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LMP2021MAEVAL Gain Calculation

Jaden Ghylin102047
Expert 1140 points
Other Parts Discussed in Thread: LMP2021, LMP2021MAEVAL

To calculate my gain and overall amplifier output of the eval board, I used both the eval board data sheet and the LMP2021/2022 data sheet.

 Using the eval board in its base configuration with R1 a DNI, R11=R12=10k, and R10 = 150 Ohms, I get a gain of 134 using the equation on page 1 of the LMP2021MAEVAL data sheet.

Is this correct?  The actual measured gain appears to be about 1/4 of this value.  

Does the use of a single supply affect the setup or gain of this circuit?

  • 0
    TI__Genius 16890 points

    Yes, you are correct, the gain should be 134.

    Here are a copule of things to check.

    Check to make sure that you do not have any problems with input or output voltage ranges.  The positive input of the LMP2021 should be within the CMVR of the op amp with the supply that you are using.  For example, if the bridge is made of four resistors with the same value, V+ of the board is 5V, V- is ground, the output of bridge will be centered at about 2.5V (1/2 of the Vref) which is within the input CMVR of the LMP2021 when using a 5V supply.  The output also needs to be within the supply range of 0V to 5V, (minus a few tens of millivolts on each side).  With the gain set for 134 the input signal can't be larger than about 35mV.  A good article that explains this is at:

    http://i.cmpnet.com/planetanalog/2010/08/C0611pt1.pdf

    and

    http://i.cmpnet.com/planetanalog/2010/08/C0611pt2.pdf

    Gain is defined as the voltage difference of pins 3 and 5 of J2 divided by the voltage difference between pins 3 and 5 of J1.

    Mike