Features: High pre-calibrated accuracy: 0.5c max @ +25c; Good linearity: 0.15C max (0C to +70C); Wide operating temperature range: –25C to +105C; Single supply operation: +4 V to + 30 V; excellent repeatability and stability; high-level output: 1 A/K m; double-ended monolithic IC: temperature input/current output; minimal self-heating error.

Product Description

The AD592 is a double-ended monolithic integrated circuit temperature sensor that provides an output current proportional to absolute temperature. The sensor acts as a high-impedance temperature-dependent current source of 1 μA/K for a wide range of supply voltages. Improved design of integrated circuit thin film resistors and laser wafer trimming have enabled the AD592 to achieve a level of absolute accuracy and non-linearity errors previously unobtainable at comparable prices.

The AD592 can be used in applications between -25°C and +105°C where traditional temperature sensors (ie, thermistors, RTDs, thermocouples, diodes) are currently being used. The inherent low cost of plastic-encapsulated monolithic ICs, combined with the low overall part count in any given application, makes the AD592 the most cost-effective temperature sensor available. The AD592 does not require expensive linearization circuits, precision voltage references, bridge components, resistance measurement circuits, and cold junction compensation.

Typical application areas include: appliance temperature sensing, automotive temperature measurement and control, HVAC system monitoring, industrial temperature control, thermocouple cold junction compensation, board level electronic temperature diagnostics, temperature readout options in meters, temperature sensors, temperature sensor and temperature sensor. Temperature correction circuits for precision electronics. The AD592 is particularly useful in remote sensing applications where it is immune to voltage drops and voltage noise on long lines due to its high impedance current output. AD592s are easily multiplexed; the signal current can be switched through a CMOS multiplexer, or the supply voltage can be implemented through tri-state logic gates.

The AD592 is available in three performance grades: AD592AN, AD592BN, and AD592CN. All devices are packaged in plastic to -92 enclosures rated for -45°C to +125°C. Performance is specified from -25°C to +105°C. AD592 chip is also available, please contact factory for details.

Product Highlights

1. AD592 provides a single power supply (4 volts to 30 volts) with a temperature measurement accuracy of 0.5 degrees Celsius.

2. Wide operating temperature range (–25°C to +105°C) and highly linear output make the AD592 an ideal replacement for older, more limited sensor technologies (i.e. thermistors, RTDs, diodes, thermocouples) Taste.

3. The AD592 has robust electrical performance; power supply irregularities and variations of up to 20 V or reverse voltage will not damage the device.

4. Because the AD592 is a temperature-dependent current source, it is immune to voltage noise pickup and IR drops in the signal leads when used remotely.

5. The high output impedance of the AD592 provides supply voltage drift and ripple rejection greater than 0.5°C/V.

6. Laser wafer trimming and temperature testing ensure AD592 units are easily interchangeable.

7. The initial system accuracy will not decrease significantly over time. The AD592 has demonstrated long-term performance and repeatability benefits inherent in integrated circuit design and construction.

theory of operation

The AD592 uses the basic characteristics of silicon transistors to achieve its temperature proportional output. If two identical transistors operate at a constant ratio of collector current density, r, then the difference in base emitter voltage (kt/q) (ln r). Since the k, boltzman constants and q charge of the electrons are constant, the resulting voltage is proportional to the absolute temperature (PTAT). In the AD592 this differential voltage is converted to PTAT current using low temperature coefficient thin film resistors. This parent will then use current to force the total output current to be proportional to degrees Kelvin. The result is that the current source output is equal to the scale factor times the sensor. A typical VI plot and extreme temperature of the circuit at +25°C are shown in Figure 1.

The factory adjusts the scaling factor to 1 μA/K at the wafer level by adjusting the temperature reading of the AD592 to correspond to the actual temperature. During laser trimming, the temperature of the IC is within a few degrees of 25°C and it is powered by a 5 V supply. The device is then packaged and temperature tested to specifications automatically.

Factors Affecting the Accuracy of AD592 System

The accuracy limits given on the AD592 specification page make it easy to apply in a variety of different applications. In order to calculate the total error budget in a given system, it is important to properly account for accuracy specifications, nonlinear errors, the response of the circuit to changes in supply voltage, and the effects of the surrounding thermal environment. As with other electronic designs, the choice of external components will have a significant impact on accuracy.

