feature

Wide Operating Range: 50µA to 10mA; Initial Accuracy: ±0.1% max; Temperature Drift: ±50 ppm/°C max; Output Impedance: 0.5Ω max; Wideband Noise (10 Hz to 10 kHz): 20µV rms; Operating temperature range: -40°C to +85°C; high ESD rating; 4kv human body model; 400V model; compact surface mount SOT-23 and SC70 packages.

application

Portable battery powered devices Mobile phones, laptops, PDAs, GPS, and DMMs; computer workstations; suitable for various video ramdacs; smart industrial transmitters; PCMCIA cards; automotive; 3 V/5 V, 8-bit to 12-bit data conversion device.

General Instructions

The AD1580 is a low cost, 2-terminal (shunt), precision bandgap reference. It provides an accurate 1.225V output for input currents between 50µA and 10mA.

The precise matching and thermal tracking of the AD1580 chip assembly allows for greater accuracy and stability. Proprietary curvature correction design techniques have been used to minimize the nonlinearity of the voltage output temperature characteristics. The AD1580 is stable under any capacitive load.

The low minimum operating current makes the AD1580 ideal for use in battery-powered 3V or 5V systems. However, the wide operating current range means that the AD1580 is very versatile and suitable for a wide variety of high current applications.

The AD1580 is available in two grades, A and B, both of which are available in SOT-23 and SC70 packages, the smallest surface mount packages. Both grades are specified over the industrial temperature range -40°C to +85°C.

Absolute Maximum Ratings

Stresses above the Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device under the conditions described in the operating section of this specification or any other conditions above is not implied. Long-term exposure to absolute maximum rating conditions may affect device reliability.

Typical performance characteristics

theory of operation

The AD1580 uses the bandgap concept to generate a stable, low temperature coefficient voltage reference suitable for use in high precision data acquisition components and systems. The device exploits the fundamental physics of silicon transistor base-emitter voltages in the forward-biased operating region. The temperature coefficient (TC) of all such transistors is about -2mv/°C and is not suitable for direct use as a low TC reference; however, extrapolating the temperature characteristics of any of these devices to absolute zero (collector current is a function of absolute temperature) ), indicating that its V value is close to the silicon bandgap voltage. Therefore, if a voltage can develop the opposite temperature coefficient sum of VBE, a zero TC reference will result. The AD1580 circuit in Figure 10 provides such a compensation voltage, V1, by driving two transistors at different current densities and amplifying the resulting VBE difference (∏VBE with positive TC). The sum of VBE and V1 provides a stable voltage reference.

Apply AD1580

The AD1580 is easy to use in almost all applications. To operate the AD1580 as a conventional shunt regulator (see Figure 11), an external series resistor is connected between the supply voltage and the AD1580. For a given supply voltage, the series resistor, R, determines the reverse current flowing through the AD1580. The value of RS must be chosen to accommodate the expected changes in supply voltage VS, load current IL, and AD1580 reverse voltage VR, while maintaining an acceptable reverse current IR through the AD1580.

The minimum value of RS should be chosen when VS is at a minimum and IL and VR are at a maximum while maintaining the minimum acceptable reverse current.

The value of RS should be large enough to limit IR to 10ma when VS is at maximum and IL and VR are at minimum. The formula for selecting RS is as follows:

Figure 12 shows a typical connection for the AD1580BRT with an operating current of at least 100 µA. This connection provides ±1 mA to the load while accommodating ±10% supply variation.

temperature performance

The AD1580 is designed for reference applications where stable temperature performance is important. Extensive temperature testing and characterization ensures device performance remains within specified temperature ranges.

There is some confusion in defining and specifying temperature reference voltage errors. Historically, references have been characterized using a maximum deviation per degree Celsius, such as 50 ppm/°C. However, due to the non-linearity of temperature characteristics in standard Zener references (such as the S-type characteristic), most manufacturers now use the maximum limit error band method to specify devices. This technique involves measuring the output at three or more different temperatures to ensure the voltage is within a given tolerance. Proprietary curvature correction design techniques are used to minimize AD1580 nonlinearity, allowing the maximum deviation method to be used to guarantee temperature performance. This approach is more useful to the designer than simply ensuring that the error band is maximized over temperature.

Figure 13 shows the typical output voltage drift of the AD1580 and illustrates the method. The maximum slope of the two diagonal lines drawn from the initial output value at +25°C to the output values at +85°C and -40°C determines the performance level of the device. For a given grade of the AD1580, the designer can easily determine the maximum total error from the initial tolerance plus temperature variation.

For example, the AD1580BRT has an initial tolerance of ±1 mV; a temperature coefficient of ±50 ppm/°C corresponds to an error band of ±4 mV (50 × 10 × 1.225 V × 65 °C). Therefore, over the operating temperature range, the unit is guaranteed to be 1.225 V ± 5 mV.

Reproducing these results requires a combination of high accuracy and stable temperature control in the test system. Evaluation of the AD1580 yields a curve similar to Figure 5 and Figure 13.

