Features: Pin-programmable 2.5 V or 3.0 V output; ultra-low drift: 3ppm/c max; high accuracy: 2.5 V or 3.0 V 1 mV max; low noise: 100 nV/√Hz; noise reduction capability; low static Current: 1 mA max; output trim capability; plug-in upgrade for current reference; temperature output pin; series or parallel mode operation (2.5 V, 3.0 V).

Product Description

The AD780 is an ultra-precision bandgap voltage reference that provides 2.5 V or 3.0 V outputs at 4.0 V and 36 V. Low initial error and temperature drift, combined with low output noise and drive capacitance make the AD780 a versatile precision reference application that enhances the performance of high-resolution ADCs and DACs. The unique low headroom design facilitates a 3.0V output input of 5.0V±10%, increasing the dynamic range of the ADC by 20%, which is too high performance when using the existing 2.5V reference voltage.

The AD780 can be sourced or sinked up to 10 mA, and can be used in series or parallel mode, thus allowing positive or negative output voltages without external components. That's it for almost any high-performance reference application. Unlike other competitors, the AD780 has no "zones" that are stable under all load conditions when a 1µf bypass capacitor is used on the power supply. A temperature output pin is available on the AD780. This provides an output voltage that varies linearly with temperature, allowing the AD780 to be configured as a temperature sensor while providing a regulated 2.5V or 3.0V output.

AD780 is for LT1019(a)–2.5 and AD680. The latter is aimed at low-power applications. The AD780 is available in three grades, plastic impregnated, SOIC, and cerdip packages. AD 780AN, AD 780AR, AD 780BN, AD780BR and AD780CR are specified to operate at -40°C to 85°C.

Product Highlights

1. The AD780 provides outputs with programmable 2.5 V or 3.0 V pins from 4V to 36V inputs.

2. Laser trimming coefficients for initial accuracy and temperature without the use of external components. The AD7800 has a maximum change of 0.9 mV from -40°C to +85°C.

3. For applications that require higher precision, fine-tuning connections can be optionally provided.

4. The AD780 has extremely low noise, typically 4μV p-p 0.1 Hz to 10 Hz, with a broadband spectral noise density of typically 100 nV/√Hz. If needed, it can be reduced further by just using two external capacitors.

5. The temperature output pin enables the AD780 to be configured as a temperature sensor while providing a stable output reference voltage.

theory of operation

Bandgap references are high performance solutions for low supply voltage and low power voltage reference applications. In this technique, a voltage with a positive temperature coefficient is combined with a negative coefficient of transistor vbe to produce a constant bandgap voltage.

In the AD780, the bandgap cell contains two npn transistors (Q6 and Q7) with emitter areas that differ by 12. Differences in their VBEs generate PTAT currents for R5. This in turn produces a PTAT voltage across R4 which, when combined with the VBE of Q7, produces a temperature-invariant voltage VBG. Using patented circuit technologies such as precision laser trimming resistors, the drift performance is further improved.

The output voltage of the AD780 is determined by the configuration of resistors R13, R14, and R15 in the amplifier feedback loop. This sets the output to 2.5 V or 3.0 V depending on whether R15 (pin 8) is grounded.

A unique feature of the AD780 is the low headroom design of the high gain amplifier, which produces an accurate 3V output from an input voltage as low as 4.5V (or 2.5V from 4.0V). The design of the amplifier also allows the part to have vin=vout when current is forced into the output. This allows the AD780 to operate as a double-ended shunt regulator, providing a -2.5V or -3.0V reference voltage output without the need for external components. The PTAT voltage is also used to provide the user with the thermometer output voltage (at pin 3), which increases at a rate of approximately 2 mV/°C.

NC pin 7 of the AD780 is a 20 kΩ V+ resistor for production testing only. Users who are currently using the LT1019 self-heater pin (pin 7) must consider the different loads on the heater supply.

Apply AD780

The AD780 can be used without any external components to achieve the specified performance. If power is supplied to pin 2, and pin 4 is grounded, then pin 6 provides 2.5 V or 3.0 V output, depending on whether pin 8 is unconnected or grounded.

If the load capacitance in the application is expected to be greater than 1nf, a bypass capacitor of 1µf (vin to gnd) should be used. The AD780 in 2.5 V mode typically draws 700 microamps of IQ at 5 V. This will add about 2 µA/V to 36 V.

Using a single 25 kΩ potentiometer connected between VOUT, Trim, and GND, the initial error can be zero. This is a coarse slight adjustment with an adjustment range of ±4% and is included in this article only for compatibility with other references. Fine-tuning can be achieved by inserting a large value resistor (eg 1–5 MΩ) in series with the wiper of the potentiometer. See Figure 2 above. For 2.5 V or 3.0 V mode, the trim range in fractional output is only greater than or equal to 2.1 kΩ/r full. The external zero resistance affects the overall temperature coefficient by a factor equal to the percentage of vout zero.

For example, a 1 mV (0.03%) output displacement (100 ppm/°C) zero resistance caused by the trimming circuit will cause the output to drift less than 0.06 ppm/°C (0.03% 200 ppm/°C, because the temperature coefficient of the AD780's internal resistance is also less than 100 ppm/°C).

