Noise measurement design of low fixed gain differential amplifier

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Overview:
Noise consisting of random small voltages can be difficult to measure, and the noise of the laboratory instrument itself complicates the measurement problem. When measuring noise, it is often necessary to use specialized techniques. For example, amplifiers are typically configured with high closed loop gain to make the amplified input noise easier to measure. However, the noise measurement of low fixed gain differential amplifiers faces a greater problem, integrating feedback and gain resistors, making it inconvenient to use high gain configurations. In addition, in order to interface with the spectrum analyzer, differential single-ended conversion is required. The second stage amplifier can provide the gain and perform differential single-ended conversion, which subtly solves the above two problems.

Figure 1 shows the optional gain (1, 2 or 3) differential amplifier ADA4950-1 followed by the low noise, low distortion op amp AD8099. The AD8099 converts the differential output to a single-ended signal with a gain of 10. Compared to the ADA4950-1, the AD8099's 1nV/√Hz equivalent input voltage noise is negligible. The output of the ADA4950-1 is amplified 10 times and its noise is proportionally amplified. Using a 0.5pF compensation capacitor and 10x gain, the AD8099 has enough bandwidth to measure the noise of the ADA4950-1; it can operate up to 10 MHz before the system's frequency response begins to roll off.