ZHCSH86B December 2017 – February 2019 LMH5401-SP
PRODUCTION DATA.
The input common-mode voltage range of the ADC must be respected for proper operation. In an ac-coupled application between the amplifier and the ADC, the input common-mode voltage bias of the ADC is accomplished in different ways depending on the ADC. Some ADCs use internal bias networks such that the analog inputs are automatically biased to the required input common-mode voltage if the inputs are ac-coupled with capacitors (or if the filter between the amplifier and ADC is a band-pass filter). Other ADCs supply their required input common-mode voltage from a reference voltage output pin (often called CM or VCM). With these ADCs, the ac-coupled input signal can be re-biased to the input common-mode voltage by connecting resistors from each input to the CM output of the ADC, as Figure 68 shows. However, the signal is attenuated because of the voltage divider created by RCM and RO.
The signal can be re-biased when ac coupling; thus, the output common-mode voltage of the amplifier is a don’t care for the ADC.