Measuring high impedance sources, such as some high impedance audio transformers, can result in poorer high-frequency performance than expected. This is due to an input shunt capacitance of around 190–300 pF that is present in almost all audio analyzers. AP’s APx500 Series has a shunt capacitance of 220–230pF, while the 2700 Series has a shunt capacitance of 95 pF in balanced mode (differential) and 185 pF in unbalanced mode. Most of this capacitance comes from an input RC filter that removes unwanted RF interference. Defeating this filter is possible, but it can result in nasty problems with demodulation by the input stages, since audio analyzers employ relatively slow, low noise and low distortion op-amps. We do NOT recommend this approach!
To see the effect of the shunt capacitance on frequency response, take a 47 kΩ high impedance source as an example. If the effective source resistance is shunted by the input resistance, you get
If you include an additional 15 pF for the stray capacitance of the test leads, the predicted bandwidth (–3 dB point) is
which will cause significant measurement roll-off in the audio band.
The best solution to this measurement challenge is to build an external attenuator that connects directly to the input of the audio analyzer. For example, a series 900 kΩ resistor shunted by approximately 25.6 pF should give a relatively flat response, with a 1 MΩ total input impedance and an effective shunt capacitance of about 23 pF. This is the same principle used in a 10:1 oscilloscope probe, except for the obvious value differences. We strongly suggest building this attenuator in a shielded box or enclosure to prevent unwanted pickup of hum and other interference. Provision should also be made for trimming the value of the shunt capacitance to achieve the best flatness.