Voltage Output Nonlinearity vs. Temperature

When a reference is used with a data converter, it is important to understand how temperature drift affects the overall performance of the converter. The nonlinearity of the reference output drift represents an additional error that is not easily calibrated outside the system. This characteristic (see Figure 14) is generated by normalizing the measured drift characteristics to the endpoint average drift. The residual drift error is about 500ppm, indicating that the AD1580 is compatible with systems requiring 10-bit accurate temperature performance.

reverse voltage hysteresis

A key requirement for manufacturers of high-performance industrial equipment is consistent output voltage at nominal temperature after operation over the operating temperature range. This characterization is generated by measuring the difference between the output voltage at +25°C after operation at +85°C and the output voltage at +25°C after operation at -40°C. Figure 15 shows the hysteresis associated with the AD1580. This feature exists in all references and is minimized in the AD1580.

Output Impedance vs Frequency

Understanding the role of the inverse dynamic output impedance in a practical application is important for the successful application of the AD1580. The voltage divider consists of the AD1580 output impedance and the external source impedance. When using an external source resistance of about 30kΩ (I=100μA), 1% of the noise of a 100khz switching power supply is generated at the output of the AD1580. Figure 16 shows how a 1µF load capacitor connected directly through the AD1580 reduces the effect of power supply noise to less than 0.01%.

Noise performance and reduction

The noise generated by the AD1580 is typically less than 5µV pp over the 0.1 Hz to 10 Hz frequency band. Figure 17 shows the 0.1 Hz to 10 Hz noise of a typical AD1580. In the 10 Hz to 10 kHz bandwidth, the noise is about 20 μV rms (see Figure 18a). If further noise reduction is required, a 1-pole low-pass filter can be added between the output pin and ground. A time constant of 0.2ms has a -3db point at about 800hz and reduces high frequency noise to about 6.5μV rms (see Figure 18b). The 960ms time constant has a -3db point at 165hz and reduces high frequency noise to about 2.9µV rms (see Figure 18c).

boot time

Many low-power meter manufacturers are increasingly concerned with the turn-on characteristics of the components used in their systems. Quick-start components typically allow the end user to turn off the power when not needed, but when the power is on, these components respond quickly. Figure 19 shows the turn-on characteristics of the AD1580.

When power is applied (cold cranking), the time it takes for the output voltage to reach its final value within a specified tolerance is the turn-on settling time. Two components usually associated with this are when the active circuit settles and when the thermal gradient on the chip settles. This characteristic results from a cold crank operation and represents the true power-on waveform after power-on. Figure 21 shows the coarse-start and fine-start settling characteristics of the device; the total settling time is approximately 6 μs within 1.0 mV, and there is no long thermal tail when the horizontal scale is extended to 2 ms/div.

When using an external noise reduction filter, the output turn-on time is modified. When present, the time constant of the filter controls the bulk settling.

Transient response

Many ADC and DAC converters provide transient current loads to the reference. Poor reference response can degrade converter performance.

Figure 22 shows the coarse and fine sedimentation characteristics of the device under loading transients of ±50 μA.

Figure 22a shows the settling characteristics of the device when the reverse current is increased by 50 μA. Figure 22b shows the response when the reverse current is reduced by 50 μA. The transient settles to 1 mV in about 3 μs.

Attempts to drive large capacitive loads (over 1000 pF) can result in ringing, as shown by the step response (see Figure 23). This is due to the additional pole formed by the load capacitance and the reference output impedance. Figure 20 shows a recommended method of driving capacitive loads. The resistor isolates the capacitive load from the output stage, while the capacitor provides a single-pole low-pass filter and reduces output noise.

Precision Micropower Low Loss Reference

The circuit in Figure 24 provides an ideal solution for making a regulated voltage reference with low standby power, low input/output loss capability, and minimal noise output. The amplifier simultaneously buffers and optionally amplifies the output voltage of the AD1580, which can deliver 1 mA of load current for output voltages up to 2.1V. A single-pole filter connected between the AD1580 and the OP193 input can be used to achieve low output noise. Nominal static power consumption is 200 microwatts.

Using AD1580 and 3 V Data Converter

The AD1580's low output drift (50 ppm/°C) and compact ultra-small SOT-23 package make it ideal for high-performance converters in today's space-critical applications.

One ADC family for which the AD1580 is well suited is the AD7714-3 and AD7715-3. The AD7714/AD7715 are charge-balanced (∑-∏) ADCs with on-chip digital filtering for measuring low-frequency signals with a wide dynamic range, such as those representing chemical, physical, or biological processes. Figure 25 shows the AD1580 connected to the AD7714-3/AD7715-3 for 3 V operation.

The AD1580 is ideal for creating reference levels for use with 12-bit multiplying DACs such as the AD7943, AD7945, and AD7948. In single-supply bias mode (see Figure 26), observe that the impedance of the IOUT2 terminal changes with the DAC code. If 3.3 volts additional linearity error results. The buffer amplifier eliminates any linearity degradation that may result from changes in the reference level.

Dimensions