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LM10 saturation, not as good as expected, any solutions?

Chris Green
Prodigy 20 points
Other Parts Discussed in Thread: LM10

I'm using an LM10CN as a simple voltage follower to buffer the input to a microprocessor ADC.  I'm running the LM10 single ended from a 1.8 volt supply because this the the maximum ADC input voltage.

The load current should be tiny (a few microamps) but the LM10 output seems to saturate at about 1.13 volts, is this what I should expect when running from 1.8 volts or have I got something else wrong?

The circuit is trivial, a 10:1 resistor divider (18k/2k) on the input to pin 3, pin 2 connected direct to pin 6 for 100% feedback and supply connections and that's it.


Any suggestions or ideas would be very welcome.

  • 0
    TI__Guru 61916 points

    Hi Chris,

    While the LM10 has a rail-to-rail output, the input is NOT rail to rail. As a buffer, the input and output voltages are the same.

    Figure 6 shows the input common mode range (allowable input voltage range). At 25°C, the upper limit is about 750mV below V+.

    With a 1.8V supply, the upper input limit would be 1.8-0.75 = 1.05V. Uh,Oh... Above this limit, the input stage gets pinched off, and the output will be indeterminate.

    So the device is working as it should. You need to keep the input voltage below 1V (or 0.8V if you are going cold).

    You could increase the division ratio to reduce the input swing. You can also add some gain to increase the resulting output swing. The gain resistors would attenuate the output swing slightly and reduce the swing on the inputs. A non-inverting gain of two would reduce the common mode voltages on the inputs by half..

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    Thanks!  That explains it of course, the upper limit about 750mV below the supply rail is just about exactly what I'm seeing.

    I've changed the division ration to 16:1 (18k and 1.2k), while that loses a little resolution in the ADC it's still quite accurate enough for what I'm doing.  I'm measuring the voltage of a 12 volt accumulators so the highest voltage I'm going to see is somewhere around 14.5 volts or maybe a little more.

    I don't think I need to add any gain but I'll bear it in mind if I find I need it in the future.


    Thanks again!  :-)