pH probe circuit - Possible problem with noise

[Lighty]

Hi all

As you can see i'm a newbie to the site, I'm got a basic grounding of electronics, enough to survive

I'm wanting to connect a glass pH probe to my Siemens PLC that runs my marine reef fish tank, it accepts 0-10v.

Found the following schematic on the net, was just about to build it but a fellow reefer said that the probe would pick up interference from the pump, lights, ETC.

what you think, and if you suspect that it will be a problem, how / what should I do to fix it.

Thanks in advance.

 

[ericgibbs]

Hi all

As you can see i'm a newbie to the site, I'm got a basic grounding of electronics, enough to survive

I'm wanting to connect a glass pH probe to my Siemens PLC that runs my marine reef fish tank, it accepts 0-10v.

Found the following schematic on the net, was just about to build it but a fellow reefer said that the probe would pick up interference from the pump, lights, ETC.

what you think, and if you suspect that it will be a problem, how / what should I do to fix it.

Thanks in advance.

hi,
As the pH will not change rapidly, it would be easy to add a simple Res/Cap filter onthe output of the 2nd stage OPA, before the PLC, to remove any noise.

 

[Lighty]

hi,
As the pH will not change rapidly, it would be easy to add a simple Res/Cap filter onthe output of the 2nd stage OPA, before the PLC, to remove any noise.

But, wouldn't the "stray voltage" (noise) be amplified through the first part of the circuit to such a point the "filter" would not work correctly. I'm need ing it to be acurate to 0.01pH.

 

[Lighty]

anyone else got some input?

 

[karmabobby]

To measure something that accurately you would need to design a circuit which implements and instrumentation amplifier otherwise the noise would be too great.

 

[Lighty]

Perhaps something like the OPA37 / OPA27? Please Help!

 

[karmabobby]

That arrangement could work or you can buy an integrated circuit so its included in one chip instead of 3. I would go with the integrated option, and do some research and read the data sheets on what you will be using it for. If all that sounds boring, welcome to my world as an E&EE undergraduate.

To use that arrangement you will need to take into account the tolerance of the resistors used and select the op-amps specifically for your use. For some tasks some op amps are great while others are terrible. This is why I would personally do some research online for and integrated instrumentation amplifier that would be best for you.

Hope this helps.

 

[Lighty]

That arrangement could work or you can buy an integrated circuit so its included in one chip instead of 3. I would go with the integrated option, and do some research and read the data sheets on what you will be using it for. If all that sounds boring, welcome to my world as an E&EE undergraduate.

To use that arrangement you will need to take into account the tolerance of the resistors used and select the op-amps specifically for your use. For some tasks some op amps are great while others are terrible. This is why I would personally do some research online for and integrated instrumentation amplifier that would be best for you.

Hope this helps.

Been Searchin for the last 2 weeks now, keep coming back to the same sites with "basic" amp circuits.

Any sugestion as to where I can find something decent?

also what chip would have that integrated circuit of the 3 chips?

Really do appreciate your help!

 

[saturn1bguy]

Any sugestion as to where I can find something decent?

Yeah, use the following. Go to pages 10-11 of the following datasheet. It's quite similar to the first circuit you posted, but this gives a little bit of help:

**broken link removed**

 

[Lighty]

thanks, will bread board some of the circuit and see how they work, just gotta wait for the probe to arrive.

 

[ericgibbs]

thanks, will bread board some of the circuit and see how they work, just gotta wait for the probe to arrive.

hi,
SatBoys OPA looks ideal.

But, wouldn't the "stray voltage" (noise) be amplified through the first part of the circuit to such a point the "filter" would not work correctly. I'm need ing it to be acurate to 0.01pH.

Considering my less elegant solution of a simple R/C filter in the final OPA, what do you think C1 is doing in the LMC6001 circuit..?

 

[Lighty]

Been doing some work, What do you think of the following?

 

[ericgibbs]

Good Morning SA.

The circuit looks unusual and a bit over complex, IMO.

Is the first stage opa [leftside] an 'ac' or 'dc' amp.?

Also the span control seems to wired incorrectly.

Where did the circuit come from.?

 

[Mikebits]

Also, for a sensitive instrument amp. You want your layout to have real short leads. Your parts seem to be too far apart which makes long leads (Can pick up noise). Also widen up the trace width a bit. Have beefy power and ground traces. Lots of ground.

 

[Lighty]

Good Morning SA.

The circuit looks unusual and a bit over complex, IMO.

Is the first stage opa [leftside] an 'ac' or 'dc' amp.?

Also the span control seems to wired incorrectly.

Where did the circuit come from.?

Does look a bit wierd, copied off an existing "off the self" manufactured pH controller, checked I had it right like 5 times!

Also, for a sensitive instrument amp. You want your layout to have real short leads. Your parts seem to be too far apart which makes long leads (Can pick up noise). Also widen up the trace width a bit. Have beefy power and ground traces. Lots of ground.

Thx, this was just a mock setup, will increase tracks and shorten lengths, and also cover it with ground traces, as with the manufactured unit.