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LM324: Low-level output voltage query

Tanmay Sutaria
Prodigy 155 points
Part Number: LM324
Other Parts Discussed in Thread: LMV324, , , , LM7705

Hello.

We wish to select an OPAMP for use with the MSP430FRxx series uCs which have an ADC input range of 0 - 1.5V. The application is very cost sensitive and hence we are considering the LM324(A) and LMV324 devices and wish to select the part which can make full use of the ADC input range.

The LMV324 'Low-level output voltage' is specified as max. 180mV for Vcc = 2.5V - 5.5V, full range of Vcc.

The LM324 'Low-level output voltage' is specified as max. 20mV for Vcc = 5V. Will this spec also hold true for Vcc = 3.3V also?

Assuming it will, would it seem that the LM324 is better suited for this application? -- as the output can swing lower hence making better use of the ADC span.

The LMV is a better, rail-rail part but for this application, the upper output swing dead zone of Vcc-1.5V of the LM324 in not a limitation, but the ability to go lower would be useful..

Please advise.

Thanks,

Tanmay

  • 0
    TI__Mastermind 25351 points

    Hey Tanmay,

    Do you have an example schematic for the application? Is swing and cost the only concern in this case?

    Have you considered the LM324LV? The output swings to 40mV from the negative rail and has an upper output swing limit of 1V from supply rail. If the device is powered from 3.3V you won't lose any ADC range if your ADC range is 0-1.5V

    Best,
    Jerry

  • 0 in reply to Jerry Madalvanos
    Prodigy 155 points

    Hey Jerry,

    Thanks for the quick response.

    The LV part looks nice, had missed it in my search, so thanks, but the low swing spec is still 75mV worst case so we still loose 0-75mV i.e. 5% of the ADC range and that is still 3.5x the 20mV specified for the LM324 which is <1.5% of ADC range. Can this spec be achieved for the LM324 or am I interpreting the spec wrong? Is it for 5V / bipolar supplies only or will it hold true for 3.3V single supply also?

    Also, the LV part is presently out of stock - only LMV324 (without A) and LM324A are available, hence the question focused on the same.

    Regarding the schematic, I do not have the segment right now but is fairly straight forward. For sake of simplification, ignore the margins for tolerance and offset. We wish to measure an AC signal and hence one input of opamp is biased at 750V (mid point of ADC) and signal of 175mVac(~250mVp-p) is applied between the inputs. With a gain of 3 this gives an output swing from 0-1.5V matching the ADC. Since the output cannot actually swing to zero, the ADC range is wasted and hence looking for an opamp with the lowest bottom dead zone in the output. In the practical application bias set point will account for the dead zone, component tolerance and over signal limits also.

    Hope that clarifies the issues. Will share the sectional schematic in a day or so,

    Thanks,

    Tanmay.   

  • 0 in reply to Tanmay Sutaria
    Prodigy 155 points

    FYI. We use/need the TSSOP-14 'IPWR' part for any/all of the devices mentioned.

  • 0
    Guru 140200 points

    Hi Tanmay,

    what about the timing? What is the aquisitaion/sampling time of ADC? And how often do you want to read the (same) ADC channel per second?

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 25351 points

    Hey Tanmay,

    You should see similar performance at 3.3V for the LM324. Even some of our newer general purpose devices only swing within 20mV of the rail. Since you are measuring an AC signal, would it be worth it to look into decoupling the output and biasing it to the middle of the ADC range? Then you can set your amplifier output common mode to keep the amplified value in linear range and the output swing wouldn't matter?

    Best,
    Jerry

  • 0
    Guru 140200 points

    Hi Tanmay,

    you should not look at the "low level output voltage" specification in this application, but at the specification range of "large signal differential voltage gain":

    When the outputs emits 20mV then the output is fully hanging in saturation and the OPAmp is no longer working in its linear operation range. More, all parameter specifications are heavily degraded then, which is especially unwanted in an application where you want to output an AC signal.

    As with everything in life, you get what you pay for and the cheapest OPAmp may not be the best choice here. Having said that, you will never get an OPAmp which can go all the way down to 0V at its output and still behave properly. When using a single supply OPAmp you will always need to use the pseudo ground technique allowing the output of OPAmp to stay well away from 0V.

    Another remedy is to use a small and harmless negative supply voltage by the help of LM7705, which was designed for exactly those applications.

    Kai