Why do single-ended to differential conversion for coax cables?

Most differential-input ADCs work better - less distortion, better s/n ratio - if fed differentially.

You can usually drive them single-ended with good results. A typical video adc will want both inputs to be biased some common-mode voltage off ground, +1.5 volts or something like that. One of these can be stiff, bypassed to ground, and you can apply the signal to the other. One usually wouldn't DC ground an input.

John

Most single ended to differential conversion techniques provide common mode rejection. Single ended coupling does not do this and is ill-advised. Cheap wideband tranformers with center tapped secondaries are readily available and provide total isolation between signal source and ADC circuit grounds.

For those types of inputs, I just tie off one input to the common with a low value non inductive resistor of some kind and use the other for the single ended feed. Some devices already employ input resisters enough to simply tie it directly to the input common.

The option is yours to choose from. It is better to use a differential line, for which the inputs are supplied, if so desired.

Applying the signal to just one input usually only lets you drive the ADC to half full-scale. If that doesn't matter, then it's fine.

It depends on your accuracy and SNR requirements. The single end connection typically can drive the ADC only to 1/2 of the scale, so here is the 6dB of degradation. Also the CM ground bounce would be fed to the input of the ADC.

If you connect the ADC like that, the path of the "hot" and "cold" inputs is going to be different. Thus the common mode noise from the ADC GND will be seen at the ADC input.

Vladimir Vassilevsky DSP and Mixed Signal Consultant