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Chapter 5. Introduction to Noise Measure... > 5.10 Measure Low Frequency Noise for...

5.10 Measure Low Frequency Noise for the OPA227

Many data sheets have a specification for the peak-to-peak noise from 0.1 Hz to 10 Hz. This effectively gives an idea of the op-amp’s low frequency (i.e., 1/f noise). In some cases, this is given as an oscilloscope waveform. In other cases, this is listed in the specification table. Figure 5.35 shows one effective way of measuring the 0.1–10 Hz noise. This circuit uses a second-order 0.1-Hz high-pass in series with fourth-order 10-Hz low-pass filter. This circuit also provides a gain of 100. The device under test (OPA227) is placed in a high-gain configuration (noise gain=1001) because the expected 1/f noise is very small and must be amplified to a range where standard test equipment can be used to measure it. Note that the total gain for the circuit in Figure 6.9 is 100,100 (i.e., 100×1001). Thus, the output is divided by 100,100 to refer the signal to the input.


  

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