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LMP2012QML-SP: (or commercial LMP2012) - What is the Max/Min Input bias Current?

user4175682
Prodigy 60 points
Part Number: LMP2012QML-SP
Other Parts Discussed in Thread: LMP2012, LMP2012QML, LMP2021, LMP2011

Hello,

We are developing  payloads for space missions (Mars rovers for 2020) and we have some important doubts regarding part LMP2012/LMP2012QML

which we need to be resolved to ensure the functionality of our designs.

We see that the typical values of bias current are -3pA (same for space model and commercial model), there is also a graph in the commercial model datasheet which implies that Ibias could be around around 100 pA depending on Vcm.  

Nevertheless, we have done some designs to measure this current (Transimpedance configuration with resistor and integrator configuration with capacitor to measure the slope of the bias current ramp) and it appears to be much higger (400-800pA), we have also found an open question (which haven’t been answer yet) regarding this issue here:

 

https://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/541206/1985485#1985485

 

We are aware that trying to measure this magnitude of bias current is a delicate issue and that maybe our test boards for this are not well optimized, however we need to be sure that the values that we sense are wrong and that the real ones are those provided by the datasheet so it is safe to ask you about it. It would be great to have some clues in how you measure this currents in order to replicate it in our lab.

 So to sum up, questions are:

1)      Are the datasheet values of Ibias correct?

2)      What are the Max/Min values for this parameter? We haven't seen it in the datasheets.

3)      If it is possible, how can we replicate the measurements of this values in our lab?

Thank you very much in advance!

  • 0
    TI__Genius 17115 points
    Due to the nature of the chopper inputs, the input bias current is not constant. The input MOSFET chopping stage creates “dynamic” transient currents on the inputs due to charge injection. These will swamp out what we would call actual “bias current”. These peaks can be several hundred pA or even nA’s. Think of it as a high, repetitive current-noise.

    The waveshape and duty cycle of these transients will change depending on the amount of “correction” needed – so the waveform and amplitude is not constant.

    What you end up seeing is an integration of the current pulses against the input impedance and capacitance. So to “null” the input currents, you actually have to balance the input capacitance. This is discussed in better detail in the datasheet for the LMP2021. While the plots would not be applicable to the LMP2012, the application issues are the same.

    This is why there is no Ibias limit…. Normal pA bias current measurement techniques cannot be used. They would be set really wide compared to normal CMOS devices.

    Basically, choppers should NOT be used for high-impedance applications (Zin < 100K). The bias currents would be less than super-beta bipolar inputs, but higher than CMOS/JFET inputs.

    There is a pspice model for the LMP2011 which is applicable to the LMP2012. It is in the Tools folder on the LMP2011 product page:
    www.ti.com/.../toolssoftware
  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 60 points

    Thank you very much for the detailed answer Kirby. I appreciate that you included the links for the pspice model and the reference to LMP2021, the input current is carefully described there.