Monthly Archives: June 2014


Electrical parameters of biological material are found to correlate with tissue structure and their (patho – )physiological changes (48). As mentioned before, muscle and blood exhibit different conductivities depending on the frequency employed. Muscle tissue is more conductive at frequencies above 12 kHz when the current is applied in a direction normal to the fiber orientation (37). Either two – or four-electrode systems have been employed for this type of tissue characterization, in combination with a bridge tech­nique or phase sensitive detector method (using a lock-in amplifier). Although simple in design, the two-electrode system is less useful because of the presence of a charge distribution at the interface of the metal electrode and the tissue. However, this undesired effect of electrode polar­ization can be eliminated by the use of the four-electrode technique, with separate pairs of electrodes for current in­jection and for sensing the resulting potential difference (49). To calculate resistivity of a particular sample vol­ume, it is necessary to select an electrical volume conduc­tor model that sufficiently describes the geometry and con­ductive properties of the medium under study. Often, the medium can be approximated as an infinite or semi-infinite structure, or as a thin layer bounded by air on both sides.

Anisotropy is an important but complex aspect of the di­electric properties of tissue (26-51), which may be accom­modated by considering a multilayered volume conductor model. (52) determined local anisotropic resistivity of ca­nine epicardial tissue (i. e., the outer muscle layer of the ventricular wall) in vivo in two orthogonal directions, with special attention to sample volume. The effective sample volume for a simple homogeneous isotropic medium pri­marily depends on the distance between the current elec­trodes, but for real anisotropic media these researchers found that in addition both longitudinal and transverse resistivity (Q-cm) varied not only during the cardiac cycle but also depended on the driving frequency studied (which was between 5 kHz and 60 kHz). Finally, it must be em­phasized that electrode construction affects accuracy (51).


The plethysmographic technique employs a device that records respiratory excursions from movements of the chest surface on the basis of electrical impedance varia­tions. Originally, the method was applied to the detection of apnea (i. e., suspension of respiration) in newborns, and for tracking changes in intrathoracic fluid accumulation (46). Other approaches such as the spirometer and the pneumo-

Sleep apnea is defined as temporary interruption of airflow to the lungs during the sleeping period and lasting for more than ten seconds. This abnormality usually results either from upper airway collapse (obstructive type) or, in 10% of the cases, from the absence of diaphragmatic contraction due to the lack of neural input (the central type) from the brain, although mixed types do occur. Such conditions are usually associated with loud snoring, while general symp­toms and signs include fatigue, hypersomnolence during

Figure 5. Cole-Cole diagram for an impedance with a single time constant.

Figure 6. Variations of left ventricular and atrial cross-sectional area in the magnetic resonance imaging (MRI) and the electrical impedance tomography (EIT) images made with the person in the supine position. (From Ref. 29, with permission.)

the day, cardiac problems, and hypertension. Snoring may lead to soft tissue damage, notably an edematous uvula. Sudden death due to sleep apnea is often related to comor­bidity and is probably rare. The standard measurement in sleep apnea includes the polysomnography repertoire, which consists of recording ECG, electroencephalogram, electro-oculogram, electromyogram of the legs, measure­ment of oral airflow with the use of thermistors, esophageal pressure, recording of snoring sound intensity, and plethys­mography to estimate movements of chest and abdomen, besides the determination of arterial oxygen saturation. Typical costs associated with these studies amount up to $1000 or even more. Treatment may include simple advice to lose weight and avoid taking alcoholic drinks or sleeping pills but may also consist of various surgical procedures or continuous positive airway pressure (CPAP).

Apnea monitoring is based on either the measurement of bioelectric impedance or on estimation of biopneumatic impedance. Obviously, the time pattern of respiration is of primary interest for monitoring in these cases, and quan­titative volume changes are of less importance, thus per­mitting simple instrumentation. The bioelectric technique is virtually identical to the ICG method, but in the present case the periodic circulatory components are regarded as perturbations superimposed on the slower respiratory sig­nal. This signal is recorded with electrodes in the midaxil- lary line, and it may typically amount to 1 Q per liter lung volume change, increasing with inspiration. The cardiac contribution (usually about 20% of the total amplitude) can (at least in humans) be determined during voluntary temporary respiratory interruption. Movement ofthe body causes artifacts, which may induce impedance changes in excess of the signal under investigation. An area that may be explored in this field concerns measurement of the mo­tion of the diaphragm: with well-positioned electrodes the motion of this good conductor is easily assessed, while the changes of the signal are clearly related to respiration.


