CM choke as signal transformer

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Yes, it does work rather well, I have used such an expedient more than once.
You have to be aware of the the limitations though: you have to choose the inductance according to your needs (impedance level/frequency range/ isolation voltage).

For example, you could be tempted to use a large value, like 47mH and a large isolation voltage, like 250VAC, just to be safe and universal, but it would be a bad idea if you need to pass high frequencies: such inductors have their windings completely separate and therefore a large leakage inductance. You thus you need to keep the magnetizing inductance to the lowest reasonably tolerable value.

If you only need a frequency range of one decade, the requirements should be easy to satisfy, provided you make sensible choices.

Of course, CM chokes used as transformers need to take into account all the parameters relevant to a transformer. In particular, the V*s product tolerated by the winding/core combination. This might be problematic, because the datasheet does normally not make mention of this parameter or any alternative like the core area or turn number, susceptible to help in computing the product.

If your lowest frequency of interest is 100kHz, this is unlikely to pose a problem, unless you need large amplitudes or use a very small component.

Anyway, I recommend you carefully verify in reality your chosen inductor under all possible conditions before you proceed to actual testing: many of the important parameters for transformer operation are in fact missing from the datasheet.

One more (obvious) thing: such "transformers" are only good for pure AC: the slightest DC component will be problematic.
 
With a volt or so and low impedance, noise sensitivity shouldn't be an issue.

Directly driving the primary from an opamp makes the inductance almost irrelevant (within reason), but it exacerbates DC problems: with 1mV offset and 0.1 ohm winding resistance, the DC current will reach 10mA, already problematic.
Possible remedies include ultra-accurate offset adjustment, inclusion of a series capacitor, or transferring 1 or 2 ohm of the 100ohm secondary to the primary.
Each method has its pro and con
 
A cap is fine, but be aware that it will add a resonance, normally at a relatively low frequency, below the intended range.

Even so, this resonance can sometimes show some influence within the useful band if it is exceptionally strong.

Sometimes, there are nastier interactions between the opamp and the primary: as the reluctance of the core is near zero, it takes very little to drive it into saturation, and since the output of the opamp also becomes non-linear when pushed too far, this can lead to relaxation oscillations or ferro-resonance-like phenomenons when the capacitor is added.
All of this is rather unlikely, but it sometimes happens, and can be puzzling, because the circuit seems to work properly most of the time, but breaks into violent oscillations after a transient, or even for no apparent reason.
If this happens, you know where to look, and the remedy is generally as simple as adding damping resistors to the cap, either parallel or series.
They will have minimal influence (parallel case), or their influence can be calculated (series case)
 
Thanks again for tip. Will test with square waves and look out for excessive ringing or odd behaviour. I generally have 'stopper resistors' on inputs and outputs depending on how reactive the ckt is. Often I include them as a safety feature even if I suspect they are overkill. This bugs some folks during the design review but then my designs usually finish with first to second revision.
 
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