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Original question:

LMH7220: Input impedance

LMH7220: IN+ / IN- current draw outside of 4V differential

Timothy Chermak
Prodigy 30 points
Part Number: LMH7220

Looking for some help clarifying the input current draw (e.g. impact on a reference voltage supply) for something like a mid-point supply on Vin- and a full-range swing at Vcc=12V (Vin+_min = 0V; Vin+_max = Vcc-2 = 10V; Vin- = fixed 5V)

Figure 4 shows the input current going exponential as the diff hits about 3V:

I realize that this shows an approx. differential input impedance of 320k, but want to be sure I understand the part's performance outside of that linear region at the center.

Thanks

  • +1
    Guru 140200 points

    Hi Timothy,

    see figure 19 of datasheet: When the differential input voltage exceeds about 1.4V, or by other words the voltage drop of the two protection diodes in series, the input bias current begins to rise exponentially until it is limited by the two 1k5 resistors in series. This scheme allows a differential input voltage of up to +/-13V without damaging the LMH7220.

    Of course, when the protection diodes begin to conduct the performance of LMH7220 might be degraded. 

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 11165 points

    Hi Kai,

    Thank you for your help on this question :) 

    Hi Timothy,

    Thank you for your post, this is a great question. I agree with the information that Kai kindly provided. Please let me know if you need further explanation on this.

    Best Regards,

    Joe

    Applications Engineer

    Linear Amplifiers Business Unit | Comparators Product Line (CMPS)

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml)

  • 0 in reply to Joe Vanacore
    Prodigy 30 points

    Agreed - thanks for the quick response! I just needed to verify that fig19 wasn't just an approximation. This gave me exactly what I needed.