This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

  • TI Thinks Resolved

current and voltage sensor circuit

Intellectual 260 points

Replies: 11

Views: 864

Hi all 

Sorry to bother you but I'm still confused .I need to measure between voltage supply  and power amplifier ( voltage range from 5- 30 V ,the current range from 150-900 mA  and the signals  bandwidth are  4MHz , 10MHz).I will probe the output voltage form the sensor.I tried to simulate the circuit but  the result is not good.

Thanks

Sattam 

  • Could you share your schematic that you used for the circuit simulation? What problem are you having?

    Regards,

    Micah Brouwer

  • In reply to sattam alshali:

    Hi Micach
    I calculated the current as follow:
    Is=Vs/Rs=0.549 A
    Is=(Vout*1k)/(Rs*Rout)=(4.15*1k)/(1*1e6)=4.15e-3A

    and the results are not the same

    Thanks
    Sattam
  • In reply to sattam alshali:

    Hi Sattam,

    The INA139 amplifies the voltage across the sense resistor according to the gain set by the load resistor. In your circuit, the voltage across the sense resistor is 0.549V, and the gain is set to 1000 with a load resistor of 1MEG. That is an extremely high gain. The output voltage is the input multiplied by the gain, so the amplifier then tries to output 549V but that exceeds the output limitations, which is specified in section 6.5 as (V+) - 0.9 (typ). With a supply of 5V, the max the device can output is about 4.1V, which is what the device is outputting. This is called railing the output.

    You need to considerably lower your gain and your sense resistor. Normally, sense resistors are in the range of 1mOhm to 100mOhms.

    Regards,

    Micah Brouwer

  • In reply to Micah B Brouwer:

    Hi Micah
    Thanks so much for your help. Now, I need to get current sensor with higher bandwidth around 10MHz.Do you have any suggestion.
    Thanks
    Sattam
  • In reply to sattam alshali:

    i Micah again 

    Sorry to bother you.  

    I tried to simulate the THS4131 which has around 15MHz but the result is not good.Would you mind check my schematic.

    Thanks for your help

    Sattam   

    THS4131 High Side Schematic.TSC

  • In reply to sattam alshali:

    Hi Sattam,

    You're welcome, I am here to help. In the future, please be specific about what kind of issue you are having. Saying the result is "not good" is not very helpful. What is not good about the circuit? What were you expecting?

    You should remove R25 in your schematic, and then you will see the output voltage (Vadc) according to what you would expect, given that there is a 50mV drop across the sense resistor and the first stage is in a gain of 2 and the second stage is in a gain of 5, so a gain of 10 in total and an output of 500mV. This is from the equation at the bottom. Currently in your schematic with R25, it is creating a voltage divider between R7, R8, and R25, and so the output is reduced (~330 mV).

    Also, it maybe be helpful in the future to simulate the voltage drop across the shunt resistor with a voltage generator. The below snip from the original TIDA-00976 file shows the signal generator being converted from single-ended to differential with voltage controlled voltage sources. TINA prefers using one voltage source and converting from single to differential rather than two separate voltage sources. This way you can run a transient simulation and see the results from a sine or square wave input.

    Regards,

    Micah Brouwer

  • In reply to Micah B Brouwer:

    Thanks a lot Micah.

    Is there any reason to use 1kOhm for R15 or can I  increase it to  20kOhm

    Wishing you the best 

    Sattam  

    TIDA-00976.pdf

  • In reply to sattam alshali:

    Hi Sattam,

    R15 is to simulate the load that the amplifier is driving. In this case, the amplifiers are designed as an input to an ADC. You could increase R15 to 20k. If you take a look at figure 27 in the THS4131 datasheet, you will see how the differential load resistance affects the output voltage of the amplifier. The amplifier might not be able to drive small loads (<1K) to the full voltage range because it will be limited by the maximum output current. Probably not a problem in your situation, but something to keep in mind.

    Regards,

    Micah Brouwer

  • In reply to Micah B Brouwer:

    Thank Micah. Sorry for late but I have some problems to access my account

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.