Dropping voltage at output of opamp

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Discussion Overview

The discussion revolves around a problem with the output voltage of an operational amplifier (op-amp) that decreases over time. Participants explore potential causes and solutions related to the op-amp's configuration, input conditions, and external components.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant reports a consistent decrease of 0.1V at the op-amp output over 3-4 minutes, despite reducing the gain to 2.
  • Another participant inquires about the precision of the digital voltmeter used to monitor the output voltage.
  • A participant suggests that the output drift could occur if the input is connected to ground or left open, potentially forming an RC circuit with stray capacitance.
  • Questions arise regarding the connection to the BNC, specifically whether it is linked to a pH probe and if the probe is immersed in a solution.
  • One participant proposes checking the type of capacitor C2 and suggests removing it to see if it affects the output voltage, noting that leakage currents in capacitors can vary.
  • Concerns are raised about the stability of the power supply voltage, particularly since it is sourced from an Arduino Nano connected via USB.
  • A recommendation is made to monitor the voltage on AOUT (IC1A Pin 1) to determine where the voltage drop is occurring, whether in the first or second stage of the circuit.
  • Further inquiries are made about the total current draw of the system and what other components are powered by the Arduino.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the cause of the voltage drop, with no consensus reached on a definitive solution or explanation.

Contextual Notes

Participants discuss various potential factors affecting the op-amp's output, including input conditions, capacitor behavior, and power supply stability, but do not resolve the underlying issues or assumptions.

Who May Find This Useful

Individuals working with operational amplifiers, particularly in experimental setups or educational contexts, may find the discussion relevant.

Minh Thanh

Homework Statement


I have problem at output of the first opamp . About 3 -4 minutes , the opamp's output voltage decreases by 0.1V . Although I reduced gain of opamp to 2, but the opamp 's output was still reducing 0.1V about 3 -4 minutes .

Homework Equations

The Attempt at a Solution

 

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How many decimal places to the right of the decimal point are displayed on the digital voltmeter you are using to monitor this OP-AMP output voltage?
 
NascentOxygen said:
How many decimal places to the right of the decimal point are displayed on the digital voltmeter you are using to monitor this OP-AMP output voltage?
It is two ..
 
Minh Thanh said:
I have problem at output of the first opamp . About 3 -4 minutes , the opamp's output voltage decreases by 0.1V . Although I reduced gain of opamp to 2, but the opamp 's output was still reducing 0.1V about 3 -4 minutes .

And this happens if the input is connected to the ground? If the input is an open circuit something like this could happen, since the input impedance is extremely high and any kind of stray capacitance could form a RC circuit with the input impedance.
If it isn't the input maybe try it without C1
 
What is connected to the BNC?
 
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berkeman said:
What is connected to the BNC?
that is pH probe.
 
Minh Thanh said:
that is pH probe.
And the probe is sitting in mid-air, not immersed in a solution, during the times you observe the OP-AMP output drifting?
 
What sort of capacitor is C2? Try removing C2 temporarily? The current through R4 is about 120uA so if C2 is leaking 10uA that would reduce the output voltage by 0.1V. Leakage currents for electrolytic capacitors can change when they are new. Might not be this but worth a try.

How stable is the power supply voltage? R6 and R7 add 1.2V to the output...

R7 * 5V/(R6+ R7) = 1.2V

..but if the 5V supply varies so does this value.
 
CWatters said:
What sort of capacitor is C2? Try removing C2 temporarily? The current through R4 is about 120uA so if C2 is leaking 10uA that would reduce the output voltage by 0.1V. Leakage currents for electrolytic capacitors can change when they are new. Might not be this but worth a try.

How stable is the power supply voltage? R6 and R7 add 1.2V to the output...

R7 * 5V/(R6+ R7) = 1.2V

..but if the 5V supply varies so does this value.
I use power supply 5v from Arduino Nano that connected to Laptop by USB
 
  • #10
I recommend you monitor the voltage on AOUT (IC1A Pin 1) to see if the 0.1V drop is introduced in the 1st or 2nd stage.
 
  • #11
Minh Thanh said:
I use power supply 5v from Arduino Nano that connected to Laptop by USB
what else is powered off the arduino?
What is the total current draw of the system?
 

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