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JFET Buffer Design Issues

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Kleinstein:
A PH sensor likes really low bias current, but usually no need for sub µV stability. There are a few Az OP (e.g. max4238, LTC2050), that may be suitable, but not all. The lower noise ones tend to have more bias, up to the > 100 pA range.

A precision CMOS OP (e.g. lmc6081) would be an option too: less bias, but more drift and low frequency noise.

madwolfe:

--- Quote from: Kleinstein on November 15, 2019, 04:54:27 pm ---A PH sensor likes really low bias current, but usually no need for sub µV stability. There are a few Az OP (e.g. max4238, LTC2050), that may be suitable, but not all. The lower noise ones tend to have more bias, up to the > 100 pA range.

A precision CMOS OP (e.g. lmc6081) would be an option too: less bias, but more drift and low frequency noise.

--- End quote ---
Great, thanks for that. I was going to use a dual opamp because I want to supply a buffered offset voltage to the pH sensor to bring it up to 1+-0.4V, so that I can read it unipolar but I think I'll just use two opamps then, because the offset voltage buffer doesn't really have the same requirements as pH reading.

Cheers

AE7OO:
Greetings,

Just make sure your sensor can handle any DC you pass it's way.  While it might have been cheap sensors, my nephew(who likes electronics AND plants) managed to destroy 2 fleabay specials, before I pointed out the maximum signal parameters,  did NOT include an input that was biased to 5V/2.  I'm still trying to teach him that paying attention to detail helps in getting stuff working the first time.  >:D

madwolfe:

--- Quote from: AE7OO on November 19, 2019, 01:29:32 pm ---Greetings,

Just make sure your sensor can handle any DC you pass it's way.  While it might have been cheap sensors, my nephew(who likes electronics AND plants) managed to destroy 2 fleabay specials, before I pointed out the maximum signal parameters,  did NOT include an input that was biased to 5V/2.  I'm still trying to teach him that paying attention to detail helps in getting stuff working the first time.  >:D

--- End quote ---
It can't handle much DC at all but, as I understand, it shouldn't be getting much. I've attached a screenshot of how I've done the bias circuitry. It is basically just a method to 'shift' up the reference voltage that the pH sensor generates by 1.024v.

I'm pretty sure this shouldn't exceed the limits of the sensor. That being said, I should probably put a RC filter on the offset voltage to reduce the inrush current when it turns on. It might not be much but it could make a difference maybe.

Kleinstein:
PH sensors don't need ultimate precision, but a really low bias helps. So the low bias CMOS versions are preferred . They are usually also avialable as duals (e.g. LMC6082).

A virtual ground like some 1 V from ground can be a good idea. However no need to get an accurate value, especially if the ADC has a differential input or one can measure both side.

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