Evaluating the error frequency responses of a fully differential amplifier can be difficult and time consuming if standard measurement techniques are used. This is because the task requires measuring small differential and common-mode error signals. For example, evaluating the amplifier’s power supply rejection (PSR) and common-mode rejection (CMR) over frequency requires measuring differential error voltages, while evaluating the amplifier’s output balance over frequency requires measuring common-mode error voltages. At low frequencies, these error voltages can be in the order of microvolts.
Traditional methods of measuring the different frequency responses of a fully differential amplifier require either expensive lab equipment (like a differential network analyzer) or specialty interface circuitry to cope with the amplifier’s differential output. The complex algebra method uses superposition to reconstruct the amplifier’s differential and common-mode output voltages from single-ended output voltage measurements. Since the measurements are single-ended, a single-ended network analyzer can be used without an interface circuit. To perform these measurements, a vector analyzer is preferred since it can output the measured waveforms in complex number form, a data representation that can be easily manipulated during the reconstruction process.
Connect the analyzer’s output port to the amplifier’s input port(s) to provide a stimulus.
Calibrate the measurement path to compensate for losses in the cables and traces.
Connect the analyzer’s input port to the amplifier’s positive output port.
Measure the frequency response at the amplifier’s positive output, making sure to set the analyzer to output a complex number data stream.
VOUTp = Rep + j×Imp
Connect the analyzer’s input port to the amplifier’s negative output port. Do not change the analyzer’s settings.
Measure the frequency response at the amplifier’s negative output.
VOUTn = Ren + j×Imn
Compute the differential and common-mode frequency response using:
Compute the magnitude and phase of the differential and common-mode frequency response:
Mag(VOUT,DIFF) = 20log(√(ReDIFF2+ImDIFF2))
Mag(VOUT,CM) = 20log(√(ReCM2+ImCM2))
Pha(VOUT,DIFF) = atan(ImDIFF / ReDIFF)
Pha(VOUT,CM) = atan(ImCM / ReCM)
Measurement Example 1
In our example, we’ll measure the PSR+ of an amplifier, the AD8476, using a Rohde & Schwartz network analyzer. Characterizing an amplifier’s PSR+ over frequency is challenging because the input excitation must be injected to the amplifier’s power bus. If the amplifier is fully differential, the challenge becomes even greater because you must measure the output voltage’s differential-mode component while rejecting the common-mode component. Luckily, the measurement can be simplified using the complex algebra method.
The Dutch are known for their love of bicycling, and they’ve also long been early adopters of green-energy and smart-city technologies. So it seems fitting that a town in which painter Vincent van Gogh once lived has given him a very Dutch-like tribute -- a bike path lit by a special smart paint in the style of the artist's “Starry Night” painting.
The UX Italia video contest recognizes Italian machinery, technology, and other experience solutions that have contributed meaningful improvements to people’s lives and production processes. If you submit a three-minute video showcasing how the quality of Italian machinery's User eXperience is essential to your company's success, you just may win a trip to Italy.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.