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OpAmp for motor speed detection

Marvn Arcangel
Genius 12615 points

Hi Team,

Good day. Can you please help with the inquiry below?

A customer is using magnetic pickup sensor and they need to measure so which op amp is suitable for that application?

The magnetic pickup sensor parameters like... Output of magnetic sensor voltage 250mv to 90vAC , sinusoidal wave form . 
2) this sensor for diesal motor speed detection so we have to find out motor speed 
3) we can't find out proper wave form AC and we have reading the value so the op amp output should be DC 
Would you kindly help further?

Thanks!

Regards,

Marvin

  • 0
    TI__Mastermind 33031 points

    Hey Marvin,

    I'm assuming this is being sensed with an ADC. This introduces a couple limits. First, general purpose amplifiers don't typically go above 40V max supply, so you will at least have to divide the sensor output by more than half to keep it in the acceptable range of a 40V amplifier. However, typical ADC voltages top out at 5V. 

    You can use a peak detector circuit as well as a buffer amp to your adc to sense this. Below is an E2E thread where another customer built a peak detector. 

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/960521/opa177-peak-detector-circuit-issue 

    I have the following questions to help with amplifier selection.

    1. What is the desired output voltage range?
    2. What resolution/input voltage range is your adc?
    3. What resolution of sensing is acceptable? 
    4. Is automotive rating required?

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    TI__Genius 12615 points

    Hi Jerry,

    Here's more information from the customer:

    1. The application is a Motor RPM speed detector in which they are using a magnetic pickup sensor.
    2. The sensor outputs a sine wave that their MCU could not read.
    3. They want to convert the sine wave output from 250mV to 90VAC.

    Are they using a correct sensor for their application? is the 250mV to 90VAC even possible?

    Thanks again.

    Regards,

    Marvin

  • 0 in reply to Marvn Arcangel
    TI__Mastermind 33031 points

    Hey Marvin,

    You can scale down the input, but doing an unscaled input of 250mV to 90V is not possible with general purpose amplifiers.

    The following circuit is assuming 90VAC is 90Vpp. If it's actually 180Vpp, you must scale down further.

    If you scale it down to 5V max, your new input range is 13.8mV to 5V, which you can do with a low voltage op amp.

    90V can be scaled down and placed at midsupply with the following configuration, then you can use a low voltage amplifier to buffer this to your ADC.

    This is if you need to keep your AC portion of your signal.

    If you just need the amplitude, you may use a peak detector as previously mentioned 

    Best,
    Jerry