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LM7121, violating specifications?

yoko19298
Intellectual 395 points
Other Parts Discussed in Thread: LM7121, MAX232

Hello friends,

We have a design which implies two cascaded LM7121, each one configured as non-inverters, G=4...5 and powered by +/-9Vdc.
The input stage is DC coupled from a PI filter LPF and in the very front, there is a balun transformer, so first amp see's ground on positive input.

The question is what if negative supply is some kind different from positive one which is well regulated at +9Vdc. In fact, we steel power from MAX232's available on board as negative rail which is obvious un-regulated and in range of -8...-10Vdc depending on loading. We downloaded spice model for LM7121 and simulations looks very nice but just to be sure. This supposed to be a very flat amplifier in 0.1...10MHz range.

Thanks in advance,

 

  • 0
    TI__Mastermind 22825 points

    Hello,

    If I understand the question correctly: You like to know what are the consequences, if any, when the VEE supply is unregulated, right?

    If the ripple on VEE is large enough and at high enough frequency, you could possibly get some error to appear on your output due to finite PSRR. However, at your low closed loop gain and the fact that the LM7121 PSRR- is specified at 78dB (good high number at DC), I think that would be very unlikely in your situation. Note that PSRR degrades at higher frequency.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Intellectual 395 points

    More precisely, I need to know the consequences if positive supply is different than negative one, say +9V and -7V because the negative is unregulated, coming from MAX232's internal charge pump.

    Simulation in PSPICE looks nice, OA performs great especially on this low loop gain, but should insert blocking capacitors at input to feed positive terminal, or leave-it as is . At output we inserted anyhow a blocking cap. I hope I described well concerns.

    Best regards,

     

  • 0 in reply to yoko19298
    Intellectual 670 points

    Hello,

    Leaving the rails unequal shouldn't cause any issues in overall operation.  After all, this part is rated to run at +5V, which has uneven rails as well.  You do change the mid-rail point, so putting blocking caps on the input would be recommended.

    That being said, in addition to Hooman's concerns about PSRR, I would be hesitant about the current that the MAX232 could nominally supply out of the charge pump.  I can't find any details on the MAX232's maximum current output, but depending on the load of the op amp, you may be exceeding how much power the MAX232's charge pump can provide, especially if you have mutliple op amps hooked up to one MAX232.

    If that isn't a concern, then you should be OK operating the amplifier with uneven rails.