FROM EDN EUROPE: SOI wafers host higher precision op-amps
By Graham Prophet -- EDN, December 5, 2005
When analogue chip designers have employed silicon-on-insulator substrates for their devices, it has often been to extend those devices' high-speed performance. When you fabricate a silicon circuit in a layer of silicon that sits on top of a further layer of (usually) silicon dioxide-rather than directly on to the bulk silicon of a wafer-you reduce or eliminate many parasitic components that are an unavoidable consequence of the fabrication process. With its VIP50 process for operational amplifiers, National Semiconductor is exploiting the same benefits in another dimension, using SOI structures to build precision amplifiers. The company's empirical definition of "precision" is an offset voltage of under 1 mV over the specified temperature range. VIP50 has complementary 4-GHz PNP and NPN transistors; a trench that reaches down to the SiO2 of the SOI wafer completely electrically isolates each active device. In addition to greatly reduced noise and cross-talk, the structure also eliminates the parasitic transistors that cause latch-up. VIP50 can operate at 12V with split-supplies for simple biasing. The process supports laser-trimmable precision resistors, with closely-matched isolated resistors that show no voltage co-efficient effects. Describing the process as "analogue-grade CMOS", National's Erroll Dietz-VP of the amplifiers product group-notes that it has low 1/f noise yielding improved accuracy at very low frequencies close to dc. He says, "In precision, wide-band noise isn't the problem, it's 1/f noise; these transistors are engineered to perform close to JFET input amplifiers."
National will build several sub-families of op-amps on the process; ultra-low-power chips-amplifiers and comparators-running from under 1 µA; precision parts for 2.7 to 12V; and others that will exploit what Dietz says is a 10× gain in speed/power ratio. An example is the LMP7711 for instrumentation front-ends. With 200 µV offset, it operates down to 1.8V and offers CMRR of 95 dB, PSRR of 100 dB, with noise of 7 nV/ 
at 400 Hz. Using just over 1 mA, it has a gain-bandwidth product of 17 MHz.
National Semiconductor, www.national.com.
