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LMP7731: Product Recommendation

Marvn Arcangel
Genius 16870 points

Part Number: LMP7731
Other Parts Discussed in Thread: LMP7721, OPA392

Hi Team,

I am having trouble finding an op-amp that would satisfy the customer requirements below. The customer finds LMP7731 is close but the data-sheet is lacking key information.

The customer needed an op-amp that is ultra-low noise (like the LMP7731) but also that has inputs with low bias current and low offset current across -40-125C when the common-mode voltage is at Vs (or Vs-20mV). the customer application is an ultra low noise current sense feedback loop and the circuit's worst-case total offset voltage (i.e., Vos + Ib_offset*Rin) needs to be low and guaranteed.

Figure 29 on page 11 of the data sheet shows the bias current on the LMP7731 rise when the inputs are within 1/2 volts of Vs and the customer would like to understand any information about what the bias currents do at the "rails" as it is advertised as an RRIO op-amp.

Can you help share information for the case where the common-mode voltage is at Vs, what the bias current and bias offset current is versus temperature and versus Vs

Similarly, what are the noise parameters (e.g., corner frequency, noise voltage density, noise current density 0.1-10Hz noise voltage) do versus temperature and versus Vs.

Can you also recommend another device that you believe might be better for the customer application?

Thank you in advance.

Regards,

Marvin

  • 0
    Guru 140200 points

    Hi Marvin,

    the datasheet contains lots of information to answer your questions. Have you seen the section "input bias current cancellation"? Looks like this OPAmp isn't well suited for input voltages close to the positive supply rail. In this situation the input bias current cancellation circuitry seems to fully turn-off, because there isn't any headroom left for the control.

    What is the maximum input bias current you can allow?

    Kai

  • 0
    TI__Guru 74561 points

    Marvin,

    LMP7731 has two bipolar input differential pairs with maximum IB of 85nA (nano-amp) at 125C - see below.  thus, this is not something I would call low IB.

    Also, it includes input bias cancellation circuitry causing increase in IB close to either rail - see below.

    If your customer needs a real low IB, LMP7721 with typical IB of 3fA (femto-amp) and max 5pA at 125C (>10,000 times lower than LMP7731) should be their choice.

    It maintains low IB all the way to (V-) but not at positive rail - see below.

    For low IB close to positive rail, please recommend RRIO OPA392 with maximum IB of 30pA at 125C - see below.

  • 0
    Prodigy 150 points

    My question is: At 125C, what is the typical and worst case Ib, Ios, CMRR and 0.1-10Hz noise, when Vcm=Vs? It would be great if you could also give me these at 25C, and 85C.

  • 0 in reply to JOHN MCCORKLE
    TI__Guru 74561 points

    We do not test LMP7731 IB at Vcm=Vs at 25C (let alone at 125C) so I cannot give you a definite answer.  However, considering that its max IQ is 4mA at 125C, and typically 5-10% of IQ is in the input stage (200uA-400uA), this would mean that without IB cancelation (and assuming min beta of 100) at Vcm=Vs the max IB could be at  2-4uA (NPN active so IB current flowing into the input terminal). As far as Ios goes, without IB cancellation it is typically 10% of IB so expect max Iso 200-400nA (into the input terminal).

    Min CMRR is not specified at 125C but considering max offset drift of 5.5uV/C, you could expect at 125C max Vos shift of 550uV, which would would translate in -6dB, thus resulting in min CMRR of 74dB (estimate).  1/f noise should pretty much be unaffected by Vcm and temp.