I actually have two questions, and I'm going to preface this by saying I'm not very good at circuits at electronics.

1. I'm given a voltage divider consisting of two 10 Mohms resistors connected in series and placed across a +5V to GND power supply and the input voltage to the buffer amplifier is connected to the midpoint of the resistors to produce a voltage relative to GND. What would the expected variation in output voltage of the buffer amplifier from 2.5V to be if you consider the impact of Buffer Amplifier’s input impedance and the op amp’s maximum bias current? Indicate the worst case variation from 2.5 V of the Buffer Amplifier’s output voltage. (in volts) from 2.5 V at both DC and 20KHz. Assume the op amp has an input resistance of 10 MOhms.

My attempt so far:

This is the link to the ADA4004 spec sheet: http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4004-1_4004-2_4004-4.pdf

From Table 2 in the sheet, I see that the max bias current is 85 nA but I'm not sure how to find the input impedance at DC or 20kHz though. For DC, would it be the input resistance multiplied by the open loop gain (10M * 400K)? To find the input impedance at 20KHz, I am plotting the open loop gain vs frequency (which has a breakpoint at 30Hz), and using then using the same formula.

  1. Design a low-pass filter with a cutoff frequency of 10KHz and a low-frequency gain of 10. In addition to plots of the open and closed loop gain, determine the maximum input amplitude (Vpp) for linear operation at 10KHz.

For designing the filter, I can use a feedback resistor of 159155 Ohm, a 100 pF capacitor, and an input resistor of 15915 Ohm. How do I plot open loop gain for a low pass filter? Is it the same no matter what type of circuit I am building (buffer, differential, inverting, etc.)?

  • \$\begingroup\$ Opamp input resistances (ignoring feedback) are usually measured in 100s of Megohms or even Gigohms; is the question a set one, or something you are trying to figure out for a design? \$\endgroup\$ – Peter Smith Apr 22 '18 at 16:19
  • \$\begingroup\$ It's a set question \$\endgroup\$ – Kevin J Apr 22 '18 at 16:52

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