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OPA333: Sallen-Key filter and level shifter

Gerhard Kreuzer
Expert 2580 points
Part Number: OPA333

Good Morning,

I have a current sensor chip (ACHS-7123) which gives 0.5V at -30A, 2.5V at 0A and 4.5V at +30A.

I only have positive currents.

My ADC (ADS1015ID) has a range of 0-4V.

I want to build a level shifter to get ~0V for 0A and ~4V for 30A to use the ADC range as good as possible.

And some filtering of the signal is also a nice idea.

Here my solution:

I_Sensor Signal Conditioning.TSC0118.I_Sensor Signal Conditioning.TSC

It gives a nice result, but maybe there is a better solution out there.

And: I have to use this chips, cause the original design used them and we have a stock.

With best regards

Gerhard

  • 0
    TI__Guru* 93105 points

    Hello Gerhard,

    Smart move staying with an op amp device that you have stock! Many ICs are in short supply right now. Besides, if you want low power, extremely low voltage offset and drift and rail-to-rail input and output the OPA333 is hard to beat.

    I did some simulations on your design and it appears to satisfy the 0 to 4 V output range needed for the ADC input and provide the low-pass filter function. I am not really sure how much simpler of circuit we could come up with. If there something more you need it to do then we could reconsider, but as it is I think you are good to go!

    Do note that the amplitude roll back seen in the amplitude vs frequency simulation is normal for a Sallen-Key low-pass filter. This occurs as the op amp begins to run out of open-loop gain with increased frequency, the output open-loop output impedance (Zo) increases with frequency and in-part is affected by the R and C values selected. The ultimate attenuation is better than -40 dB and according to the simulation and is no less than -20 dB at higher frequencies. The Multiple Feedback (MFB) topology exhibits the effect to a lesser amount, but is an inverting configuration.

    Regards, Thomas

    Precision Amplifiers Applications Engineering