A precise, temperature-compensated voltage reference circuit lies at the heart of every analog-to-digital converter. Voltage-reference technology hadn't changed fundamentally in decades until last year, when Intersil announced a single reference chip based on precharging a floating gate. The company now has announced several additional voltage references. But first, let's look at the technologies.
"Bandgap" voltage references have been around since Bob Widlar developed the LM113 for National Semiconductor. The "bandgap" terminology derives from the need for an output voltage equal to the bandgap voltage of silicon extrapolated to 0° Kelvin, which is approximately 1.24 V.
There are two voltage sources in a bandgap reference. One is the base-emitter drop of a forward biased bipolar transistor. It has an inherent –2-mV/°C temperature coefficient that's balanced by a "proportional to absolute temperature" (PTAT) circuit in which the difference in base-emitter voltage between matched transistors operating at different current densities has a +2-mV/°C temperature coefficient.
It's also possible to make a bandgap-like voltage reference using two junction field-effect transistors, one with an extra channel implant that increases its pinch-off voltage. Analog Devices, which developed the technology, calls them XFET voltage references.
"Buried zener" references take advantage of the +2-mV/°C temperature coefficient demonstrated by Zener diodes in the 5- to 8-V range. The "buried" part of the name comes from the need to implant the Zener below the surface of the IC die for noise reasons.
Intersil's new voltage references store a precise voltage on a floating gate. A CMOS amplifier buffers the floating-gate voltage (see the figure). Typically, the trapped charge on the floating gate can remain without loss for greater than 10 years.
Intersil's ISL21009 voltage reference is available in 1.25- and 2.5-V versions. It features very low noise (4 µV p-p for 0.1 to 10 Hz), low operating current (180 µA, max), and options of 3-, 5-, or 10-ppm/°C temperature drift. Versions are available with guaranteed initial accuracies of ±0.5, ±1.0, and ±2.0 mV.
In 1000-unit quantities, representative pricing for the ISL21009 is $4.04 for the 3-ppm/° B version, $3.74 for the 5ppm/° C version, and $2.65 for the 10ppm/° D version.