Question about the "LMC6001 pH probe amplifier" circuit

RadioCrack · May 17, 2007

Hi, i have a question about the pH probe amplifier circuit in the Typical Applications section of the LMC6001 data sheet. I don't understand why Z1 is a 2.5v reverse breakdown zener and not a 5.0v reverse breakdown zener instead. I understand that the LMC6001 multiplies the signal from the ph probe by 1.7 and that the LMC6041 provides the phase inversion and offset needed to display the correct pH. According to the diagram, the non-inverting input of the LMC6041 is being set at .35v, but it actually needs an offset of .7 volts, am i correct? If D1 was to be a 5.0v zener, then the non-inverting input would be set at the correct offset of .7v, right?

I'm sorry, but i'm kinda new to electronics so i dont know if what i'm saying is true or just gibberish. Thank you for your time.

Russlk · May 17, 2007

In the first place, the power supply is 5 volts, so a 5 volt zener will not work, there will be no current thru it. In the second place, a 5 volt zener is a lousy voltage regulator. The LM4040 is not a zener, it is a voltage reference which provides a temperature stable voltage for the bias on the LMC6041. If the op amp has a gain of 1.7 and you offset the input by 0.7 volts, the output will be offset by 1.7*0.7 = 1.19 volts.

Roff · May 17, 2007

RadioCrack said:
Hi, i have a question about the pH probe amplifier circuit in the Typical Applications section of the LMC6001 data sheet. I don't understand why Z1 is a 2.5v reverse breakdown zener and not a 5.0v reverse breakdown zener instead. I understand that the LMC6001 multiplies the signal from the ph probe by 1.7 and that the LMC6041 provides the phase inversion and offset needed to display the correct pH. According to the diagram, the non-inverting input of the LMC6041 is being set at .35v, but it actually needs an offset of .7 volts, am i correct? If D1 was to be a 5.0v zener, then the non-inverting input would be set at the correct offset of .7v, right?

I'm sorry, but i'm kinda new to electronics so i dont know if what i'm saying is true or just gibberish. Thank you for your time.

To the noninverting input, the second stage has a gain of 2 (Av=1+Rf/Rs). 0.35V on the noninverting input yields an output offset of 0.7V.

Question about the "LMC6001 pH probe amplifier" circuit

RadioCrack

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Question about the "LMC6001 pH probe amplifier" circuit

RadioCrack

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