The design project involves the task of measuring the current of charge collected by a rocket probe, converting the current to a voltage, removing an unwanted common-mode signal (the bias
voltage applied to the probe for charge collection), and finally doing the necessary signal processing (voltage scaling and shifting) to ensure compatibility with the input to a telemetry system (typically, a PCM encoder). It also is desirable to know the probe collection voltage’s polarity since we expect positive charge collection to result when the probe has a negative bias voltage, and vice versa.
This was a two-week design project within my EE 310 Electronic Circuit Design I Lab.
The following overall specifications applied to this design effort:
- Input current magnitude (| I | ) ≤ 10 μA
- Small-signal transresistance gain (ΔvO3/ΔI) = 200 V/mA
- No vP contribution at output (i.e., essentially perfect common-mode rejection)
- Output voltages (vO3 and vO4) compatible with PCM encoder’s input (0-5 V)
- Voltage polarity monitor accurate to ±10 mV
- Power supply voltages: ±15 V
The following overall design limitations were also applied to the design:
- Keep your design as simple as possible and keep the number of components minimal (especially op amps)
- Care must be taken to not load down the op amp’s output, so choose external components so that the output current value does not exceed 2.5 mA
- Power supply bypass capacitors should be applied to all op amps
This is the full design schematic within MultiSim. Each yellow box shows the voltage values at each step of the overall circuit.