Noise I'm getting in my summing amplifier

[Minh Thanh]

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

I am measuring EC of solution . I got Vec is 0.98V . After that, I took it to summing amplifier to reduce error from measurement . However, I saw a problem is that Vout of summing amplifier was continuously variable from 4.V to 4.5V, with value of potentionmeter is 180k. I hope I receive your help . Tks you !

 

[berkeman]

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

I am measuring EC of solution . I got Vec is 0.98V . After that, I took it to summing amplifier to reduce error from measurement . However, I saw a problem is that Vout of summing amplifier was continuously variable from 4.V to 4.5V, with value of potentionmeter is 180k. I hope I receive your help . Tks you !

Welcome to the PF.

What is the "EC of solution"? What is this circuit supposed to do?

On the question of the output voltage, what is Vout when the potentiometer is full CW (grounded wiper)? Is it zero reliably?

 

[Baluncore]

A summing amplifier sums currents at a node that has a fixed voltage = virtual earth. Resistors convert voltages to currents. I see no resistor that will convert the voltage on the RV1 wiper to a current at the summing node of the amplifier.

To offset a signal you must add or subtract a reference. The –9.3V reference must be stable and quiet to keep any supply voltage drift or noise out of the signal path.

The input impedance of EC? on R2 is 10k. Is the EC source impedance low? Is a continuous current of (0.98V/10k) = 98uA available from the signal source ?

 

[Tom.G]

Oops! Deleted. I didn't see this was homework and I gave too much information

 

[Minh Thanh]

A summing amplifier sums currents at a node that has a fixed voltage = virtual earth. Resistors convert voltages to currents. I see no resistor that will convert the voltage on the RV1 wiper to a current at the summing node of the amplifier.

To offset a signal you must add or subtract a reference. The –9.3V reference must be stable and quiet to keep any supply voltage drift or noise out of the signal path.

The input impedance of EC? on R2 is 10k. Is the EC source impedance low? Is a continuous current of (0.98V/10k) = 98uA available from the signal source ?

I see problem is that the negative power is not stable... I am using IC 7660 to get negative power. Do you have any idea about IC that can give stable negative power ?

 

[Baluncore]

Do you have any idea about IC that can give stable negative power ?

A 2.5 volt reference would work OK. Consider LM336-2.5 or LT1004-2.5 or others.

See the attached circuit diagram.
RV1 is a 10k potentiometer, simulated as two parts a and b.
RV1(ab) will offset input voltages between 0 and 2.5 volts.
RV1 will not be linear as there is no voltage buffer between RV1 and R4.