9103 USB Picoammeter Theory of Operation
A low noise BNC cable provides the input current to the 9103. A low leakage double diode protects the 9103 amplifier from over voltage with respect to ground.
The current is then input to a low femtoampere input current precision amplifier where the DC current is converted to a voltage. The converted voltage is fed into a precision high input impedance buffering amplifier before finally being converted to a digital value by the 24 bit digitally filtered ADC. The A/D converter is fed with a 2.50V reference. This produces a conversion of -8388608 to +8388607 for the maximum differential input of [-2.50V, +2.50V) (223 = 8388608). With no error, an input of 0.000000000000A will produce an output conversion of 0. The margin between maximum differential input Voltage ±2.50V and nominal range ±2.00V allows offset calibration to accurately place the 0 conversion for 0 input current. Thus the nominal range is -6710886 for -2.0V to +6710886 for +2.00V. The filtering is user selectable with the effect of increased noise performance of the current measurement with a corresponding increase in the acquisition time.
The ADC continually measures the current but the output can be averaged depending on the acquisition time selected by the user. The fastest acquisition time is 15mS with a typical acquisition time of 500mS or longer.
The USB interface circuitry utilizes and industry standard flash microcomputer for ease of installation. Typical current draw for the 9103 is 160mA.
Magnetic isolators are used to provide signal isolation between the micro controller and USB interface, and the current measurement circuitry and power supplies. Ten signals transition the isolation barrier, eight of which act as controlling inputs to the current measurement circuitry, and two of which provide feedback to the micro controller.
Bias option – The bias option incorporates a low-noise 90 VDC supply, with a 100k ohm resistor to limit the current to the 9103 in the event that the input becomes shorted while the 90V bias is applied.
How the +90 Volt DC Bias Improves the Accuracy of Electron and Ion Current Measurements:
When the electron or ion beam impacts the target, secondary electrons are ejected. When measuring the target current the secondary electrons will subtract from the total current, resulting in an inaccurate current measurement.
Adding an ultra-low noise fixed +90 V DC supply in series with the picoammeter will bias the target and substantially reduce the number of secondary electrons that leave the target; greatly increasing the accuracy of the target current measurement.
BIAS Options Explained
The internal +/- fixed 90V bias option is used to prevent secondary electrons from leaving the target in a vacuum system and results in more accurate electron and ion current measurements. The bias is comprised of two 45 V batteries, so it is possible to use standard 9V batteries as well and give you options for 9V, 18V, 54V and 90V.
External bias is selected if you want to bias the target with your own low noise isolated programmable DC voltage supply and measure current as a function of bias voltage.
BIAS options are not required, but they are mutually exclusive - you can only have one bias option installed on the 9103. If you need both a 90V bias and an external bias, then select the external bias option and set your own low noise DC voltage supply to 90V when needed.