XFET ReferencesLow noise, lower voltage than Zeners, Micropower, better than bandgaps
by Roya Nasraty
In order for an analog signal to represent (or be represented by) a digital number, a reference, usually voltage, is necessary to translate the scale. Thus, an A/D converter produces a digital number proportional to the ratio of an analog signal to a reference voltage; and a D/A converter produces an output that is a fraction of the full-scale voltage or current, established by a reference. If the reference signal develops an error of +1%, it will cause a proportional system error: the analog output of a DAC will increase by 1%, and the digital output of an ADC will decrease by 1%.
In systems where absolute measurements are required, system accuracy is highly dependent on the accuracy of the reference. In high-resolution data-acquisition systems, especially those that must operate over a wide temperature range, high-stability references are a must. The accuracy of any converter is limited by the temperature sensitivity and long term drift of its voltage reference. If the voltage reference is allowed to contribute an error equivalent to only 1/2 of a least-significant bit (1 LSB = 2-n of full scale), it may be surprising to see just how good the reference must be, even for small temperature excursions. And when temperature changes are large, the reference design is a major problem.
For instance, an autocalibrated true 16-bit A/D converter has an LSB of 15.2 parts per million (ppm) of full scale. For the ADC to have an absolute accuracy of 16 bits, the voltage-reference error over the entire operating temperature range must be less than or equal to 1/2 LSB, or 7.6 ppm. If the reference drift is 1 ppm/°C, then (neglecting all other error sources) the total temperature swing must not exceed 7.6°C to maintain true 16-bit accuracy. Anther sources of error, often overlooked, is reference noise; keeping it low (typically less than 1/4 LSB) is critical for high accuracy. Nonlinearity of the reference's temperature coefficient and large thermal hysteresis are other sources of error that can significantly affect overall system accuracy.
TYPES OF REFERENCES
Zener* diodes: Widely used for many years is the temperature-compensated Zener diode, produced by the reverse breakdown of the base-emitter junction at the surface of the device. Zeners have constant voltage drop, especially when used in a circuit that can provide a constant current derived from a higher supply voltage. Zeners are available in a wide range of voltage options: from about 6 V to 200 V, tolerances of 1.0% to 20%, and power dissipation from a fraction of a watt to 40 or 50 W. However they have many shortcomings. They often require additional circuitry to obtain low output impedance, the voltage tolerance of low-cost devices is generally poor; they are noisy and very sensitive to changes in current and temperature, and they are susceptible to change with time.
The buried, or subsurface Zener is the preferred reference source for accurate IC devices. In a subsurface Zener reference, the reverse breakdown area is covered by a protective diffusion to keep it well below the impurities, mechanical stresses and crystal imperfections found at the surface. Since these effects contribute to noise and long term instability, the buried breakdown diode is less noisy and more stable than surface Zeners. However, it requires a power supply of at least 6 V and must draw several hundred microamperes to keep the noise to a practical level.
*Note: Reference diodes can use two types of breakdown phenomena, Zener and avalanche. Most reference diodes employ the higher-voltage avalanche mode, but all have come to be called "Zener" diodes.
Bandgaps: Another popular design technique for voltage references uses the bandgap principle: the Vbe of any silicon transistor has a negative tempco of about 2 mV/°C, which can be extrapolated to approximately 1.2V at absolute zero (the bandgap voltage of silicon). The difference in base-emitter voltage between matched transistors operating at differing current densities will be proportional to absolute temperature (PTAT). This voltage, added to a Vbe with its negative temperature coefficient, will achieve the constant bandgap voltage. This temperature-invariant voltage can be used as a "low-voltage Zener diode" in a shunt connection (AD1580). More often, it is amplified and buffered to produce a standard voltage value, such as 2.5 or 5 V. The bandgap voltage reference has attained a high degree of refinement since its introduction and is widely used; yet it lacks the precision demanded by many of today's electronic systems. Practical bandgap references are not noted for good noise performance, exhibit considerable temperature hysteresis, and have long-term stability dependent on the absolute value of at least one on-chip resistor.
