Noise, composed of small, random voltages, can be difficult to measure. Lab instruments add their own noise, further complicating the measurement. Special techniques are often used when measuring noise. For example, amplifiers are typically configured with high closed-loop gains, multiplying their input noise to make it easier to measure. Low-fixed-gain differential amplifiers present a greater challenge, however, as their integrated feedback and gain resistors preclude the use of a high-gain configuration. Additionally, differential-to-single-ended conversion is needed to interface with available spectrum analyzers. A second amplifier stage can provide gain and the differential-to-single-ended conversion, neatly solving both of these problems.
Figure 1 shows an
ADA4950-1 selectable-gain (1, 2, or 3) differential amplifier followed
AD8099 low-noise, low-distortion op amp. The AD8099, configured for a
gain of 10, converts the differential output into a single-ended signal. Its
Figure 1. The AD8099 low-noise, low-distortion op-amp is used to measure the noise of the ADA4950-1 selectable-gain differential amplifier.
The output voltage of the AD8099 is simply: (1)
The noise contribution of the AD8099, measured with inputs grounded, is treated as the noise floor of the measurement system. The total output noise including the ADA4950-1 was then measured, with the contribution from the AD8099 subtracted using root-sum-square math, as shown in Equation 2, where Vn1 is the output noise of the ADA4950-1 and Vn2 is the output noise of the AD8099.
Total output noise: (2)
A few other techniques were implemented to accurately measure the system noise:
The AD8099 itself contributed only a small amount of noise:
where vn is the input voltage noise; and ni+ and ni- are the input current noise of the AD8099.
Measuring the ADA4950-1ís current noise is impossible because a large feedback resistor is needed to amplify the noise, but the value of the internal feedback resistor cannot be changed.
The Stanford Research Systems SR785 was used to measure noise up to 100 kHz, while the Agilent E4440 PSA spectrum analyzer was used for noise beyond 100 kHz.
Figure 2. Test Results
Download this article as a PDF: Measuring Noise of Low-Fixed-Gain Differential Amplifiers
Copyright 1995- Analog Devices, Inc. All rights reserved.