As mentioned above, many kinds of ingenious internal archi­tectures have been used for bandgap references. The actual topology of the bandgap elements has been arranged in many ways—ways that may (or may not) be transparent to the user. However, there are also features that are useful to some users and of no value to other users. A list of typical features is provided here, along with a brief list of typical ICs that include those features. This list of ICs is by no means in­tended to be exhaustive, but merely indicative of what fea­tures one might look for.

• Low Power. Many users like the advantages of low power, but often a low-power bandgap reference is noisy, because transistors operated at small currents tend to have voltage noise inversely proportional to the square root of the emitter current. LM185-1.2 (10 ^A), ADR291 (15 ^A).

• Low Noise. A bandgap reference operated at higher cur­rent tends to be less noisy. LM336, ADR291.

• Shutdown. Sometimes it is important to turn the device off for minimum system power drain.

• Startup. Not all references start up very quickly.

• Shunt-Mode Operation. In some applications, a shunt­mode reference (which looks like a Zener) is easier to apply. LM4040, LM336, LT1009, AD1580.

• Series-Mode Operation. For applications where the load current may vary, series mode can be advantageous. AD581, LT1019, many others.

• Output Can Sink or Source Current. Sometimes it is con­venient if the output can drive load currents in both di­

rections. If you need this, be cautious, as load regulation when sinking is usually inferior to when sourcing. MAX6341, AD581, LT1019, many others.

• Output Is Trimmable—in a Narrow Range. Beware that if you use this feature to adjust Vout a significant amount, the tempco may be degraded.

• Output Is Adjustable—over a Wide Range. This is some­times a nice feature, but the accuracy, stability, and tempco of external resistors must be considered. LM385 (adjustable), LM4041-ADJ.

• Filter Pin. As band-gap references are fairly noisy (some­times comparable to 1 LSB of a 10- or 12-bit system) a provision for an external noise filter capacitor is some­times very useful. LM369, AD587.

• Temperature Sensor. Some units provide a temperature sensor output at —2.2 mV/°C. REF01, LT1019, many oth­ers.

• Heater. Some units provide a resistor on-chip that can be used to heat the unit to a constant warm temperature. LT1019.

• Low-Dropout Regulator. Many modern references need a supply voltage only 0.1 V or 0.2 V larger than Vout: a pop­ular feature. See also below.

• Requirement for Capacitive Load. Most low-dropout ref­erences require a capacitive load on their output to pre­vent oscillations: an unpopular feature, as sometimes the capacitor is bigger or more expensive than the IC.

• Tolerance of Cload. Some references will tolerate any ca­pacitive load, and may not require any load capacitor at all. LM385, LT1009.

• Low Tempco. A very desirable feature.

• After-Assembly Trim. This is a procedure for optimizing the low tempco of a reference. However, it uses pins for connection to in-circuit trims such as fuses or Zener zaps—so that these pins cannot be used for other fea­tures. LT1019, LM169.

• Small Packages. Many small systems require surface – mount devices. Packages such as SO-8 or SOT-23 are popular. However, tiny plastic packages tend to cause stresses which may degrade long-term stability.

• Long-Term Stability. A very desirable feature, but not trivial to find, and not easy or inexpensive to test for. In fact, stability is just about the most expensive specifica­tion that one can buy on a reference.

• Compromises. No reference can provide every advan­tage, so priorities and tradeoffs must be engineered.

• Price. Any combination of excellent features and/or spec­ifications is likely to command a high price. This leads to compromises; see above.


The concept of the bandgap reference was first published by David Hilbiber of Fairchild Semiconductor in 1964 (20). If a suitable circuit is used to add voltages with both positive and negative temperature coefficients, until the output voltage is approximately the same as the bandgap voltage of the mate­rial used (1.205 V for silicon, 0.803 V for germanium) a stable, low-tempco reference can be achieved.

