Voltage references 101 - Embedded.com

Voltage references 101

Many embedded systems require a stable, low-noise DC reference. This reference can be a stable current or voltage source. These references are used in many ways. Some typical uses are listed as follows:

  1. Provide a stable excitation voltage or current to a sensor or array of sensors
  2. A reference level for an analog to digital, (A/D) converter
  3. A reference level for a digital to analog, (D/A) converter
  4. Provide a stable reference for a timing circuit

In this Roundtable, I will go over the basic DC voltage source or a voltage reference. Also, some discrete designs will be reviewed. A very brief compendium of some of the off-the-shelf IC’s will be discussed.

Desirable characteristics

In general, a voltage reference must provide a stable low noise DC level to some part of a system. In some cases, a high accuracy is desired. However, a high accuracy usually entails some increase in costs. These costs can be manifested as an increase in unit costs, due to a more expensive part, or an increase in direct labor during manufacturing, due to manual or automated trimming during final assembly. Some of the desired characteristics of a voltage reference are as follows:

  1. High precision
  2. Low temperature coefficient
  3. Good power supply rejection
  4. Very good load regulation
  5. Low power consumption
  6. Reasonable cost

What NOT to do
Given the desired characteristics of a voltage reference, we can see that not achieving or straying from these characteristics may lead to undesirable properties. I have seen a lot of ‘strange’, or actually, I shall say, ‘lazy’ designs when it comes to the application of circuitry for a voltage reference.Figure 1 shows some of the ‘simple’ not-to-be-recommended designs for a voltage reference. In short, I call the examples shown in this figure the voltage reference “Hall of Shame.”


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Figure 1 Voltage Reference “Hall of Shame”

The Zener diode
Review / Overview: One of the oldest and well known methods to create a stable voltage reference is to reverse bias a Zener diode. Going back to EE101, I show the IV curve for a typical Zener diode in Figure 2 .


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Figure 2. IV characteristic of a Zener Diode

One of the reasons why I show this well-known curve is to point out that the Zener voltage is really dependent on the current flowing through the reverse biased junction. In the classical case, as we were taught in university, we modeled the behavior as a constant Zener breakdown voltage for all currents flowing through the junction. This is simply not true. There is a small, yet significant slope in the Zener voltage vs. current. This slope is modeled as a reverse Zener impedance. This impedance is usually in the 30-100 ohm range.

So, let us say that we are using a typical 5.1-volt Zener diode biased using a 12-volt DC power supply. Let us also say that we are running about 10 milliamperes of reverse current through this thing. Let us also say that the Zener impedance is approximately 50 ohms. A 1% variance in the DC power supply will yield a variance of around 120 millivolts. Using the voltage divider relationship means this 1% variance will yield approximately a ± 8 millivolt variance in the reference voltage. At 5.1 volts we get roughly .008/5.1 which yields a 0.1% variance in the reference voltage. This equates to approximately a 20 dB improvement in the PSRR for our simple Zener reference.

Temperature characteristics
There are two effects that affect the temperature dependence of a Zener diode. These two effects have opposite temperature coefficients. The 1st characteristic is the Zener effect, which has a negative temperature effect. The 2nd characteristic is the avalanche breakdown. This effect has a positive temperature effect. In fact, for Zener voltages in the 5.1 to 5.6 volt range, both of the temperature effects nearly cancel each other out. The net result of this can be clearly seen in Figure 3 .


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Figure 3 Temperature coefficient of the various Zener diodes

VREF generation using a Zener Diode
Zener diodes are relatively inexpensive. Using a reverse biased Zener diode along with an op amp buffer is shown in Figure 4 . This figure shows that one can get a reasonable voltage reference for approximately $0.60.


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Figure 4 generating a voltage reference using a Zener Diode with a resistor biasing scheme

We get an even better PSRR by replacing the bias resistor with a constant current source. This circuit is shown in Figure 5. In this circuit, we show how to create a simple constant current source using a cheap 3-terminal adjustable regulator.

Figure 5A shows a simple voltage reference using a current source to bias the Zener diode. In Figure 5B, we get a bit fancier by adding a non-inverting gain stage. This allows us to attain a reference voltage different from the Zener breakdown voltage.

