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LMP8602 no offset voltage in output

Other Parts Discussed in Thread: LMP8602

Hi all,

I'm worried this is going to be a completely trivial problem, but here I am.

I'm working with the LMP8602 in a current shunt monitoring application. We're breadboarding the circuit before we make PCBs to verify the design, and I can't seem to cause the LMP8602 to output an offset output voltage (i.e., 0V in differential = Vs/2 output). The amplifier appears to be working as expected otherwise: it behaves linearly with linear input within the range we've tested. But even if I tie the two differential inputs directly together, I get precisely 0 V out.

Here is the relevant portion of the circuit as it is breadboarded:

*Note: The schematic has one error: pins 8 and 5 are swapped (8 should be +IN, 5 should be OUT). The circuit is wired correctly on the breadboard.

I can probe directly to pin 7 of the device, and I measure +Vs there, as expected.

Any advice on what might be going wrong? Thanks in advance for your time.

  • Hi Aaron,
    Thanks for posting your problem. The idea of how the schematic is connected is correct (keeping in mind that the pin numbers are not correct; pins 6 and 7 are also incorrect). Please verify the correct pin-out from the datasheet.

    I tested an LMP8602 as in your circuit and it worked fine; this is what I did... can you try it on your breadboard? Just for now remove the shunt and the load and use standard voltage supplies.
    Connect VS (pin 6) to 3V.
    Connect OFFSET (pin 7) to 3V.
    Connect GND (pin 2) to ground.
    Connect A1 and A2 (pin 3 and 4).
    (a) Connect both +IN and -IN (pin 8 and pin 1) to ground. Applying power and measure OUT (pin 5) versus ground. This gave me 1.5V at the output.
    (b) Connect +IN to +30mV and leave -IN connected to ground. This gave me 3V at the output.
    (c) Connect +IN to -30mV and leave -IN connected to ground. This gave me 0V at the output.
    Let me point out that you will not be able to achieve the +/-75mV input range in this configuration; the most you can do with a 3V supply and mid-rail offset for a gain of x50 is +/-30mV. I noticed that the part on your schematic is labeled as 8601; are you moving from the 8601 to the 8602?
    Regards,
    Sergio
  • Sergio,

    Thanks so much for your prompt reply! I'm sorry I've taken so long to follow up.

    The problem with the LMP6801 ended up being a bad solder joint on the breakout board we were using to test the part. (Yes, we are in fact using the LMP6801 - I'd been staring at the datasheet too long and read the part number incorrectly.) Although I measured appropriate voltages at the pins of the component, it appears that the connection was marginal- it's possible that pushing down on the pin with the probe of my multimeter connected the pin for just long enough to measure.

    The LMP6801 is now functioning properly, and I've corrected my schematic :)

    Thanks again! 

  • Hey Aaron,

    I am glad to read that you found the cause of your problem.

    Regards,

    Sergio