An opamp supply limit in the differential amplifier mode

Thread Starter

SAM-Mo

Joined Jan 19, 2019
32
Hi,

I have a sensor which is connected to +12V and it shows some small variations at the output.

I have an opamp which I have configured in AC input, inverting amplifier. therefore I should supply it with the load voltage and then find the best value for + rail (by default the half of the VCC which is 6), something like the figure below, but the opamp maximum supply is 3.3V. I am bounded to use this opamp and I can not change the sensor voltage.



is there any method to fix this problem?
 

ericgibbs

Joined Jan 29, 2010
8,885
Hi Sam,
Why do you see a problem with that OPA circuit when using a 3.3v [ assuming your OPA will work with a3.3v supply]
Pin 3 will sit at 3.3v/2 and the inverting input is AC coupled, so the any DC component from the 12V powered sensor will be blocked.
E
 

Thread Starter

SAM-Mo

Joined Jan 19, 2019
32
Hi Sam,
Why do you see a problem with that OPA circuit when using a 3.3v [ assuming your OPA will work with a3.3v supply]
Pin 3 will sit at 3.3v/2 and the inverting input is AC coupled, so the any DC component from the 12V powered sensor will be blocked.
E
Actually, I don't see the desired wave at the output. Yes, it looks like to be Okay but in practice, it just gives me a flat line in the oscilloscope, except I use an opamp with at least +12 supply and adjust the + voltage for the suitable output. I think it is wise because it makes the difference.
 

Thread Starter

SAM-Mo

Joined Jan 19, 2019
32
Thank you but in my case the input signal is +12V referenced, not ground referenced. that's make the trick
 

Thread Starter

SAM-Mo

Joined Jan 19, 2019
32
it doesn't work because the differential signal at the + rail is low, it only works when I change the opamp which I could apply the higher voltages to the + rail, because actually it is a differential amplifier

I'll test again anyway
 

AnalogKid

Joined Aug 1, 2013
8,156
What is the part number of the opamp you are required to use?

What type of sensor is it, as in what is it sensing?

What is the typical output voltage or current range of the sensor?

How fast does the sensor signal change? Your input time constant is 100 us, for a high-pass corner frequency of 1.6 kHz. That probably is too high for a capacitive sensor.

ak
 

DickCappels

Joined Aug 21, 2008
5,950
Actually, I don't see the desired wave at the output. Yes, it looks like to be Okay but in practice, it just gives me a flat line in the oscilloscope, except I use an opamp with at least +12 supply and adjust the + voltage for the suitable output. I think it is wise because it makes the difference. (Emphasis added for clariity)
Do you suppose that your not seeing the desired wave at the output might be related to the input AC coupling being a single pole high pass filter at 1.5 kHz?
 

Thread Starter

SAM-Mo

Joined Jan 19, 2019
32
Do you suppose that your not seeing the desired wave at the output might be related to the input AC coupling being a single pole high pass filter at 1.5 kHz?
No, actually in my circuit the RC high pass filter values are 10uF and 100R. but anyway even with the same circuit and just changing the opamp which can accept the higher voltage, it works.
 
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