The cost of goods in this implementation has increased. In general, it will have increased by around $0.15 to about $0.75 and change.


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Figure 5 Voltage reference generated by a constant current biased Zener Diode

We can do away with the constant current source and save the $0.15 for the current regulator. In this case, we must find a way to “self-bias” the Zener diode. A simple way to do this is shown in Figure 6 .


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Figure 6 Voltage reference generated by a constant current biased Zener Diode

Before we move on, some explanations of this circuit are in order. First of all, there is a small capacitor in the feedback loop of the amplifier. Doing this will improve the stability of the circuit. We need to do this because we have added a self-bias network on the positive loop of the amplifier. Adding this positive feedback reduces the stability of this circuit and may cause oscillations.The RBIAS resistor will self-bias the Zener diode. The appropriate value of this resistor is dependent on the desired Zener current. Equation 1 shows how to properly calculate the value of the bias resistor.

Equation 1: How to calculate the bias resistor value for the self-biased reference network

The source resistor, RS, is used to ‘bootstrap’ the circuit into operation. It is also needed to give a ‘boost’ to the OP AMP current output. The value of the ‘bootstrap’ resistor is determined by the Zener voltage, (VZ), Zener bias current, (IZ), desired reference voltage, (VREF) and the amount of sourcing current desired, (IS). Equation 2 shows the inequalities needed for calculating the proper value for the 'bootstrap' resistor.

Equation 2: Inequalities needed for calculating the 'bootstrap' resistor valueShunt Voltage References

We have seen, in the previous section, that we can get a pretty decent voltage reference using a biased Zener diode circuit for a relatively reasonable cost. However, in this modern age of single supply, low voltage and low power operation, the traditional shunt-biased Zener diode circuit may not be appropriate. In essence, there are two restrictions to using a Zener that may prove untenable in modern circuit designs. These restrictions are as follows:

1. We need at least 10 mA of current to properly bias the Zener diode. Even this small current may be too large for some low power applications.

2. We have found the optimum Zener breakdown voltage is nominally 5.1 to 5.6 volts. This voltage is too high in the era of 1.8 volt or 3.3 volt system operation.

To solve this problem, the silicon IC manufacturers introduced the band-gap reference many years ago. One of the most notable of these devices is the National Semiconductor LM336 device. This device operates in very nearly the same mode as a Zener diode. However, the reverse impedance of these devices is in the tenths of an ohm or less. Plus, these devices have the advantage of operating at very low power levels. These devices have a shunt voltage that can be ‘configured’ by the manufacturer to many voltage less than 3.3V.

Figure 7 shows a circuit realization of the LM336-2.5 shunt regulated reference. The advent of new, lower power precision OP-AMPS has made using the newer shunt regulated references very attractive.


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Figure 7: Using an LM336-2.5 as a low-power shunt voltage reference

Series voltage references
We can save considerable printed circuit board real estate by using a series voltage reference. The series voltage reference is wired up in the same way as a 3-terminal regulator. Many of these references are very accurate, precise, show good PSRR and reasonable in cost.

Table 1 shows a representative sample of the costs for a 2.5V series voltage references by different manufacturers. A quick look at this table shows that this option will cost you about $1.50 and change.

Table 1: A small sample of various 2.5v series voltage references

Manufacturers
In general, there are many effective ways to create a low-cost reference voltage with a reasonable level of performance. We have shown that even adding a simple resistor bias to a Zener diode can increase the PSRR by at least 20dB.

However, there are some problems with the biased Zener diode approach given the modern age of low-power, low-voltage operations. The most effective way to solve this problem is to go to any one of the linear manufacturers to purchase a dedicated voltage reference. Table 2 shows a summary of costs and comments for the various reference voltage generation methods.

Table 2

Ken Wada is president and owner of Aurium Technologies, an independent product design and consulting firm in California's Silicon Valley. Ken has over 25 years of experience architecting and designing high-tech products and systems, including the FASTRAK vehicle-sensing system for toll roads and bridges. His expertise includes industrial automation, biotechnology, and high-speed optical networks. Ken holds four patents. You may reach him at .

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