The invention of the LM113 bandgap reference IC (21) by Robert J. Widlar in 1971 was rather like the birth of a baby— just a beginning. While this small IC was useful for instru­ment makers who needed a reference that would run on low voltages (such as 4.5 V or 3 V or even 1.5 V), it definitely did not have superior performance. The standard LM113 had an output voltage of 1.220 V with a tolerance of ±5%, a fairly broad spread of temperature coefficients, and mediocre long­term stability. Still, the principle and the feasibility of the bandgap reference had been proved, and the development of new bandgap reference circuits with needed improvements soon commenced. These efforts have continued for well over 25 years without much diminishment.

The bandgap reference was first used in the NSC LM113 reference circuit (1971) and the LM109 Voltage Regulator (1969) (22). The bandgap circuit employs transistors op­erating at different current densities, such as 10 p, A flowing through one transistor with one emitter, and the same amount of current flowing through another transistor 10 times as big. This generates a voltage (AVbe) of perhaps 60 mV, which is a voltage proportional to absolute temperature (VPTAT). This voltage is amplified and added to a voltage proportional to the transistor’s base-emitter voltage Vbe, which decreases fairly linearly with temperature. The addi­tion is scaled so that the total voltage is about 1.24 V dc. When the reference voltage is set or trimmed to this voltage, a low tempco is obtained.

The principle of the bandgap reference relies on a good un­derstanding of the Vbe of transistors. Widlar’s paper (23) on this subject clarified the mathematics and physics of Vbe and corrected various misconceptions.

We refer to the example of the LM113. In Fig. 2, the LM113 schematic diagram shows a basic bandgap circuit. When V+ is around 1.22 V dc, Q1 runs at a high current den­sity, about 230 nA/^m2. Q2 is operated at a low density, about 15 nA/^m2, and so its Vbe is much smaller, by about 70 mV. Now, let’s assume that the circuit is at balance and the output is near 1.22 V. Then the 70 mV across R5 is magnified by the ratio of R4 to R5, about 8.5: 1, up to a level of 600 mV. This


Figure 2. Schematic diagram, LM113 (simplified).

voltage is added to the Vbe of Q4 (about 620 mV at room tem­perature) to make a total of 1.22 V, as required. Q4 then am­plifies the error signal through Q7 and Q9, which provide enough gain to hold the V+ bus at 1.22 V. The beauty of the bandgap reference is the summation of the Vbe term, which decreases at the rate of about —2 mV/°C, and the AVbe term, which grows at about +2 mV/°C, to achieve an overall tempco that is substantially zero. All bandgap references employ this summation of a growing and a shrinking voltage to make a stable low-tempco voltage. Further, it has been shown (23) that when a circuit has been trimmed to the correct voltage, the correct tempco will follow, despite process variations in parameters such as Vbe, 3, sheet resistivity, etc. Consequently, bandgap circuits are often trimmed to their ideal voltage so as to provide a low tempco.

Since the LM109 and LM113 were introduced, many other circuits and configurations have been invented for bandgap references. The Brokaw cell is often used for output voltages larger than 1.25 V, as its Vout can be scaled by the ratio of two built-in resistors, R3 and R4. This unit, the AD580, was intro­duced in 1974 (24) (Fig. 3).

A similar circuit is used in the LM117 (25), whose output can be set by two external resistors R7 and R8 to any voltage in the range 1.25 V to 37 V (Fig. 4).

The above information was excerpted from an invited pa­per at the IEEE Bipolar Circuits and Technology Meeting, 1990 (26). The paper includes much information on how to



Figure 4. Schematic diagram, LM117 (simplified).

engineer a bandgap reference badly, and a little information on how to do it right. The paper is accessible on the World Wide Web at: http://www. national. com/rap/Application/


The bandgap reference was soon introduced into many kinds of voltage regulator ICs, instead of the old Zener refer­ences. Many of these ICs showed improved performance in one aspect or another. But most of these regulators had low accuracy, and will not be considered here. Our study here will concentrate on precision references with much better than 1% accuracy and tempcos much better than 50 X 10—6/°C.

Paul Brokaw at Analog Devices, Wilmington, MA, de­signed a curvature-correction circuit that canceled out the normal quadratic dependence of the bandgap’s output on tem­perature. The temperature drift over a 70°C span was re­duced below 5 X 10—6/°C. These were introduced in the AD581, about 1976. The related US patent (27) showed how to use different types of IC resistors, with different tempcos, to correct for the curvature.

