Requirement: Normal Voltage Levels at Input
Input voltages shall normally range from +0.8 to +3.5 V DC. Specifications of this section shall be met over a full scale voltage swing. A full scale voltage swing shall be 2 V and shall range from 0.8 to 2.8 V. The input shall be DC coupled to the SCA output.
Justification:
The SCA op-amp works best within the range of 0.8 to 3.5 V. The PASA output over the required dynamic range shall be 0.8 to 2.8 V. The output stage of the proposed PASA is capable of driving up to 3.5 V, though the range above 2.8 V can have significant distortion.
Requirement: Offset Voltage Level
The DC level out of the SCA when a voltage representing a null charge level from the PASA is applied at the input shall range from 700 to 900 mV and shall have a maximum fluctuation of 0.35 millivolts RMS across samples within any channel. As a goal, the offset fluctuation from channel to channel or between SCA devices shall not exceed 0.5 mV RMS.
Justification:
This requirement avoids unnecessary pedestal corrections by keeping pedestal fluctuations below the system noise level. By keeping the offset below .35 mV rms the pedestal fluctuation for each channel can be the same for all samples. If channel to channel and inter-SCA offset fluctuations can be kept below 0.5 mV, then pedestal correction will be a constant number for the entire system and so would be trivial and not require any special hardware (e.g., DAQ ASICs).