Wideband A/D Converter Front-End Design Considerations

Transformers are used for isolation and to convert signals from single-ended to differential. A factor often overlooked when using transformers in the front-end circuitry of high-speed A/D converters is that they are never ideal. With sinusoidal input signals, any imbalance introduced by the transformer delivers an imperfect sinusoidal wave to the input of the ADC, and results in overall data-conversion performance worse than the ADC could otherwise provide. We consider here the effects of input imbalances on ADC performance and provide examples of circuitry to achieve improved results.