Program from digital logic circuits-Trimming potentiometers-pcb

- Feb 10, 2017-

Program from digital logic circuits-Trimming


Trimming potentiometers

Often designers use potentiometers, which are adjusted during end testing until the desired function of the circuit is reached. In many applications, the end user of the product would prefer not to have potentiometers, as they can drift, be mis-adjusted or develop noise. Therefore manufacturers determine the needed resistance or capacitance values by measurement and calculation methods and afterwards solder the suitable component into the final PCB; this approach is called "Select on Test" (SOT) and is quite labor-intensive.

It is simpler to substitute the potentiometer or the SOT part with a trimmable chip resistor or chip capacitor, and the potentiometer adjusting screwdriver is replaced by the laser trimming. The achieved accuracy can be higher, the procedure can be automated and the long term stability is better than with potentiometers and at least as good as with SOT components. Often the laser for active trimming can be integrated in existing measurement systems by the manufacturer.

Program from digital logic circuits

A similar approach can be used to program digital logic circuits. In this case, fuses are blown by the laser, enabling or disabling various logic circuits. An example of this is the IBMPOWER4 microprocessor where the chip contains five banks of cache memory but only requires four banks for full operation. During testing, each cache bank is exercised. If a defect is found in one bank, that bank can be disabled by blowing its programming fuse. This built-in redundancy allows higher chip yields than would be possible if all cache banks had to be perfect in every chip. If no bank is defective, a fuse can be blown arbitrarily, leaving just four banks.

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