Calibration error, absolute accuracy and

Nonlinear specification

The three main error limits for the AD592 are given to make it easy to select the correct grade for any given application based on the overall level of accuracy required. They are the calibration accuracy at +25°C, and the error over the 0°C to +70°C and -25°C to +105°C temperature ranges. These specifications correspond to the actual error the user sees, whether the current output of the AD592 is converted to a voltage with precision resistors. Note that at room temperature, the maximum error over a commercial IC temperature range or an extended range including the boiling point of water can be read directly from the spec sheet. All three error limits are a combination of initial error, scale factor change, and nonlinear deviation from the ideal 1 μA/K output. Figure 2 graphically depicts the guaranteed accuracy limits of the AD592CN.

Compared to older technology sensors (ie, thermistors, RTDs, and thermocouples), the AD592 has a highly linear output, so the nonlinear error specification is decoupled from the given absolute temperature accuracy. As the maximum deviation from the best-fit straight line, this specification represents the only error that cannot be eliminated. Figure 3 is a graph of a typical AD592CN nonlinearity over the entire specified temperature range.

Fine tune for better accuracy

Calibration errors at 25°C are eliminated with a single temperature trim. Figure 4 shows how to adjust the scale factor of the AD592 in a basic voltage output circuit.

To fine-tune the circuit, the temperature must be measured by the reference sensor, and the R value should be adjusted so that the output (VOUT) corresponds to 1 mV/K. Note that the trimmer should be as close as possible to the highest temperature accuracy required. In most applications, if a single temperature trim is required, it can be implemented where the AD592 current-to-output voltage conversion occurs (eg, output resistor, offset to op amp). Figure 5 illustrates the effect on total error when using this technique.

If more accuracy is required, the AD592 in the circuit of Figure 6 can be used to eliminate initial calibration and scale factor errors.

When the sensor is at 0°C, adjust R1 for the 0 V output to zero, the initial calibration error will be zero, and move the output from K to °C. The scale factor error is eliminated by adjusting R2 to adjust the gain of the circuit at high temperature. The only error in the temperature range is nonlinearity. Figure 7 presents two typical plots of fine-tuning accuracy.

Influence of supply voltage and thermal environment

The power supply rejection feature of the AD592 minimizes errors due to voltage irregularities, ripple, and noise. If a power supply other than 5V is used (for factory trimming), the power supply error can be eliminated with a single temperature trimming. The PTAT properties of the AD592 will remain unchanged. The general insensitivity of the output allows the use of lower cost unregulated power supplies, which means that series resistances of several hundred ohms (eg CMOS multiplexers, instrumentation coil resistances) do not degrade overall performance.

The thermal environment in which the AD592 is used determines two performance characteristics: the effect of self-heating on accuracy and the sensor's response time to rapid changes in temperature. In the first case, the temperature of the integrated circuit junction above the ambient temperature is a function of two variables: the power dissipation level of the circuit and the thermal resistance (θ) between the chip and the environment. Multiplying the power dissipation by θ gives the self-heating error in °C. Since this type of error varies widely for environments with different heat dissipation capabilities, it is necessary to specify θ in several cases. Table 1 shows how the magnitude of the self-heating error varies with the environment. In a typical +25°C, 5V supply, free air application, the error is 0.2°C or less. A common clip-on heat sink will reduce the error by 25% or more in critical high temperature, large supply voltage situations.

notes

(1), τ is the average of the five time constants (99.3% of the final value). In cases where the thermal response is not a simple exponential function, the actual thermal response may be better than indicated.

(2) Use thermal grease.

The response of the AD592 output to sudden changes in ambient temperature can be simulated with an exponential function of time constant τ. Figure 8 shows typical response time graphs for several media of interest.

The time constant τ depends on θ and the thermal capacity of the chip and package. Table 1 lists the effective τ (time to 63.2% of the final value) for several different media. However, if the copper printed circuit board connections are ignored in the analysis, they will absorb or conduct heat directly through the AD592's solder-dipped Kovar leads. When a faster response is required, thermal grease or glue should be used between the AD592 and the surface temperature being measured. In free air applications, a clamp-on heat sink will reduce output settling time by 10-20%. Youth Achievement Organization

Installation Precautions

If the AD592 is thermally connected and properly protected, it can be used in any temperature measurement situation where the maximum temperature range encountered is between -25°C and +105°C. Due to the use of plastic IC packaging technology, when using a clamp or screwing on the heating plate. Thermally conductive epoxy or glue is recommended for typical installation conditions. Any electrically isolated metal or ceramic well can be used to shield the AD592 in wet or corrosive environments. Condensation at low temperatures can cause errors related to leakage current and should be avoided by sealing the unit with insulating epoxy paint or impregnation.

application

Connecting multiple AD592 devices in parallel increases the current through them and produces a reading proportional to the average temperature. The series AD592s will indicate the lowest temperature because the coldest device limits the series current flowing through the sensor. Both circuits are shown in Figure 9.

