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LM324 for Subtractor Circuit

Brian Wang0928
Genius 12930 points
Other Parts Discussed in Thread: LM324

Hello team,

Customer using the LM324 to make a subtractor circuit. We found that the output low level is clamp by the 0.6V. it's caused by the device limitation, or not? In the Datasheet it say the low-levle output voltage is 20mV,max only. Why it's clamping by 0.6V? Please find the encloure for the measurements of critical point. Thanks.

Also, what's other OPA suggestion to solve this issue? Is OP4171 good one to solve it with single bias power of +15V? 

Condition: +15Vcc to Grounding (Single bias power).

Circuit.pdf

  • 0
    TI__Guru**** 152966 points

    Brian,

    Yes, VOL is less than 20mV when load to 10k to GND pin. This means LM324 doesn't have to sink any current. This doesn't sound impressive, but some op amps won't go to GND even with a pull down resistor (usually OP_AMP where GND pin is labeled as "VCC-")

    See output current specification "VID = –1 V, VO = 200 mV 25°C 12uA MIN 30uA TYP"
    VOL near GND is quite weak.

    When VOL rises to about 0.6V then the PNP sink driver can provide mili-amperes of current.

    Keep in mind that feedback resistors are also a load for the output. Large value load/feedback resistors, negative supply, and change OPAMP are options.

     

  • 0 in reply to Ron Michallick
    TI__Genius 12930 points
    Ron, Thanks. I now understand what happened.
    However, i wonder to know if there is any OPA option able to minimize the VoL level with specific sink current that is better than LM324?


    Regards,
    Brian
  • 0 in reply to Brian Wang0928
    TI__Guru**** 152966 points

    Brian,

    OP4171 looks good for 25C specs. However, temperature spec looks bad for output to rail.

    30mV typical for 25C and 350mV maximum from rail at temperature.