Electrometer

          The electrometer is implemented using an AD844 operation amplifier. This product is suited very well to electrometer applications where it is important to have very high input impedance and very low input bias current. The typical input resistance for the model chosen is 10 MΩ and the input bias current is typically 200 nA. This is suitable for current collection on the order of μA. [8], [9] The output will be passed to a 12-bit analog to digital converter (ADC). This component converts the analog signal to a digital result for evaluation by the microprocessor. The microprocessor will then record the digital data for either on-orbit analysis or for analysis on the ground after a telemetry downlink. The HPP electrometer will also feature selective gain. Three different feedback resistors will be selectable on the electrometer using an ADG201HS analog multiplexer device. This allows the microprocessor to automatically scale the data being collected if necessary. For example, if the microprocessor finds that the Langmuir Probe voltage sweeps are not producing the highest resolution data possible, it could increase the gain factor by selecting a higher value feedback resistor. The fourth feedback resistor attached to the multiplexer is to be used exclusively for PFP operation, as it must match a reference feedback resistor on another AD844 op amp.

Please click on the picture for the full-size schematic