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Typical probe sweeper circuit.

Here are the basic features of the probe sweeper:

1. A dc-to-dc converter generates + - 100 V dc voltages from a floating, rechargeable 12 V battery. With a high impedance potentiometer a variable dc voltage is generated. It is connected in series with a ramp generator (described below) and applied to the input of a high voltage operational amplifier. The latter operates as a voltage follower and produces enough current to drive probes.
   
   
2. The output voltage of the probe sweeper is applied to the probe. The probe current is obtained from the voltage drop on a series resistor between the internal ground of the sweeper and the external ground (chamber wall). The current signal is applied to the Y-axis of a recorder or to a analog-to-digital (AD) converter in a computer. The voltage from the probe to the internal ground is applied to the X-axis.
   
   
3. In order to minimize measurement errors the following considerations are necessary: The measured voltage differs from the applied probe voltage by the voltage drop across the resistor to ground. Hence, the latter should be minimized. If the X-axis were connected between probe and external ground the current through the internal resistance of the recorder would be measured in addition to the probe current. The effect is that the I-V characteristics without plasma is a straight line of slope 1/R_recorder. The present connection of the XY recorder inputs avoids the undesirable tilting of the I-V curves for small probe currents. However, the AD converters have a common ground such that the the resultant slant has to be subtracted out by a calibration procedure.
   
   
4. The ramp generator produces a voltage which varies linearly in time. A charged capacitor is discharged at a constant current flowing through the field-effect transistor (FET). Sweep times from seconds to minutes are adjustable by varying the gate voltage which controls the drain-source current.
   
   
5. For measuring the characteristics of double probes the sweeper has to float with respect to ground. This is possible with the analog XY recorder but not the available computer. The floating AD converters cannot withstand high common mode voltages to ground.
   
   

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