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FEATURE TEST & MEASUREMENT


Accurately measuring differential signals


Engineers working on power device designs involving GaN and SiC technologies and other high-speed applications struggle to accurately measure differential signals when common mode interference is present. To address this challenge, Tektronix has introduced IsoVu technology. Dean Miles, a senior EMEA technical marketing manager at Tektronix, comments


differential signal by rejecting any signal that is common to both test points (i.e. the common mode signal). CMRR is defined by: CMRR = | ADiff


/ACM


where: ADiff


| = the voltage gain for the


difference signal ACM


mode signal Ideally, ADiff


= the voltage gain for common- would be large and ACM


would be zero, resulting in an infinite CMRR. In practice, a CMRR of at least 80dB (10,000:1) is considered quite good. One amplifier that meets demands is the LeCroy DA1855A. In Figure 1, the DA1855A’s CMRR exceeds the 80dB level at low frequencies up to a few MHz. However, the CMRR capability of this amplifier quickly derates and is only 20dB or 10:1 at 100MHz. This means that a common-mode input signal of 10V at 100MHz will induce a 1V error signal in the differential measurement. It should be noted that the plot is for the amplifier only as performance further degrades when using probes with the amplifier.


F


or those working on power device designs involving GaN and SiC


technologies and other high-speed applications, there has been no way to accurately measure differential signals when common mode interference is present. As a result, these signals are essentially hidden, resulting in designers ending up with misleading information on their scope. Unable to make the measurement, they


will turn to alternative methods to try and make sense of what is actually occurring in the circuit – such as extensive simulation, and measuring adjacent points and extrapolating the results. Often, however, this doesn’t reflect what is actually occurring and can lead to the circuit failing or operating sub-optimally. In particular, engineers are struggling to evaluate the performance of their gate drivers on both half-bridge or full-bridge designs where a small differential signal is floating at a high common-mode voltage (e.g. 40V or higher). In cases where the switch node voltage is switching between ‘ground’ and the input supply voltage, the gate-source voltage is


42 NOVEMBER 2016 | INSTRUMENTATION


impossible to measure without adequate common mode rejection. For these types of measurements,


the key performance parameter of the measurement system is the common- mode rejection ratio (CMRR). This determines a measurement system’s ability to accurately measure the true


Figure 1: CMRR Plot for LeCroy DA1855A differential amplifier


A NEW APPROACH When the best amplifier available fails to deliver repeatable results, a new measurement system is needed with the ability to completely reject the common mode signal and reveal the true differential signal. Unlike other probes, Tektronix IsoVu technology uses an electro-optic sensor to convert the input signal to an optical signal. This electrically isolates the device-under-test from the oscilloscope. IsoVu incorporates four separate lasers, an optical sensor, five optical fibres, and employs sophisticated feedback and control techniques. The IsoVu architecture with galvanic


Figure 2: IsoVu block diagram


isolation provides common mode withstand voltages of >2000Vpeak across its frequency range with no derating. The electrical limitation for an optically isolated solution such as IsoVu is many thousands of volts. Because IsoVu achieves galvanic isolation through its fibre optic connection, the only limitation in its common mode voltage rating is due to


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