Robert Widlar designed the NSC LM10 IC bandgap refer­ence (28). This circuit had a reference output of 0.200 V, which was easily scalable by external resistors up to 39 V. The basic reference was able to run on a 1.1 V power supply. It included curvature correction (29).

At NSC, Carl Nelson designed a quadratic curvature cor­rection circuit suitable for curvature correction of bandgap references and temperature sensors, as described in a US pa­tent (30). This was first introduced on the LM35 temperature sensor (31). While at LTC, Nelson later designed an improved logarithmic curvature-correction scheme (32). This circuit was introduced in the LT1019 (33).

Derek Bowers at Analog Devices designed a modified type of reference IC circuit that does not rely on the AVbe of transis­tors operated at different current densities. It depends on the offset of the threshold voltages of two junction field-effect transistors (JFETs) made with different implant doses. This IC was introduced in 1997 as the ADR291 (34). It has a typi­cal long-term stability of only 100 X 10—6 per 1000 h at + 150°C. The ADR291 as introduced does not have sufficiently low tempco or gain error to permit such excellent long-term stability to be fully appreciated. But since the feasibility of this principle has been demonstrated, it is foreseeable that ICs with fully optimized performance will be available soon.

Most of these IC references are not usable directly as stan­dards. However, the ones with the best specifications have adequate stability and are suitable for portable standards in small systems. They may need auxiliary circuits for trims, calibration, tempco correction, and so on—just as a Weston standard cell needs an oven to be useful.


For many years, makers of hybrid ICs were able to include chips of different technologies, wired together on a small ce­ramic substrate. For example, the old NSC LH0075 (intro­duced in 1975 but now out of production) included a quad opamp chip, an LM329 IC reference, and several laser – trimmed resistors. The trimming provided advantages of im­proved output accuracy, and a convenient 10.00 V output volt­age level rather than the 6.95 V of the reference itself.

Likewise, modern hybrid IC references such as Thaler Corp.’s Models VRE100/101/102 use chips of different tech­nologies (Zener diodes, precision trimmable resistors, opera­tional amplifiers, and thermistors) to provide the same kinds of advantages, such as output voltages trimmed to ±0.005%, and temperature coefficients better than ±0.6 X 10—6/°C or ±0.3 X 10—6/°C. Output voltages such as +10 V or —10 V dc (or both) to as low as +1.5 V or —1.5 V are available (35).


The LX5600 temperature-sensor IC, designed by Robert Dob – kin at National Semiconductor Corp. (NSC) and introduced in






Tempcos (10—6 °C—’)








Motorola, APD






Motorola, APD






Motorola, APD








” Tolerance ±5%.

1973 (13), had a hidden agenda: in addition to the tempera­ture sensor, this chip had an experimental IC Zener refer­ence. The Zener diode was connected in series with a transis­tor’s base-to-emitter voltage Vbe, and a buffer amplifier was provided. The reference’s actual performance was fairly medi – ocre—the tempco was typically +30 X 10—6/°C, and the long­term stability was stated as 1000 X 10—6 per 1000 h at +85°C. Still, this temperature-sensor IC went into production as a test bed, and the engineers were able to evaluate a large num­ber of the diodes. Best of all, the construction of a tempera­ture sensor on the same chip as the Zener reference made it easy to operate the temperature sensor as a temperature controller, and to make a little oven around the Zener, hold­ing it at a very stable temperature (such as +88°C). It was easy to evaluate a large number of these references, operating at a constant temperature.

This study soon led to the LM129 and LM199 IC refer­ences (14).

The LM129 was an improved, simplified, upgraded version of the LX5600’s reference, with a series resistance better than 1 П, and a tempco typically in the range 10 X 10—6/°C to 60 X 10—6/°C. These ICs could be tested (and graded in produc­tion test) for 50, 20, or 10 X 10—6/°C.

The LM199 was a new design. It used an integrated tem­perature controller on the die, to hold the die temperature at +88°C. It was housed in a four-lead TO-46 package (similar to a low-profile TO-18). A small plastic thermal shield over the package was used to minimize the power needed to hold the whole IC at that temperature. Under these conditions, the LM199’s reference could show a usable tempco better than 2 X 10—6/°C, 1 X 10—6/°C, or even j X 10—6/°C, selected and tested, over a temperature range from —55° to +85°C. Of course, this temperature-controlled IC did require a signifi­cant amount of power for the heater (typically 260 mW at 25°C, and even more at low ambient temperatures) to hold that +88°C temperature. But this was an acceptable require­ment in many systems.