Noise performance

If desired, the AD780's impressive noise performance can be further improved by adding two capacitors: a load capacitor C1 between the output and ground, and a compensation capacitor C2 between the temperature pin and ground. Suitable values are shown in Figure 3.

C1 and C2 also improve the settling performance of the AD780 under load transients. The improvement in noise performance is shown in Figures 4, 5 and 6 below.

noise comparison

The wideband noise performance of the ad780 can also be expressed in ppm. The typical performance of C1, C2 is 0.6ppm, and the typical performance without external capacitor is 1.2ppm. This performance is 7 and 3 respectively lower than the specified performance of the LT1019.

temperature performance

The AD780 combines patented circuit design techniques, precision thin-film resistors, and drift trimming to provide excellent performance over temperature. Temperature performance is expressed in ppm/°C, but due to the non-linearity of temperature characteristics, parts are tested and specified using the box test method. The nonlinearity takes the form of the characteristic S-shaped curve shown in Figure 7. The box test method forms a rectangular box around this curve, enclosing the maximum and minimum output voltages over the specified temperature range. The specified offset is equal to the slope of the diagonal of this box.

Temperature output pin

The AD780 provides a "temperature" output (pin 3) that varies linearly with temperature. This output can be used to monitor changes in system ambient temperature and initiate system calibration if required. The voltage VTEMP is 560 mV at 25°C, and the temperature coefficient is about 2 mV/°C. Figure 8 shows a typical VTEMP temperature characteristic at the output of the op amp with an irreversible gain of 5.

Since the temperature voltage is obtained from the bandgap core circuit, the current drawn from this pin will have a significant effect on vout. Care must be taken to buffer the temperature output with a suitable op amp, such as an op07, ad820 or ad711 (all of which cause less than 100 microvolts of change in vout). The relationship between item P and VOUT is as follows:

Note how sensitive the current dependency factor is to vout. Large amounts of current, even tens of microamps, drawn from the temperature pin can cause VOUT and the temperature output to fail.

The choice of c1 and c2 is primarily driven by the need for a relatively flat response that disappears very early in the high frequency noise at the output. But there is considerable leeway in choosing these capacitors. For example, the user can actually put a giant C2 on the temperature pin but not the output pin. However, little or a lot of capacitance must be placed at the temperature pins. Intermediate values of capacitance can sometimes cause oscillations. In any case, users should follow the advice in Figure 3.

temperature sensor circuit

The circuit shown in Figure 9 is a temperature sensor that amplifies the temperature output voltage by a factor of a little more than 5 to provide a wider full-scale output range. A trimmer can be used to adjust the output to change by exactly 10mV/degree Celsius. To reduce the change in resistance with temperature, resistors with low temperature coefficients, such as metal film resistors, should be used.

Supply current temperature is too high

The quiescent current of the AD780 will vary slightly over temperature and input supply range. Test limits are 1 mA for the industrial temperature range and 1.3 mA for the military temperature range. Typical performance over input voltage and temperature variation is shown in Figure 10 below.

opening time

The time required for the output voltage to reach its final value within a specified error range is defined as the turn-on settling time. The two main factors that affect this are the settling time of the active circuit and the thermal gradient settling time on the chip. Typical settling properties are shown in Figure 11 below. The AD780 settled to within 0.1% of its final value within 10 microseconds.

Dynamic performance

The output stage of the AD780 is designed to provide superior static and dynamic load regulation. Figure 12 shows the performance of the AD780 driving a 0 mA to 10 mA load.

Dynamic loads can be resistive and capacitive. For example, the load can be connected by a long capacitive cable. Figure 13 below shows the performance of the AD780 driving a 1000 pF, 0 mA to 10 mA load.

Accuracy benchmark for high resolution +5v data converters

The AD780 is ideal as a reference for most +5V high resolution ADCs. The AD780 is stable under any capacitive load, it has excellent dynamic load performance, and the 3.0V output provides the converter with maximum dynamic range without the need for additional expensive buffer amplifiers. One of the many ADCs that the AD780 fits into is the AD7884, a 16-bit high-speed sampling ADC. (See Figure 15.) This part previously required a precision 5.0V reference, resistor divider, and buffer amplifier to perform this function.

The AD780 is also ideal for high resolution converters such as the AD7710/AD7711/AD7712. (See Figure 16.) Although these parts use a 2.5V internal reference, the AD780 in 3V mode can be used to improve absolute accuracy, temperature stability, and dynamic range. Shown below are two optional noise reduction capacitors.

+4.5 V reference voltage for +5 V supply

Some +5V high-resolution ADCs can accommodate references up to +4.5V. The AD780 can provide an accurate +4.5V reference from a +5V supply using the circuit shown in Figure 17. This circuit will provide a +4.5 V output regulated from a supply voltage as low as +4.7 V. High quality tantalum 10µF capacitors in parallel with ceramic 0.1µF capacitors and 3.9Ω resistors ensure low output impedance up to about 50 MHz.

Negative (–2.5 V or –3.0 V) reference voltage

The AD780 can generate a negative output voltage in parallel mode by simply connecting the GND pin of the AD780 to the input and output ground of the negative supply through the bias resistor shown in Figure 18.

Using the following bootstrap circuit, the AD780 can implement an accurate -2.5 V (or -3.0 V) reference in series mode capable of delivering up to 100 mA to the load.