Assistance required…

Hi. I hope you are well. I am working on a circuit (see attached). It is a thermistor going through a passive High Pass Filter and into a non-inverting op amp. The output of the op amp has a dc offset of about 2V. With the ac part of the signal rising and falling as the temperature of the thermistor changes. I don’t want the output to have any dc and just be the ac part of the signal. Can the op amp be configured to remove the dc part of the signal?

 

Assistance required…

Hi. I hope you are well. I am working on a circuit (see attached). It is a thermistor going through a passive High Pass Filter and into a non-inverting op amp. The output of the op amp has a dc offset of about 2V. With the ac part of the signal rising and falling as the temperature of the thermistor changes. I don’t want the output to have any dc and just be the ac part of the signal. Can the op amp be configured to remove the dc part of the signal?

Juat add an output DC blocking cap.

 

Something wrong if you've got an LM358 - they can get almost to ground, so I would not expect any offset.
Why are you high-pass filtering the output from a temperature sensor? You have a time constant of 1 second, but an LM35 temperature sensor in ideal conditions (oil bath) also has a time constant of 1 second, so I doubt you would see any output whatsoever.

 

How fast does the temperature change that you are trying to measure?

 

How fast does the temperature change that you are trying to measure?

Hi. Temperature change is about a 1Hz sine wave…

 

Juat add an output DC blocking cap.

Hi. That gets rid of the dc after the amplifier, but I was hoping I could reconfigure the amplifier to only amplify the ac part of the signal and ignore the dc part? Many thanks…

 

Hi. Temperature change is about a 1Hz sine wave…

Will your sensor respond that quickly?

 

Something wrong if you've got an LM358 - they can get almost to ground, so I would not expect any offset.
Why are you high-pass filtering the output from a temperature sensor? You have a time constant of 1 second, but an LM35 temperature sensor in ideal conditions (oil bath) also has a time constant of 1 second, so I doubt you would see any output whatsoever.

Hi. The HPF is to remove the dc offset as from the thermistor. I only want to measure changes in temperature…

 

Hi JK,
Using your OPA and a LM335 temperature sensor, the response of the circuit is not linear if the rate of change of temperature is less than 1Hz.
These are LTSpice simulations of your circuit.

E

 

Will your sensor respond that quickly?

Yes… it does

 

Hi JK,
Using your OPA and a LM335 temperature sensor, the response of the circuit is not linear if the rate of change of temperature is less than 1Hz.
These are LTSpice simulations of your circuit.

EView attachment 318865View attachment 318866View attachment 318866

Interesting… would you suggest a different thermistor?

 

Hi JK,
As you did not specify a temp sensor, I chose a suitable model.

Tell us what the application is for the temp sensor, then we can suggest ideas, it is the 1uF that limits the response.
E

 

Yes… it does

So what are you now getting on the output?

 

Instead of a high pass filter, use a difference amplifier and subtract the DC level.

 

So what are you now getting on the output?

Hi. At the op amp output I am getting a dc offset of about 2V with an ac sine wave on top as the temperature changes. I would like to increase the gain of the amplifier to make the sine wave bigger, but when I do, the dc offset increases and the amplifier saturates…

 

Hi JK,
As you did not specify a temp sensor, I chose a suitable model.

Tell us what the application is for the temp sensor, then we can suggest ideas, it is the 1uF that limits the response.
E

The temperature changes are created by breathing in and out…

 

Hi JK.
Which type of OPA's do you have available?
Also, do you have dual power supply?
And which type of sensor do you want to use?
E

 

Hi JK.
Which type of OPA's do you have available?
Also, do you have dual power supply?
And which type of sensor do you want to use?
E

Op amp LM358. Single 5V supply. MCP9701 thermistor

 

Op amp LM358. Single 5V supply. MCP9701 thermistor

So, might the problem be that you only have a single 5V supply, yet your circuit diagram shows a bipolar power supply?

 

Hi JK,
This is a clip from the d/s, which suggests for a human body temperature a possible Vout max of say
38C 38*19.5mV = 741 mV.
What range of temperatures would you expect from a human?
Also have you consider the ambient temperature range?
How is it going to be mounted, and what is its purpose?
E

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