The temperature sensitivity of any temperature-stabilized circuit depends at least as much on the layout of the heat – sensitive components, and on the gradients caused by the heater, as on the tempco of the circuit. Thus the LM199’s good tempco is related to good die layout.

Further disadvantages of the LM199 were its tolerance (±2%) and the fact that its nominal voltage (6.95 V) was not as convenient as 5.000 V or 10.000 V or even 6.20 V. And unless a charge pump was added, the LM129 or LM199 could not run on 5 V—it needed at least 8 V, at 1 mA for the refer­ence and at 50 mA for the heater. These disadvantages led to efforts to develop improved circuits that avoided some of these drawbacks.

The other significant advantage of the LM129 (and LM199) resided in its buried (subsurface) Zener diode. In most Zener references the breakdown occurred at the surface, where the concentration of impurities (doping) was maximum. Thus, sur­face contamination (even with high-quality planar processing) and electron charging of the oxide caused some degradation of noise and of long-term stability. The invention by Carl Nelson and Robert Dobkin of shallow diffusion layers with decreased concentrations at the surface caused the Zener breakdown to occur about a micron below the surface of the IC, where it is im­mune to surface conditions. This allowed superior consistency of low noise and better long-term stability.

Extensive testing of large numbers of LM199s showed that a large fraction of the units exhibited reference stability con­sistently better than 10 X 10—6 or 5 X 10—6 per 1000 h, when sampled once a week. (However, some units were consistently worse than 20 X 10—6 per 1000 h.) The units that tested better than 20 X 10—6 per 1000 h were designated LM199AH-20 and LM299AH-20, and were used in many precision systems as stable references. Also, they were popular in high-resolution DVMs. The LM199 is still the only successful temperature – stabilized IC in the industry.

Several good selected LM299AH references were evaluated by the National Bureau of Standards (NBS, now the NIST). They found that the long-term drift tendency was about —1 X 10—6 per 1000 h, a fairly consistent drift, presumably re­lated to operation at the die temperature of +88°C.

Other researchers found that if one group of LM299AHs were kept around room temperature, with their heaters off, and another group allowed to run at their normal tempera­ture of +88°C, the long-term drift trend of the units at room temperature was considerably lower than that of the warm units. The room-temperature units could be heated up to their normal +88°C on a specified schedule, perhaps one day per 3 months, and used to calibrate out the long-term drifts of the units kept at +88°C.

The use of buried Zener diodes has spread to other ICs. The LT1021 (15), designed by Carl Nelson at Linear Technol­ogy Corp. (LTC), used a buried Zener diode with temperature – compensating circuits and after-assembly trims to achieve 2 X 10—6/°C, without any heater. The LM169 (16) was engi­neered by Robert Pease at NSC to do likewise.

The LTZ1000 is a buried Zener designed by Robert Dobkin of LTC for laboratory standard use. All resistors are off chip, so that high-stability resistors can be utilized. Its on-chip heater can be activated for thermal stabilization. The die at­tach uses bubble material for high thermal impedance, as high as 400°C/W. Long-term stability approaching 1 X 10—6 is claimed (17).

The Analog Devices AD534 Multiplier IC was designed by Barrie Gilbert (18) using a buried Zener diode. The Analog Devices AD561 was a complete digital-to-analog converter (DAC) IC designed by Peter Holloway and Paul Brokaw, uti-

lizing a buried Zener diode and tempco trim circuits to achieve a gain tempco better than 10 X 10—6/°C (19).

Further research and development into circuits with bur­ied Zener diodes has waned, due to the improvements in bandgap references and to the concentration of research on low-voltage circuits and on CMOS technology, which pre­cludes the use of buried Zener diodes.

The design of a good reference on a large CMOS chip is not trivial. In many cases, a mediocre on-chip bandgap reference is adequate for a system on a chip. If a superior reference is needed, an external (off-chip) reference is often added. This can often provide cost, performance, and yield advantages.