Timeline for PH Probe Unstable Measurement
Current License: CC BY-SA 4.0
15 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Nov 1, 2018 at 4:12 | vote | accept | Platytude | ||
| Nov 1, 2018 at 4:11 | answer | added | Platytude | timeline score: 4 | |
| Sep 28, 2018 at 4:49 | comment | added | Transistor | It's 'pH' with a small 'p'. The exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH stands for "power of hydrogen". Wikipedia pH. | |
| Sep 27, 2018 at 23:34 | comment | added | Scott Seidman | If your low freq drift is in the 400mV range, aliasing is a very likely suspect | |
| Sep 27, 2018 at 21:30 | comment | added | Platytude | I've scoped out various points on the board. There is definitely more noise in the circuit than I had hoped. I expected the output of the probe to be noisy simply due to how high impedance the node is, but apparently some noise is also leaking into my reference buffer, and the op-amp output (400mVp-p which seems way too high). This might be due to the environment I'm in (lots of extensionn cords, CCFLs, laser equipment, etc), I'll post back once I've sniffed around some more. | |
| Sep 27, 2018 at 18:38 | comment | added | Scott Seidman | Before you go too much further, I suggest looking at the output of U1B with a real scope. | |
| Sep 27, 2018 at 17:57 | comment | added | Platytude | The PCB isn't exactly lab-grade clean, but it was rinsed with isopropyl alcohol after soldering. I'll look into adding a filter before sampling :) I'm using FloraSol pH 7 solution. It doesn't say on the packaging if it's buffered or not. Yes JP7 is unpopulated, I was using it on an earlier prototype to measure probe offse voltage | |
| Sep 27, 2018 at 17:55 | comment | added | Scott Seidman | You are leaving JP1 empty, right? It looks like connecting it will do nasty things. | |
| Sep 27, 2018 at 17:55 | comment | added | crj11 | Also, are you are using a buffered pH 7 solution? | |
| Sep 27, 2018 at 17:54 | comment | added | Scott Seidman | You might be looking at classic aliasing. Filter before you sample! | |
| Sep 27, 2018 at 17:53 | comment | added | crj11 | Have you cleaned your PCB extremely well? There could be contamination messing with the high impedance input. You could eliminate PCB issues by lifting the center pin on the BNC and the input on the op amp and soldering a wire directly between the two. Make sure to clean the area thoroughly after soldering. Also, are you sure that the pH probe works? I've had issues with low quality pH probes. | |
| Sep 27, 2018 at 17:52 | comment | added | Platytude | I've added a link to a datasheet in the post body. I don't think a reaction is happening due to the +2.5V bias, as the probe can't "tell" that it's floating on top of it (ie. probe common is connected to +2.5V, probe output is connected to the high impedance op-amp terminal). | |
| Sep 27, 2018 at 17:51 | history | edited | Platytude | CC BY-SA 4.0 |
added 310 characters in body
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| Sep 27, 2018 at 17:46 | comment | added | rdtsc | What composition probe is being used? Perhaps a chemical reaction is taking place with that 2.5v potential. Try an RC filter on the input? | |
| Sep 27, 2018 at 17:32 | history | asked | Platytude | CC BY-SA 4.0 |