Requirement: Capacitance Across Input Terminals
The capacitance across the input terminals shall not exceed 10 pf.
Justification:
The PASA ability to drive capacitance at low power levels and wide bandwidth is limited. Low capacitance reduces the drive current that produces noise and can, at some level, contaminate the high gain input of the PASA
Requirement: Inactive Drive Output Characteristics
The output pad isolation to analog signal line shall be at least 10 M
and not
more than 1 pf capacitance to ground or any power line when the corresponding channel is
not active.
Justification:
This is typical of what CMOS circuits can achieve, so the risk is low. It is necessary because 15 SCA devices are daisy chained to the analog signal bus. Any load from an inactive channel must be driven by the active channel.
Requirement: Active Drive Characteristics
The output of the active signal channel shall be capable of driving a 30 pf capacitance (inactive outputs, buffer op-amp input and trace capacitance).
Justification:
The output must reach final levels by the end of the channel read interval while driving the necessary circuit loads. Failure to settle within specified time could degrade system accuracy. Because of ADC settling time, the full settling interval of 150 ns must be shortened.