A new principle--the XFET: With the proliferation of systems using 5-V supplies and the growing need for operation at and below 3 volts, designers of ICs and systems need high-performance voltage references that can operate from supply rails well below the >6 V needed for buried-Zener diodes. Such a device must combine low-power operation with low noise and low drift. Also desirable are linear temperature coefficient, good long-term stability and low thermal hysteresis. To meet these needs, a new reference architecture has been created to provide this much-desired voltage reference. The technique, dubbed XFET (eXtra implanted FET), yields a low-noise reference that requires low supply current and provides improved temperature coefficient linearity with low thermal hysteresis.
The core of the XFET reference consists of two junction field-effect transistors, one of which has an extra channel implant to raise its pinch-off voltage. With both JFETs running at the same drain current, the difference in pinch-off voltage is amplified and used to form a highly stable voltage reference. The intrinsic reference voltage is about 500 mV, with a negative temperature coefficient of about 120 ppm/K. This slope is essentially locked in to the dielectric constant of silicon and is closely compensated for by adding a correction term generated in the same manner as the proportional-to-absolute temperature (PTAT) term used to compensate bandgap references. However, the intrinsic temperature coefficient of the XFET is some thirty times lower than that of a bandgap. As a result, much less correction is needed. This tends to result in much less noise, since most of the noise of a bandgap reference comes from the temperature-compensation circuitry. The temperature correction term is provided by a current, IPTAT, which is positive and proportional to absolute temperature (Figure 1).
Figure 1. Simplified schematic diagram of ADR29x reference.
The ADR29x series are the first of a growing family of references based on the XFET architecture. They operate from supply rails from 2.7 to 15 V and draw just 12 µA. Output voltage options include 2.048 V (ADR290), 2.5 V (ADR291), 4.096 V (ADR292), and 5 V (ADR293).
Fruits of the new technology: The XFET circuit topology has significant advantages over most bandgap and Zener references. When operating at the same current, peak-to-peak noise voltage from a XFET reference at frequencies between 0.1 and 10 Hz is typically 3 times less than that for a bandgap (see comparison between the REF192 and ADR291). Alternatively, a bandgap reference needs to run at typically 20 times the supply current of an XFET reference in order to provide equivalent peak-to-peak noise performance (ADR291 vs. AD680). The XFET reference has a very flat or linear temperature coefficient over the extended industrial operating temperature range. The best bandgap and Zener voltage references typically have non-linear temperature coefficients at the temperature extremes. These nonlinearities are not consistent from part to part, so a simple ROM/software look-up table cannot be used for temperature coefficient correction. Temperature coefficient linearity is a very important specification for DVM applications. Another major advantage of the XFET is its excellent long term stability. Its drift is less than one-fifth that of a bandgap reference and comparable to that of Zener references (see Table).
Despite the low quiescent current, the ADR29x family are capable of delivering 5 mA to the load from a low-dropout PNP output stage; and there is no requirement for an output decoupling capacitor. Thermal hysteresis with the XFET design is much better than with bandgaps. Production devices exhibit approximately 200 µV of recoverable and non-cumulative shift when subjected to a 100-kelvin thermal shock vs. a 500 to 1000-µV shift in comparable bandgaps. The overall performance advantage offered by ADI's proprietary XFET architecture in portable systems requiring precision, stability, and low power is unmatched by existing bandgap or Zener references.
Application--current source: The ADR29x Series are useful for many low-power, low-voltage precision reference applications, including negative references and "beefed-up" precision regulators using external low-quiescent rail-to-rail amplifiers with Kelvin feedback connections. The low and insensitive quiescent current (about 12 ± 2 µA over temperature) permits the ADR29x family members to serve as precision current sources, operating from low supply voltage.
Figure 2 shows a basic connection for a floating current source with a grounded load. The precision regulated output voltage causes a current of (VOUT/RSET), to flow through RSET, which is the sum of a fixed and an adjustable external resistance. This current, <5 mA, adds to the quiescent current to form the load current through RL. Thus, predictable currents from 12 µA to 5 mA can be programmed to flow through the load.
Figure 2. Precision current source.