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LMH6702: Large DC Offset with Non-Inverting Amplifier Topology

Christopher Tocci
Prodigy 10 points
Part Number: LMH6702

Folks,

I am having a very large negative DC offset voltage at the output of a simple non-inverting topology.  See the circuit below:

The input to the far left, just before the fuse, F1, is a 500 OHM resistor to AGND.  With the Relay RL1 OPEN (as shown) and Relay RL8 CLOSED, the gain of this circuit is about 40.  The problem is the DC offset at TP2 (the output of U4) is about -2.5VDC with 0VDC input at TP1.  When you OPEN RL8, the gain is unity (+1) and the DC offset output voltage at TP2 is -50mVDC.  According to the expression for total output DC offset voltage from this amplifier (LMH6702), I should not see more than 32mVDC, so I am not shure what is happening with this amplifer.  I can live with the unity gain DC offset voltage, but cannot deal with the -2.5VDC offset voltage at TP2 when I switch the gain the +40 by closing Relay RL8.

Why am I getting such a huge DC offset voltage at TP2 with that gain.  The datasheet computation indicates a worst-case of about 552mVDC.  I am getting 5 times that!  Why??

Thanks,

Chris Tocci

cstocci@comcast.net

  • 0
    TI__Genius 15975 points

    Hi Chris,

    The main contributor for the large output DC offset voltage seems to be the R5 of 4.53k along with the non-inverting input bias current of -15uA. If you just try to calculate the output DC offset voltage due to non-inverting input bias current, it comes out to be -2.74V. The other two contributors (Vos and Ib-) just try to cancel out this offset voltage due to Ib+, but are very small. 

    Below shows the output Offset voltage individual contributors and the total output offset voltage:

    Vout_Vos = Vos*(1+Rf/Rg) = 4.5mV*40 = 0.1817 V

    Vout_Ib+ = Ib+*Rin*(1+Rf/Rg) = -15uA*4.53k*40 = -2.73V

    Vout_Ib- = Ib-*Rf = 30uA*500 = 0.015V

    Total output offset voltage = Vout_Vos + Vout_Ib+ + Vout_Ib- = 0.1817 + (-2.73) + 0.015 = -2.57V

    Are you making sure that you are grounding the other side of the fuse with 500-ohms to AGND? From the above calculation, it does not look like the 500-ohm resistor is being grounded at all when the gain is set to 40V/V. Would it be possible to attach a schematic on the other side of the fuse as well?

    Best Regards,

    Rohit