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TALES FROM THE CUBE: The eyes have it

In the mid-1970s, I joined a company that designed and manufactured scientific instruments for studying plants. LI-COR (www.licor.com) provided instruments such as the world’s only portable leaf-area meter, used to measure the area of a leaf without detaching it from the stem of a plant. My first assignment was to develop a new light-measuring instrument—a quantum radiometer photometer. The core wasto be a chopper-stabilized amplifier used in a transimpedance configuration. Back then, there wereno monolithic ICs to perform this task.The design involved using MOSFETand JFET transistors as the choppingswitches along with some monolithicamplifiers surrounded by resistors andcapacitors. The design was challenging.An early problem was that the offsetof one of the amplifiers drifted for thefirst 100 hours of operation, causing theinstrument to lack long-term stability.The cause was an ion-contaminationproblem at the IC manufacturer. Aftersolving this problem, the final amplifierachieved about 50 pA of input current,offset drift of less than 50 nV/°C, andnoise of approximately 30 nV/√Hz.

LI-COR manufactured various light sensors using different-colored filter glass in front of a photodiode to shape the spectral response. We input the photodiode output to the chopper-stabilized amplifier configured as a transimpedance amplifier. The amplifier had several gain options that a rotary-switch selection of various resistor settings determined, ranging from approximately 1 kΩ to more than 50 MΩ. The output of the amplifier drove a precision analogmeter as an indicator.

Later, a production technician asked whether I would help him troubleshoot a bunch of the amplifier boards that had failed to pass their performance test. He had been unable to determine their cause of failure and had accumulatedabout 40 boards in a box.

The first amplifier had a peculiar behavior. When the gain was set with a feedback resistor greater than 10 k, the amplifier output would saturate to its supply rail. With a feedback resistor of less than 10 k, the amplifier would behave within specification. Failures on the production line are always more challenging to troubleshoot, because each such case is a circuit that has never worked normally. This one ended up being a microscopic solder short between the positive andnegative inputs of the amplifier.

However, most of the boards that had failed in testing seemed to be fully operational, but had excess noise. In examining signals on all the nodes throughout the circuit, everything looked normal, but the meter readout was unstable. Although its mean reading was correct, it erratically jumped around. I examined the circuit operation, then looked at the defective board alongside one that was working properly. Then I saw it: aresistor installed backwards.

To minimize board space, the assemblers mounted the resistors—mostly axial-leaded 1⁄8W resistors—on the board standing up. The first amplifier stage had two resistors to set its gain. The resistor going from the negative input of the amplifier to ground had been assembled with the lead going to the amplifier input facing up in the air. In this configuration, the lead became an antenna that allowed the injection of interference directly into the input of the first amplifier stage—the source ofthe excess noise.

We inspected the rest of the amplifier boards; most had the resistor in backward. We added a note to the board’s test procedure to carefully check the position of this resistor before beginning the electricaltests.

Jerome Johnston is a principal engineerat Cirrus Logic.