Design Considerations for pH Electrode Amplifier

[JJJJJJJJJJJJJ1]

I am designing an amplifier to interface with a pH electrode. 

As I've read, pH electrodes are devices with high output impedance (>100MOhm) which are susceptible to small leakage currents and/or coupling to other low frequency noise sources.

In my case, I am reading the pH in a hydroponic reservoir whose contents are fairly conductive (~1000uS).  The reservoir is plastic but a grounded wire is placed in solution (connected to the ground plane of my PCB which in turn is connected to an earth grounded metal enclosure).

The pH electrode is Chinese made, with an output impedance in the tens or hundreds of MOhm.  I am looking to achieve an amplifier/ADC with performance to read +/-0.05pH (approximately 3mV).

I would think a twisted pair or triax cable would be ideal - since twisted pair can easily remove common mode signals via an instrumentation amplifier (such as the AD8220) and Triax provides additional shielding.  In my case, the pH electrode is connected through a coax cable, the shield of which is connected to the reference electrode and the center conductor connected to the measurement electrode.  The end of the coax is terminated in plain BNC.     

Is this strategy advised?

• The voltage produced by the pH electrode is read by a low bias-current buffer such as the LT1793 or ADA4627.  The outer shield of the BNC will be attached to PCB ground with as low-impedance path possible.  Should I compensate for average bias current, with a low leakage thin film type resistor?
• Because guarding the shield is impossible (connected to reference electrode) guarding can only be used on the PCB near the opamp input.  Is it worth guarding this short of a distance?
• The boards will be meticulously cleaned in IPA in ultrasonic cleaner and then baked at a low temperature to increase insulation resistance.
• Since voltage can easily be induced on the shield of the BNC, I think a low impedance connection is necessary. Is any advantage gained by producing a virtual ground to drive the reference electrode?
• Since the response of the pH electrode is slow (10s of seconds) analog filtering can be used.  Would a small capacitor directly across the pH electrode suffice (film/low leakage type) or is it better to leave the electrode directly connected to buffer input and have a second stage filter for AC >~1Hz (active butterworth?)
• Is it advised to shield the amplifier circuit on-board?  I will already try and employ best EMC/noise practices: solid ground plane, mindful to separate digital and analog sections, rise time limiting. Should I use ferrite beads where signals enter/exit the shield?
• I am planning to use a boost converter to generate +/-18V supply (https://www.ti.com/lit/an/slua288/slua288.pdf) with an LC filter to reduce switching noise (I am using ADP1613 with output current of ~100mA per supply).  That supply will be placed physically far from the analog section.  Near the analog section, +/-15V LDO with good noise performance will be used to supply the opamps.  Are ferrite beads, or additional filters required in this application?
• Output of amplifier is read with a 16 bit (hope for ~12 bits ENOB) A/D integrated into a Renesas MCU.  Should I set amplifier gain/offset to maximize A/D range assuming there is no noise or DC offset on my signal? Or should I leave some breathing room at the sacrifice of resolution?

Basically, I'm not sure what matters.  I will try and breadboard all that I can, but also try to get it right the first time.


Any help or experience with these types of probes is appreciated.  Thank you

[moffy]

I remeber designing a PH meter many years ago, and one of the issues was battery like induced voltages on the electrode. I decided to go with AC excitation to get around that effect. Never finished the project though, circumstances intervened.

[KoRba88]

• The voltage produced by the pH electrode is read by a low bias-current buffer such as the LT1793 or ADA4627.  The outer shield of the BNC will be attached to PCB ground with as low-impedance path possible.  Should I compensate for average bias current, with a low leakage thin film type resistor?

Bias compensation works only in opamps without internal compensation! (only in simple bipolar opamps). So don't do that.

• Because guarding the shield is impossible (connected to reference electrode) guarding can only be used on the PCB near the opamp input.  Is it worth guarding this short of a distance?

Yes, its even more important since there is short distance to low impedance voltage sources. See https://www.analog.com/en/design-center/reference-designs/circuits-from-the-lab/cn0407.html

• The boards will be meticulously cleaned in IPA in ultrasonic cleaner and then baked at a low temperature to increase insulation resistance.

Ok

• Since voltage can easily be induced on the shield of the BNC, I think a low impedance connection is necessary. Is any advantage gained by producing a virtual ground to drive the reference electrode?

You can measure then in pseudodifferential manner, so might help to cancel ground noise effects.

• Since the response of the pH electrode is slow (10s of seconds) analog filtering can be used.  Would a small capacitor directly across the pH electrode suffice (film/low leakage type) or is it better to leave the electrode directly connected to buffer input and have a second stage filter for AC >~1Hz (active butterworth?)

Direct connection is better. Cap adds area where current can leak.

• Is it advised to shield the amplifier circuit on-board?  I will already try and employ best EMC/noise practices: solid ground plane, mindful to separate digital and analog sections, rise time limiting. Should I use ferrite beads where signals enter/exit the shield?

