ADS1115: AMP for pH and ORP Probe Interface

Jesse,


To be honest, I'm not familiar enough with ORP or pH probes to make a very good recommendation.

If the measurement is falling off because of the decay of charge being drawn away from the input of the ADC, it might be sufficient to increase the capacitance on the inputs of the ADC. That way the decay is slower. The ADS1115 has a rather high input impedance (710 kΩ in the highest gain), so it normally wouldn't be a factor in the measurement decay. Is buffering the ADS1115 input going to be enough to prevent the probe's output voltage decay?

If you still need an amplifier, I'd need a bit more information about what sort of speed you need, what supplies you'll have access to, and other technical information about the probe. However, you might want to look at the OPA333 as a buffer. It has a low bandwidth and very low offset. It should run on a low supply, so you may be able to wire something together quickly.


Joseph Wu

Joseph,

According to a brochure for one inexpensive pH probe "The input impedance requires that it only be interfaced with equipment with high impedance circuits. The input impedance required is greater than 10^13 ohms."

I'm only taking readings every 15 seconds or so.  I find though that the readings are less noisy when I take them every second.  When I pushed the readings to once a minute to try to overcome the impedance issue, the readings became very noisy, and not very representative of the solution the probe was in.

I like that the OPA2333 has two channels, would that interface well to allow two differential signals to be analyzed?  Will an OPAMP work for a differential signal? Someone on another forum recommended I try an INAMP, but I'm not very familiar with either.  The readings are going to typically be between +/-200 mV.  There is also the LMP91200, but it's SPI, and I really would prefer i2c and more than one probe input.

Jesse,

I did a quick search on pH probes and came across the following application note. It describes using the LMP7721 as a high-impedance buffer for pH electrodes. 

www.ti.com/.../snoa529a.pdf

While I think that the OPA333 may work as well, the input bias current might high enough to be an issue, because of the high output impedance of the probe.

Are you comparing two different measurements? If you want to try to make differential measurements, it might be better to duplicate the circuit and make a differential measurement with the ADC.

Joseph Wu

Joseph,

Thank you for the link to the application note. It was a good read and led me to try figure out how to use the site and to look at the variety of amps that have pH in their notes.  There are quite a few, and I found "The Signal" PDF so I'll probably read through that to get a better understanding of amplifiers.

My basic understanding is that reading the voltage differential from the probe is the only accurate way to read the negative voltages it produces?  The ground ring on the BNC connectors isn't really a ground, it's connected to the electrode in the reference solution.  I could be wrong though, I'm way out of my depth here.  If that is the case though, to be able to connect two different probes to the same ADS1115, I would need to have two separate differential measurement circuits.  Correct me if I'm misunderstanding this, but that would allow the actual voltage of the probes to "float" a little up or down relative to the voltages of the microcontroller and ADS while maintaining accuracy.  Perhaps though I only need an amp on the center tap, as the reference solution should always be 0 V?

That being said, I found some open hardware schematics for two separate interface boards, one for pH and one for ORP,  to convert to 0-+5v for Arduino analog pins.  Both schematics bring the outer ring to ground and create a -5v reference though.

I'm trying to keep the component count down, as I'll likely make a few just for myself.  If there are any components that reduce the number of separate amps that I need on the board that would help.  

Jesse,


I'm not sure what you mean by allowing the voltage of the probes to "float". However, I think you should be ok making measurements as long as there is a common ground for the system. That would mean that your two different reference solutions must have the same ground. Note that the ADC requires that the input voltage measurement is between the supplies. If you're using a 0 to 5V supply both AINP and AINN needs to be within that range.

I'd also note that I've never worked with pH probes, so I don't know much about their construction and what's required in the circuits used to measure them. I generally support data converters so my ability to help in this particular application may be limited.


Joseph Wu

Joseph,

Thank you, I appreciate the help. Do you think this thread should be moved over to the amplifiers section or should I start a new thread there?

Jesse,


Just start a new thread. Also, I'd describe what you want out of the measurement. I wasn't sure if you want two separate measurements or if you want the differential measurement between the two samples.

I think asking about the particular devices used in the applications notes would help connect you to the right people to ask about these probes. Once you settle on a front end, post back if you have questions about the ADS1115.


Joseph Wu