Matched MOSFETs provide in-circuit trim
By Joshua Israelsohn -- EDN, April 1, 2004
Despite hyperintegration and systems on chips, many designs require bits of glue logic to complete or customize an interface. Similarly, many linear circuits use discrete transistors to implement or optimize functions, particularly within or near I/Os. Well-matched transistor pairs and quads, among the most common tool in the analog-IC designer’s kit, are particularly handy for making simple and scaled current mirrors, cascodes, and discrete differential stages.
The ALD1221E and ALD1123E from Advanced Linear Devices are matched dual and quad n-channel MOSFETs, respectively, available in plastic SOIC and plastic or ceramic DIP packages (Picture). The matched MOSFETs provide for postpackage trimming, which allows the manufacturer to dial out offsets resulting from package-induced die stress. You can trim the threshold voltage, initially 1V ±10 mV, upward to 3V in 100-μV steps as a no-moving-parts in-circuit trim. The device’s maximum long-term drift is 50 μV in 1000 hours; typical 10-year drift is less than 2 mV.
If you use the part as the manufacturer trims it, the maximum offset is 5 mV. The on-resistance, typically 500Ω with gate-to-source voltage 4V over the threshold, matches from device to device within a pair to 0.5%. Prices start at 75 cents (1000).
Advanced Linear Devices, 1-408-747-1155, www.aldinc.com.
