Nuts and Volts - September 2013

ESD Instrument Circuits

QI have looked for used ESD (electrostatic discharge) instruments on eBay with little success. I have also been unsuccessful at finding circuits for these handheld instruments.

At this point, I would like to make the following, but need a source for circuits: Surface resistance tester (100 VDC with order of magnitude resolution in ohms per square to beyond a teraohm); Static field meter (roughly ± 50-10,000V, compatible with a charge plate that I will build); and Deionizing circuit for an air nozzle (either AC or chopped DC).

I have substantial experience using this equipment, and want to integrate it into my home lab. I just don’t want to start from scratch with design due to time constraints and concerns over the safety aspects of making the voltages necessary to accurately measure surface resistance, and to generate positive and negative ions.

Given the ever decreasing line-to-line spacing in ICs (leading to less than 50V sensitivity for many new designs), I would think this would be a popular set of equipment to make.

— Bob Crain

AI will present my idea for a surface resistance tester here and work on the static field meter and deionizing circuit for a later column.

The probes for the tester should be spherical so as not to penetrate the surface, and should be gold or nickel plated for good contact. Surface resistance will read the same regardless of where you probe, providing the probe is not near an edge and the probes are close together relative to the size of the total area. I’ll assume probes of 10 mils diameter spaced one inch apart will be used.

I had to look up what a teraohm is; it is 10 12 ohms. The design could use constant current and measure the voltage, or constant voltage and measure the current. If the voltage is limited to 100V, the current is 102/1012 = 10-10 = 100 picoamps. I want to use an AC source because I don’t want to deal with a high gain DC amplifier. I think a voltage source will be easier to make than an AC current source.

The frequency will be 20 kHz in order to be inaudible and low enough that inexpensive op-amps can be used. The signal source is shown in Figure 1 and is a SwitcherCAD simulation circuit. All the transistors are available from Mouser. I did not have an op-amp capable of full output at 20 kHz, so I built one.

The first stage is a “long tail pair” which has wide bandwidth and low distortion. Q5 provides rail-to-rail signal, and Q6 and Q7 provide the drive to the following stages. R4 and R3 give positive feedback to produce plus and minus two volts of hysteresis. Negative feedback through R6, R7, and C1 makes it an oscillator.

Q1 and Q2 are high voltage transistors to drive the MOSFET gates. The current is limited by R12 to about 8 mA peak; 8 mA 100V = 0.8 watts, but it is only driving half of the time so the transistors should be okay. There is a shoot through current in the output transistors which is limited by R13 and R14 to less than two amps.

Figure 2 is the probe circuit. The sense resistor is 100K for teraohm sensitivity which generates a voltage of 100-10*100+ 3 = 100-7 = 10 µV. The first stage is an instrumentation op-amp (INA128) with a gain of 100.

The next two stages are a

In this column, Russ answers questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions. Send all questions and comments to: Q&A@nutsvolts.com

16 September 2013

■ WITH RUSSELL KINCAID Q & A

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