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LM8358: Reference Bias Issue

Edmond Chiu
Prodigy 170 points
Other Parts Discussed in Thread: LMP8358, LMP7701

Hello,

I currently have my LM8358 connected the way it is in Figure 61 on page 32 of the LM8358 datasheet. This would make it so that the bias reading I get from this amp to 2.5V. I'm trying to run the diagnostic in which i send the AMP 0x0082, meaning that I want to set the MUX in the configuration where I ground IN- and attenuate IN+ by 50 and set the gain setting to 50. When I take this reading, I get 4.993377 V. Because of the Reference Bias, I subtract 2.5 V from this to get around 2.5V. However, let's say that the IN+ is shorted to V+. Theoretically, it would be 7.5V due to the Reference Bias but the chip is unable to send out a voltage over 5V. What configuration would I need to do in order to determine whether or not the IN+ is shorted or working properly? Thank you.

  • 0
    TI__Genius 16890 points

    Hello,

    If you are using figure 61, VBridge = 5V,  the circuit is working correctly (no faults) and you send 0x0082 to the LMP8358 you should get 2.5V on the output.  With no faults IN+ and IN- will be at 2.5V.  With the register set to 0x0082, IN- is grounded, the signal at IN+ is divided by 50 and the multiplied by 50 so the output should be at 2.5V.  If you are getting 4.99V on the output that means the input is at 4.99V.

    Mike

     

  • 0 in reply to Michael Stout
    Prodigy 170 points

    Thank you for your response Mike. I have a follow up question. When I try to do a gain setting of 100 by sending 0x0003, I get the reading of 2.6V when I have a 1mV signal connected to +IN terminal and ground connected to the -IN terminal. I also get a reading of 2.5V when I have both +IN and -IN grounded together. I was under the assumption that because of this, my values were being biased by 2.5V so I had to subtract 2.5 from my readings in order to get the correct values. Am I using this chip properly? Thanks again.

  • 0 in reply to Edmond Chiu
    TI__Genius 16890 points

    Hi Edmond,

    Yes, you are using it correctly.  I overlooked the information that you were using a 2.5V reference, so you do need to subtract 2.5V.  In the original post above, you should be getting 4.99V.  In the post directly above, you are getting the correct values. 

    Sensing a short of the input to V+ would be more complicated with the reference at 2.5V.  You could set the reference lower, maybe 2V.  This would give you a 4.5V output with no fault and 4.99V with a fault.  Another possibility is to run the bridge at 4V. 

    Mike

  • 0 in reply to Michael Stout
    Prodigy 170 points

    Thank you for your response, that makes sense. However, which solution would be easiest to implement ,i.e. changing as little hardware as possible?

  • 0 in reply to Edmond Chiu
    TI__Genius 16890 points

    If you have the LMP8358, the bridge, and the reference all connected to +5V, the easiest change would be to change the value of one of the 10k resistors connected to the LMP7701 positive input to make that point 2V.

    Mike