impedance matching needful?

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janfred

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Hi there,

I designed a circuit for a measurement-task.

230VAC will be transformed into ± 12VDC (normal procedure.. transformer-> bridge rectifier -> 78XX/79XX. Of course there are some filter to eliminate noise.

After voltage regulation, the ± 12VDC will supply an operation amplifier which works as an impedance converter (IConv).
For the output of the IConv, I have to use a coax-cable (because of a DAQ-card and very high frequencies).

Now, my question...
Do I need a HF-transformer (50 Ohm) between output of the IConv and BNC-connector to match the impedance?
Is that necessary or just like breaking a butterfly on a wheel?
 

Almost certainly no. It would be better to use an op-amp capable of driving a 100 Ohm load, then wiring a 50 Ohm resistor in series with it's output and the input end of the cable. If the DAQ card doesn't aready have one, wire another 50 Ohm resistor across the output end of the cable. The op-amp will see a combined resistance of the two resistors in series (100 Ohms) and the cable will see 50 Ohms at both ends so it gives a good impedance match. It will halve the signal level because you are extracting from the center of a 50 + 50 Ohm potential divider but you can overcome that by using x2 gain in the op-amp or by doubling the readings from the DAQ card.

Brian.
 

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