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LMP7721 Amp with LMV796

Other Parts Discussed in Thread: LMP7721, LMV796, LMP7715

Hi Paul,

I have a question regarding LMP7721 application. 

Recently I found below application note of the LMP7721 used as a charge amplifier from a third party.

(https://sites.google.com/site/exergylabs/pca-boosterpack/schematic)

My question is, they used LMV796 before signal being sent to ADC. Is there any side effect of this circuit, specially when LMP7721 is implemented as a Trans-impedance configuration for low current measurements? Anyway, what are the advantages of using output buffer circuit ? 

Thanks

~Ishara

  • Hi Ishara,

    I do not see any advantage to having the LMV796 buffer. The LMP7721 has plenty of output drive, quite capable of driving the ADC by itself. The buffer just adds another source of offset and drift. In fact, the LMV796 is based off the LMP771x core, of which the LMP7721 is also based - so the outputs are nearly identical.

    There should also be a small series resistor (~200 ohms or more) and small reservoir cap (3x the internal A/D sampling cap) between the amp output and the A/D.

    I would also add a series resistor between the bias E-Pot (U3) and the LMP7721 input, to create a lowpass filter with C12, to reduce the noise/glitches on the reference line. And note that the reference voltage (set by U3) should not be more than 1V below "Analog Power".

    Regards,

  • Thanks Paul. What I understood from the cct, E-pot (U3) doing the same job you explained in the LMP7721 Eval. board manual section 4.2.1 Vref and Vbias. is it?

    Still my electronics knowledge is not enough to understand what is Amplifier biasing means and its advantages?

    By the way I have some other question regarding the LMP7721 eval. board.

    1. Page 15 of the manual, you have used R3 (10k) as a feed back resistor to LMP7715. Why can't we use this Op-amp as a follower? ( without R3 and C5)

    2. Does R3 value depend on Rgd value?

    3. What is the purpose of R7 ( I think this is a typo, it should be R4. right?) in the cct. 

    Thanks Again

    ~Ishara

  • Hi Ishara,

    The way they have the capacitors is "sub-optimal". The way the capacitors are placed, they will actually inject noise into the pot. Remember that capacitors are "shorts" to AC signals. So any AC signal (noise) on the power or ground line, will then go directly into the pot. I would eliminate C10 and C11 and make C12 larger.

    1. It is a follower. The R/C feedback creates a lowpass filter, and also helps U2's stability driving capacitive loads. You do not want the buffer to "peak", otherwise the gain goes over 1 and it could cause an input to output oscillation.

    2. No. Rgd can either be a resistor or a 0-ohm jumper. Though if Rgd is in the 2-1 position, Rgd should be a large value (>10K) to isolate U2's input capacitance from the node.

    3. Yes, it is a typo. It should be R4. R4 (R7) is there to isolate the output from the guard capacitance, and to provide some current limiting should the guard be shorted.

    This is all covered in the manual. Please re-read section 4.2.2

    Regards,

  • Thanks for the detailed explanation Paul. 

    ~Ishara