Description
Problem 1: Feedback amplifier noise
Ri
Rf
Vout
Vin
Figure 1. Inverting amplifier
Rf = 10k and Ri = 1k. The opamp has a gain-bandwidth product (fT) of 10MHz, input voltage noise
density (en) of 10nV/Hz, and input current noise density of 1pA/Hz.
a) (10 points) Determine the input-referred noise voltage density (in V/Hz) of the amplifier,
including contributions from all noise sources.
b) (5 points) Determine the equivalent noise bandwidth (fENB) and input-referred RMS noise
voltage.
c) (5 points) Check your answers to a) and b) by simulating the circuit in Ltspice using the
UniversalOpamp2. Be sure to set the noise voltage density ‘en’ and current noise density ‘in’ to
the appropriate values. To verify the RMS noise voltage, you can ctrl-click on the plot title (or
export the data to MATLAB/Python).
Problem 2: Opamp noise characterization
Rs Vs
Figure 2. Opamp noise testbench
Characterize the noise performance of the AD8691 and ADA4898 opamps. For the AD8691, use a single
supply of 5V. For the ADA4898, use a split supply of +/-5V. Be sure to observe any test conditions
provided in the datasheet(s) when running your tests.
a) (5 points) Using the unity-gain feedback configuration shown, verify the 3dB bandwidth of each
amplifier. Use this to calculate their equivalent noise bandwidths.
b) (5 points) Setting Rs = 0, perform noise simulations to verify the input voltage noise density of
each opamp at the frequencies specified in their datasheets.
c) (5 points) For the ADA4898 only, set Rs to a value large enough such that the opamp’s current
noise density should be 10 times that of its voltage noise. Verify the current noise at the output
of the amplifier using the relationship en,out = inRs. Be sure to use the term ‘noiseless’ after the
resistor value to ensure its noise doesn’t affect the simulation results.
d) (5 points) Calculate the expected output RMS output noise in a 1MHz bandwidth for each
amplifier and perform a noise simulation to verify. Note/explain any discrepancies.