best regards!

  • Hello Marko,

    Can you please give us the part number of your pH electrode(s) so we can look at the data sheet(s)?

  • Marko,

    I believe that your circuit will function. A resistor is not required on the inverting input because it is directly connected to the 6V pseudo ground. This node supplies the necessary input bias current path.

    I agree with your concern about the floating 6V on the BNC shield connection. Accidental grounding of this node is unlikely to damage the amplifier but it would certainly create an incorrect output. I might also greatly increase the input bias current of the INA116 on the non-inverting input. Though this might be a temporary condition, it could easily polarize the pH probe and permanently affect its accuracy.

    You might want to consider some other type of connector, one that would not look as if the outside connection is ground. Also consider that the connector and wire used to connect to the pH probe should be very low leakage type--probably insulated with Teflon.

    Another possible hazard involves the possible grounding of the liquid being measured. The measured liquid becomes part of the circuit and would be floating at somewhere near 6V potential. Any conduction path from the measured liquid to ground would disturb circuit operation and potentially damage the probe. A finger touching the measured liquid, for example, would most likely be at ground potential and could damage the probe. This is one of the reasons I believe it's best to use a bipolar power supply with ground-referenced inputs.

    Look at figure 7 in the INA116 data sheet. Notice that the measured liquid is grounded and the two pH probe connections are measured differentially. I believe that most commercial pH probes provide three connections and one is an electrical connection to the measured liquid, intended to be grounded. Your circuit would need to "float" this connection (leave it disconnected).

    Regards--  Bruce

  • Dear mister Pete Samig and mister Bruce Trump,

    Thank you for your answers, this clears up a lot of things. Unfortunately I cannot easily use another connector than BNC because the pH electrode already has a coax cable with a BNC connector attached. As an addition, I plan to use some other electrodes including an industrial ORP probe to measure the redox potential. This one has a lower internal resistance than the pH probe, and is able to produce voltages from -2000 to 2000 mV (according to the datasheet). It has a S8 head, but I already have a proper S8-to-BNC adapter cable ready. The specs for the two electrodes can be found here:

    pH: Hanna Instruments HI 1230B
    http://www.hannainst.com/usa/prods2.cfm?id=030&ProdCode=HI%201230B

    ORP: Hamilton Oxytrode Pt 120
    http://www.hamiltoncompany.com/products/sensors/c/764/


    While accidentally shorting the BNC shield with the ground may not damage the opamp itself, it can easily damage the reference/pseudoground circuitry. This is why I originally wanted to include a resistor (i.e. 10 MOhm) between the reference/vground terminal and the opamp negative input. My electrodes offer only two connections, so there does not seem to be any grounding. The ORP is an industrial probe, but the pH probe was from a handheld pH meter (HI 83141), which was battery-powered.

    I currently only have a +12VDC 2Amp single supply from a mains switching adapter. The reason I decided to use it was due to simplicity, however, switching to a bipolar supply would likely require me to invest into a custom power supply circuitry. I will also need the 12VDC to power a bipolar stepper motor for a peristaltic dosing pump. There's also a microcontroller that will drive the motor, and read/send the measurements to a PC. I will consider having separate power supplies for analog/motor/digital parts.
     
    P.S. would the circuit above still work, if I disconnect the +6VDC reference/vground terminal from the opamp input?

    Kind regards!

  • Marko,

    If you do not use a bipolar power supply the 6V power supply is required to properly bias your circuitry.

    The additional resistor you propose seems like a good idea. This will protect the 6V power supply if it could be damaged by a short-circuit. This resistor may not provide significant protection for the probes. A 10M resistor is nearly as bad as a short circuit to a probe that has greater than 100M effective series resistance.

    So, once again, it is important to assure that the measured liquid does not acquire another stray current path to ground. Fluids that are flowing through your pump, for example, may be at ground potential and this could create an invalid reading or damage the probes.

    Regards--  Bruce

  • Mr. Bruce,

    Does impedance matching matter between pH probe (or electrode ) and input of INA116 amplifier?

    If so, can you please shed some light on this matter. 

  • Ambar,

    No, impedance matching is not desirable with a pH probe. In fact, in this case it is very undesirable. It is very rare in sensor or precision signal acquisition that impedance matching is desirable.

    For others who might read this and be confused with regard to impedance matching of other types of sensors:  The exception to this case may be where high-frequency transmission line effects could cause reflections, distorting pulses or creating standing waves. There are other cases where certain sensors, such as current transformers, are designed to operate with a specific load impedance. This is not a case of "impedance matching" but rather providing the expected load for a sensor. Consult the manufacturer of your sensor.

    Regards-- Bruce

  • Mr. Bruce, Thank you for clarification.

  • It sounds like you may have leakage current issues. To confirm if your problem is leakage current you can "skywire" the input (from the electrode) directly to the input pin on the amplifier (the Amplifier pin must also be in the air), this will result in making the only possible leakage current path the air around the connection(s) to the amplifier. If the problems go away then you have some PCB re-design to do. If you filter the signal it should only be done at the output of the initial amplifier.

    I assume that your pH electrode is manufactured by a electrode manufacturer, if this is the case then the cable used on the pH electrode should minimize the effect of triboelectric current generation which will happen when the electrode cable is moved. If the incorrect cable is used it could generate enough current to result in more than 1pH of error.

  • Dear Mr. Trump,

    Than how can we prevent the inputs of the amplifier from swinging to some unexpected voltage before submersing them into the solution?

    Will it be helpful to ground one of the inputs of INA116 (and which preferably?) through some big resistor, say 1 GOhm, so there is no current path between the inputs?

    I need to measure the voltage between two contacting immiscible liquids and the impedance of their interface can be comparable to that of a pH probe.

    Regards,

    Alex