That could help. If you will see excess unknown offset, that could be a EMI pickup (demodulation) by opamp.

• I am planning to use a boost converter to generate +/-18V supply (https://www.ti.com/lit/an/slua288/slua288.pdf) with an LC filter to reduce switching noise (I am using ADP1613 with output current of ~100mA per supply).  That supply will be placed physically far from the analog section.  Near the analog section, +/-15V LDO with good noise performance will be used to supply the opamps.  Are ferrite beads, or additional filters required in this application?

Probably not necessary. However if your environment is water tank with salt at some industrial facility I will strongly recommend to fully isolate power and communication signals. I saw many times full phase voltage on that tanks.

• Output of amplifier is read with a 16 bit (hope for ~12 bits ENOB) A/D integrated into a Renesas MCU.  Should I set amplifier gain/offset to maximize A/D range assuming there is no noise or DC offset on my signal? Or should I leave some breathing room at the sacrifice of resolution?

Leave 10%, set maximum oversampling to increase ENOB, add 50/60Hz postprocessing filter, delta-sigma ADC are superior for that applications.

[Terry Bites]

It needs ultra-low bias and low short term drift. Long term drift doesn’t matter so much because the calibration is local and frequent. An ADA4530 or ADA4692 would be a good choice. The pcb design and probe cable needs to carefully managed with regard to current leakage paths and EMI. Galvanic isolation will boost the effective CMRR by removing the ground reference altogether. Driven guards lowers EMI susceptibility. For this you need Triax. A cheaper way is to put a cable driver at the probe end as seen here. Use a coin cell for power.

www.maximintegrated.com/en/design/technical-documents/app-notes/3/34.html]https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/34.html


TI have a ref design https://www.ti.com/lit/pdf/snoa529?keyMatch=SNOA529A so do AD https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0326.pdf  I'm sure there are many more

[CaptDon]

Most of the PH probes we used had specialized connectors and were 'single ended' or 'unbalanced' with one side being grounded via the chassis mount BNC connector. Maxim made a specialized chip just for PH probe amplifier/buffer use. It could be powered with a 9vdc transistor radio battery and the battery easily lasted over a year although we replaced them yearly. I don't recall the part number but look in the Maxim catalog. We built the little buffer/amplifiers into a small plastic RadioShack supplied housing and they were just a dumb little black box with BNC in / BNC out. They were perfect for driving a PH meter located about 30 feet away. Many PH probes act like a battery and the lower of an impedance load you put on their output the faster the probe will age and become inaccurate.
 

[Marco]

I'd just go for coax and single ended, keeps things simple. Do most of the integration on the MCU and integrate over a common multiple of 1/50 and 1/60 seconds.

[JJJJJJJJJJJJJ1]

• The voltage produced by the pH electrode is read by a low bias-current buffer such as the LT1793 or ADA4627.  The outer shield of the BNC will be attached to PCB ground with as low-impedance path possible.  Should I compensate for average bias current, with a low leakage thin film type resistor?

Bias compensation works only in opamps without internal compensation! (only in simple bipolar opamps). So don't do that.

• Because guarding the shield is impossible (connected to reference electrode) guarding can only be used on the PCB near the opamp input.  Is it worth guarding this short of a distance?

Yes, its even more important since there is short distance to low impedance voltage sources. See https://www.analog.com/en/design-center/reference-designs/circuits-from-the-lab/cn0407.html

• The boards will be meticulously cleaned in IPA in ultrasonic cleaner and then baked at a low temperature to increase insulation resistance.

Ok

• Since voltage can easily be induced on the shield of the BNC, I think a low impedance connection is necessary. Is any advantage gained by producing a virtual ground to drive the reference electrode?

You can measure then in pseudodifferential manner, so might help to cancel ground noise effects.

• Since the response of the pH electrode is slow (10s of seconds) analog filtering can be used.  Would a small capacitor directly across the pH electrode suffice (film/low leakage type) or is it better to leave the electrode directly connected to buffer input and have a second stage filter for AC >~1Hz (active butterworth?)

Direct connection is better. Cap adds area where current can leak.

• Is it advised to shield the amplifier circuit on-board?  I will already try and employ best EMC/noise practices: solid ground plane, mindful to separate digital and analog sections, rise time limiting. Should I use ferrite beads where signals enter/exit the shield?

That could help. If you will see excess unknown offset, that could be a EMI pickup (demodulation) by opamp.

• I am planning to use a boost converter to generate +/-18V supply (https://www.ti.com/lit/an/slua288/slua288.pdf) with an LC filter to reduce switching noise (I am using ADP1613 with output current of ~100mA per supply).  That supply will be placed physically far from the analog section.  Near the analog section, +/-15V LDO with good noise performance will be used to supply the opamps.  Are ferrite beads, or additional filters required in this application?

