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lmp2022 Doubt

fco.javier quiros jacobo
Prodigy 140 points
Other Parts Discussed in Thread: LMP2022, OPA2277, LMP2021

Hello everyone, 

I'm going to design the adaptation part of the ADC peripherical of F28M36. I want to use a two steps, one for high impedance input and the other step with a second order amplifier filter (with the necessary gain to adapt 5V signal to 3.3 V for the DSP).

I used to use OPA2277 amplifier for this applicattion and it works very well. However I have been looking for new amplifiers (time goes by and maybe there are cheaper amplifiers with same features). I found lmp2022 but I have some doubts.

I see that voltage supply is between 2.2 to 5.5 V and it has rail to rail output. I thought to supply it between 5V and 0V. It means that otuput voltage can  be exactly 5V or 0V ? And, the same for the input? Sorry for these questions but I can't find this information on its datasheet (http://www.ti.com/lit/ds/symlink/lmp2022.pdf) .

In addition to these questions I would like to know your opinion for this amplifier for this kind of application. It will work such good as opa2277 amplifier?

  • 0
    TI__Guru 57786 points

    Hello Javier,

    The "supply voltage" to the op amp is the total voltage between the V+ and V- pins. The "2.2 to 5V" is the minimum voltage that must be between the power pins. Of course, it could not work with "0V" between the pins...

    So if you have V- at 0V (ground), and V+ at +5V, then you have a 5V supply - which is fine.

    The output is "rail to rail", which means the output will swing within millivolts of the rail, but it will NOT swing exactly to the rail due to voltage drop across the output transistors. ALL op-amps do this - not just the 2022  (some better than others). How close the output swings to the rails is documented in the electrical table specs (Output Swing High and Low), as well as in the graphs (Figs 27-30). Output swing depends on the current drawn from the output. The more current drawn, the farther away from the rail the output will be.

    The input is NOT "rail-to-rail". The input can go from 0V (V-) to about 1V below V+, so on a 5V supply, that would be 0 to 4V total input range. You need to be aware of this when operating as a buffer or other very low positive gains (<1.5).

    Note that the OPA2277 is a 30V part and has a narrow input voltage range at 5V that extends between 2V above and 2V below the supplies, so the legal input range for a +5V supply is between +2 and +3V - so the LMP2022 would provide significant common mode headroom.

    If your circuit has been operating satisfactorily with the OPA2277, then the LMP2021 will work just fine. However, a word of caution; The LMP2021 is a "chopper" amp, and has a chopping frequency around 35KHz. Some of this chopping noise energy will appear above 35KHz. If the BW of your circuit includes >35KHz, then be prepared for a little noise. Also, the LMP2021 is not suitable for very high (>1Mohm) source impedance due to the charge injection on the inputs due to the "chopper" switches in the input.

    So order a few samples and give it a try.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 140 points

    Hello Paul Grohe

    Thanks a lot for your reply. My doubts have been resolved :D. Now I know how works the rail-to-rail amps.

    Your advise about LMP2021 which works as a Chopper surprise me. I don't see that information on the datasheet. Thanks for this ;).

    I would like to clarify that OPA2277 I supplied to +-12V. That's why it works me fine. Now I'm looking for new amps and I must chose how to supply them.  Thanks to you I see that LMP2022 is not for me. My inputs must be between 0 to 5V. Then I need to supply LMP2022 between 0 to 6V  but Its maximum supply is 5.5V. 

    I will keep searching for new options.

    Thanks again Paul.