Non-inverting op amp configuration

Thread Starter

JonnyK

Joined Mar 30, 2024
9
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?
 

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eetech00

Joined Jun 8, 2013
4,004
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.
 

Ian0

Joined Aug 7, 2020
10,005
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.
 

Thread Starter

JonnyK

Joined Mar 30, 2024
9
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…
 

ericgibbs

Joined Jan 29, 2010
18,990
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.

EEG57_ 1587.pngEG57_ 1586.pngEG57_ 1586.png
 

Attachments

ericgibbs

Joined Jan 29, 2010
18,990
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
 

Thread Starter

JonnyK

Joined Mar 30, 2024
9
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…
 

Thread Starter

JonnyK

Joined Mar 30, 2024
9
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…
 

ericgibbs

Joined Jan 29, 2010
18,990
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
 

ericgibbs

Joined Jan 29, 2010
18,990
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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