The circuit of Figure 10 demonstrates a method for deriving the voltage output in a differential temperature measurement.

r1 can be used to trim the inherent offset between two devices. Temperature measurements can be made with higher resolution by adding a gain resistor (10 kΩ). If v+ and v- are not the same size, the difference in power consumption between the two devices can lead to differential self-heating errors.

Cold junction compensation (CJC) used in thermocouple signal conditioning can be implemented using the AD592 in the circuit configuration of Figure 11. No more expensive analog ice baths or difficult to trim, inaccurate bridge circuits.

The circuit shown can be optimized for any ambient temperature range or thermocouple type by simply choosing the correct value for the scaling resistor R. The AD592 output (1 μA/K) multiplied by R should approximate the line (slope in V/°C) that best fits the thermocouple curve over the most likely ambient temperature range. In addition, the output sensitivity can be selected by the choice of resistors rg1 and rg2 to select the desired non-inverting gain. The offset adjustments shown refer to the AD592 to °C only. Note that the referenced TC and resistor are the main contributors to the error. A 40-to-1 temperature suppression can be easily achieved using the techniques described above.

Although the AD592 provides a noise immune current output, it is not compatible with process control/industrial automation current loop standards. Figure 12 is an example of a 4–20 mA temperature transmitter used with a 40 V, 1 kΩ system.

In this circuit, the 1µA/K output of the AD592 is amplified to 1mA/C and offset so that 4mA equals 17C and 20mA equals 33C. At an intermediate reference temperature, fine-tune RT for correct readings. Any temperature range within the operating range of the AD592 can be selected by selecting the appropriate resistors.

In a microprocessor-based system, reading temperature with the AD592 can be implemented with the circuit shown in Figure 13.

Using a differential input A/D converter and proper selection of current-to-voltage conversion resistors, the AD592 can measure any temperature range (up to the 130°C range that the AD592 is rated for) at any point center with a minimum number of components. In this configuration, the resolution of the system will be as high as 1°C.

In the circuit of Figure 14, a variable temperature controlled thermostat can easily be built using the AD592.

rhigh and rlow determine the temperature limits controlled by potentiometer rset. The circuit shown operates over the full temperature range (–25°C to +105°C) that the AD592 is rated for. The reference voltage maintains a constant setpoint voltage and ensures that approximately 7 V appears on the sensor. If necessary to protect and prevent additional noise hysteresis, it can be increased by connecting a resistor from the output to the ungrounded terminal of RLOW.

Due to the device's current mode output and supply voltage compliance range, multiple remote temperatures can be measured using a few AD592s or a series of 5v logic gates with a cmos multiplexer. As long as 4V is maintained across the sensor, the on-resistance of the FET switch or the output impedance of the gate does not affect the accuracy. The mux and logic driver circuits should be chosen to minimize leakage-related errors. Figure 15 shows a locally controlled multiplexer for switching signal currents from several remote AD592s. CMOS or TTL gates can also be used to switch the AD592 supply voltage, and the multiplexed signal is carried over a twisted pair to the load.

To minimize the number of muxes required when using a large number of AD592s, this circuit can be configured in a matrix. That is, the decoder can be used to switch the supply voltage to the AD592s column, while the mux is used to control the sensor row being measured. The maximum number of ad592s that can be used is the product of the number of channels of the decoder and the mux.

An example circuit controlling 80 AD592s is shown in Figure 16. All that is required to select one of the sensors is a 7-digit word. The enable input of the multiplexer turns off all sensors for minimal losses at idle.

To convert the AD592 output to °C or °F, an inexpensive reference and op amp can be used, as shown in Figure 17. Although this circuit is similar to the two temperature trimming circuits shown in Figure 6, there are two important differences. First, the gain resistors are fixed to reduce the need for high temperature trimming. Acceptable accuracy can be achieved by choosing inexpensive resistors with the correct tolerances. Second, AD592 calibration errors can be eliminated by a single-pot trim at a known convenient temperature (ie, room temperature). This step is independent of gain selection.