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 amplifierís power supply rejection (PSR) and common-mode rejection (CMR) over frequency requires measuring differential error voltages, while evaluating the amplifierís 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 amplifierís differential output. The complex algebra method uses superposition to reconstruct the amplifierís 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 analyzerís output port to the amplifierís input port(s) to provide a stimulus.
- Calibrate the measurement path to compensate for losses in the cables and traces.
- Connect the analyzerís input port to the amplifierís positive output port.
- Measure the frequency response at the amplifierís positive output, making sure to set the analyzer to output a complex number data stream.
VOUTp = Rep + j◊Imp
- Connect the analyzerís input port to the amplifierís negative output port. Do not change the analyzerís settings.
- Measure the frequency response at the amplifierís negative output.
VOUTn = Ren + j◊Imn
- Compute the differential and common-mode frequency response using:
VOUT,DIFF = (RepĖRen) + j◊(ImpĖImn) = ReDIFF + j◊ImDIFF
VOUT,CM = (Rep/2+Ren/2) + j◊(Imp/2+Imn/2) = ReCM + j◊ImCM
- 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, weíll measure the PSR+ of an amplifier, the AD8476, using a Rohde & Schwartz network analyzer. Characterizing an amplifierís PSR+ over frequency is challenging because the input excitation must be injected to the amplifierís power bus. If the amplifier is fully differential, the challenge becomes even greater because you must measure the output voltageís differential-mode component while rejecting the common-mode component. Luckily, the measurement can be simplified using the complex algebra method.