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Wideband transformers
In spite of lack of appeal to many, perhaps due to some misled reputation of being old-fashioned, transformers really has a place in measurement setups when it comes to ensure signal integrity. They offer a simple means to combat unwanted signal loops, and make it easier to connect instruments that are powered from different power sources. The advantages of transformers include low added noise, zero offset, the ability to apply impedance matching, the ability to include multiple secondary and/or secondary windings, and robustness. On the downside, transformers have limited frequency range, and do obviously not allow the passage of DC, and the response will depend on the source and load impedances. The TC8235GIT "Ground Isolation Transformer" (seen with Bosch, Philips and Burle labels) is an example of an unbalanced wideband transformer with 1:1 ratio, originally marketed to remove hum interference from analog video signals by breaking an unwanted ground loop. However, it can be used for other applications where you need to separate two circuits, and not just for 75 Ohm environments. The graph to the left shows the measured frequency response for 50 Ohm source and load impedance. The response is well-behaved over a large range of frequencies. The parasitic capacitance between primary and secondary is close to 2 nF, so at high frequencies the transformer may not necessarily offer the common-mode isolation you would expect. |
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Tektronix A6909 Two Channel Isolator
If you need a response down to DC and/or a wide frequency range covering the MHz range, then the Tektronix A6909 could be something to consider. If the 2 channels are not enough, go for the A6907 with 4 channels. The A6907 and A6909 feature internal optical links for the signals, and RF transformers for the power supplies. The unit shown on the left has been modified so that the two isolated input are equipped with standard BNC connectors, rather than the original connectors for the special probes furnished with the instrument. Also, the outputs are located on the front, rather on the back. For applications where touching the input connector could pose a safety issue, I would replace the standard BNC inputs with a touch-protected variant from Stäubli (check them out at https://www.staubli.com/sk/en/electrical-connectors/products/t-m-products/high-frequency-measurement/touch-protected-bnc-probes.html) The isolator's parasitic capacitance between any input to any output GND is measured to between 54 pF and 57 pF which is very fine, but may still result in too strong a coupling in some cases. At 10 MHz, the 54 pF equals an impedance of about 300 Ohms, so common mode voltages at high frequencies on the input may propagate through the A6909. In fact, Tektronix specifies a derating curve for the common mode voltage, and a maximum input common mode slew rate of 20 kV/µs, and that's when using the special probe with its common mode suppression box. Without the special probes, it's likely a good idea to add your own common mode filter on the input to protect against large dV/dt common mode voltages. |