I want to amplify the noise generated by a MAX8069 voltage reference, using direct coupling in order to preserve the low frequency components. The frequency band of interest is from 1Hz to 44Khz. I'd like to be able to measure the power spectrum at different frequencies, so minimising filtering and distortion is important. The output of the op-amp will go to the input of an AVR ADC, which is high impedance. The application is a random number generator.
I had the idea of measuring the difference between two similar noise sources; this should yield plenty of noise whilst allowing the direct coupled design.
The op-amp chosen is LM358, noise is supplied into both inputs from two 1.22v MAX8069 voltage references. These should generate around 20µV noise. Supply voltage is 5v and current into each reference is limited to around 1ma by 3k3 resistors. This should maximise the noise each reference generates - see Terry Ritter's junction noise measurements.
The expected behaviour is to see large voltage noise at the output of the op-amp. What I'm actually seeing is the output steady at 3.75v. This changes to 3.8v if the inputs are swapped.
What is the explanation for the fixed output voltage, and what modifications would be needed to see the expected behaviour?