Probably not necessary. However if your environment is water tank with salt at some industrial facility I will strongly recommend to fully isolate power and communication signals. I saw many times full phase voltage on that tanks.

• Output of amplifier is read with a 16 bit (hope for ~12 bits ENOB) A/D integrated into a Renesas MCU.  Should I set amplifier gain/offset to maximize A/D range assuming there is no noise or DC offset on my signal? Or should I leave some breathing room at the sacrifice of resolution?

Leave 10%, set maximum oversampling to increase ENOB, add 50/60Hz postprocessing filter, delta-sigma ADC are superior for that applications.

Yes, thank you for your feedback.  I will try and select an opamp with internal guard drive, or add guarding externally.  I do wonder - is it worthwhile to bend up the leads of the Opamp input and make the connection to BNC directly in-air.  Certainly would reduce leakage, although I am not sure this is necessary for pA range of leakage.  I do have access to insulation resistance tester where I can measure intrinsic leakage of my board.  Dielectric polarization of the FR4 must also contribute, no?

My MCU can measure in so called pseudo-differential but the acquisitions are a few ms apart.  This would remove DC common mode voltages, no?

ADC is 16 bit, sadly not S/D (and the S/D resources on this MCU are already sampling output from another amplifier circuit) - https://www.renesas.com/us/en/document/apn/16-bit-ad-converter-performance-ra2a1?language=en.

The tank environment is very salty - up to 2000mS!  There are pumps submerged into the tank - operating at line voltage but switched with a relay.  These pumps will be off during acquisition, although when they are on they must contribute significant noise.  There is also another sensor in the tank - an EC sensor which comprises two stainless steel electrodes: one "grounded" through a voltage to current amp and another excited with a 100Hz +/-10V square wave to prevent electrode polarization.  This too will not be powered during pH acquisition, but the voltage to current amp is always connected.  I based the circuit loosely off of this app note:  https://www.analog.com/media/en/analog-dialogue/volume-50/number-4/articles/fully-automatic-self-calibrated-conductivity-measurement-system.pdf.  I do wonder what interference and/or ground loops will be formed from this strategy... the amplifier directly after the pH electrode is powered +/-15V so should have plenty of headroom.

Unfortunately isolation would be difficult, since the MCU which has the A/D resources is not isolated.  I hope the "grounded" EC electrode will wick away the large potentials (AC, DC, or electrostatic as the container is plastic) but there still may be a considerable 60Hz noise component.  Also, second stage filtering will hopefully have sufficient rejection to those higher frequency disturbances.  I don't have a good intuition...  Perhaps this is one advantage to an instrumentation amplifier - wouldn't that kind of common mode noise be removed?  I do still worry about any DC offsets that might change between calibrations. 

[JJJJJJJJJJJJJ1]

It needs ultra-low bias and low short term drift. Long term drift doesn’t matter so much because the calibration is local and frequent. An ADA4530 or ADA4692 would be a good choice. The pcb design and probe cable needs to carefully managed with regard to current leakage paths and EMI. Galvanic isolation will boost the effective CMRR by removing the ground reference altogether. Driven guards lowers EMI susceptibility. For this you need Triax. A cheaper way is to put a cable driver at the probe end as seen here. Use a coin cell for power.

www.maximintegrated.com/en/design/technical-documents/app-notes/3/34.html]https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/34.html


TI have a ref design https://www.ti.com/lit/pdf/snoa529?keyMatch=SNOA529A so do AD https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0326.pdf  I'm sure there are many more

Thanks - in my research for this project I've read both of those documents.  Unfortunately moving the driver closer to the probe is not possible, since the probe comes as an already encapsulated unit with lead attached.  Battery powered system would certainly be less noisy, though, and without the long BNC acting as an antenna.  Galvanic isolation is difficult - my MCU with A/D resources is not isolated, and sadly the project is already locked in w/ this microcontroller.  A quick google search brings up "analog isolation amplifiers" but I have never used them.

Sucks that most Triax pH probes are absurdly expensive!  I can only find the coax style for an appropriate price. 

[Marco]

You can buy a coax probe with an amplification module for 12 bucks BTW. Just see if it's good enough before using more effort?

[JJJJJJJJJJJJJ1]

You can buy a coax probe with an amplification module for 12 bucks BTW. Just see if it's good enough before using more effort?

Yes, I have a few of those cheap Aliexpress pH probe + amplifier combos for testing.  They are ok.  Most of the probes have a liquid or gel reference electrode, which drift pretty bad over a timescale of about 1 year.  But the polymer reference probes are pretty good: with calibration they can maintain +/-0.05pH reasonably.  This is probe by itself - using a cheapo ADC and measuring the pH of a solution in a beaker. 

My concern is when I integrate all these parts into a system - with switching power supply, digital signals, reservoir itself is conductive with motors and what not. 

[Marco]

It's impossible to say without trying, if you really want to overengineer it cut off the existing cable and epoxy a circuit with AMC3306xxx or ADE1201 on top of the sensor.