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Original question:

INA114 voltage output range

OPA177: measurement 0-30mv analog signal

elektroboard
Prodigy 105 points
Part Number: OPA177
Other Parts Discussed in Thread: OPA192,

Hi.

I need to increase the value measured by a sensor giving 0-30mV output by 100 times and process it with an MCU.

I built the circuit below. However, when I connect the sensor to the input of the opamp, the sensor output voltage drops.

How can i solve this problem.

  • 0
    TI__Mastermind 22137 points

    Electroboard,

    1. Your problem is possibly due to a device limitation called "output swing to the rail".  This parameter measures how close the amplifier can get to the power supplies.  This device is specified to have a swing of 13.5V on a 15V supply for a 10k load.  That means that the swing to the rail is 1.5V from a 15V rail.  So on a 5V rail you should be able to get to 3.5V.  Your output is 30mV x 100 = 3V, so technically you should be ok, but you are close.  Nevertheless, the device is only specified for a 15V supply so the assumption of 1.5V from the rail may not be true at 5V.  Furthermore, the swing will get worse if the load is smaller than 10k.  Do you have a load not shown in the schematic?  This video covers this concept:  https://www.youtube.com/watch?v=q9jz1cY_1ts 
    2. Can you measure the supplies?  Any droop below 5V can be a problem for output swing.  Can you adjust your supply to a larger value and see if it solves the issue?
    3. If your issue is swing to the rail, you may consider a device like OPA192.  This device is similar to OPA177 but has better swing to the rail and common mode range.
    4. Do you use decoupling capacitors in your actual design?  This article explains the importance of decoupling: https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/the-decoupling-capacitor-is-it-really-necessary 
    5. Can you give more details on the error.  A table of Vin vs Vout would be helpful so that we can see if the error happens only when the input signal is at certain levels or if it happens across the range.

    I hope these questions get your trouble shooting moving in a good direction.

    Best regards, Art

  • 0 in reply to Art Kay
    Prodigy 105 points

    Hello Art. Thanks for the valuable information I have provided. I will make these measurements in more detail.

    The problem is that if the output voltage of the sensor is lower than 1mv, I can't get the output as much as the gain.

    My project is to make a 0-10v converter for a pyranometer with 20uV/W/m2 sensitivity.

    Maybe the circuit I gave above is wrong. Do I need to change the topology? Your suggestions are very valuable to me.

  • +1 in reply to elektroboard
    TI__Mastermind 22137 points

    Elecktrobord,

    Can you provide a data sheet on your sensor?  The main thing I would like to know is the impedance of the sensor.  

    Thanks,

    Art

  • 0 in reply to elektroboard
    TI__Genius 12445 points

    Hi,

    I'll jump in here - can you clarify when you said: "The problem is that if the output voltage of the sensor is lower than 1mv, I can't get the output as much as the gain."

    Was this a measurement using the exact circuit you have shown above?  Meaning the supply voltages are +/- 5 V?  If so, there's no way you will get a full 10 V swing.  Can you post the circuit that you measured and the results you are getting?

    Regards,
    Mike