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AD580TH Datasheet(PDF) 5 Page - Analog Devices |
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AD580TH Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page AD580 Rev. B | Page 5 of 8 THEORY OF OPERATION The AD580 family (AD580, AD581, AD584, AD589) uses the bandgap concept to produce a stable, low temperature coef- ficient voltage reference suitable for high accuracy data acqui- sition components and systems. The device makes use of the underlying physical nature of a silicon transistor base-emitter voltage in the forward-biased operating region. All such tran- sistors have approximately a –2 mV/°C temperature coefficient, unsuitable for use directly as a low TC reference. Extrapolation of the temperature characteristic of any one of these devices to absolute zero (with an emitter current propor-tional to the absolute temperature), however, reveals that it will go to a VBE of 1.205 V at 0 K, as shown in Figure 3. Thus, if a voltage could be developed with an opposing temperature coefficient to sum with VBE to total 1.205 V, a 0 TC reference would result and operation from a single, low voltage supply would be possible. The AD580 circuit provides such a compensating voltage, V1 in Figure 4, by driving two transistors at different current densities and amplifying the resulting VBE difference (∆VBE—which now has a positive TC). The sum, VZ, is then buffered and amplified up to 2.5 V to provide a usable reference-voltage output. Figure 5 shows the schematic diagram of the AD580. The AD580 operates as a 3-terminal reference, meaning that no additional components are required for biasing or current setting. The connection diagram, Figure 6, is quite simple. 1.5 1.0 1.205 0.5 0 –273 °C –200 °C –100 °C 100 °C 0 °C 0K 73K 173K 373K 273K TEMPERATURE FOR BOTH DEVICES REQUIRED COMPENSATION VOLTAGE– SAME DEVICES VBE VS. TEMPERATURE FOR TWO TYPICAL DEVICES (IE α T) CONSTANT SUM = 1.205V Figure 3. Extrapolated Variation of Base-Emitter Voltage with Temperature (IEαT), and Required Compensation, Shown for Two Different Devices R7 R8 R2 R1 I2 ≅ I1 2I1 = I1 + I2 +VIN COM R4 R5 Q2 8A Q1 A V1 = 2 ∆V BE R1 R2 ∆V BE VBE (Q1) VOUT = VZ 1 + = 2.5V R4 R5 = VBE + 2 ∆V BE R1 R2 = VBE + 2 ln R1 R2 kT q J1 J2 = 1.205V VZ = VBE + V1 Figure 4. Basic Bandgap-Reference Regulator Circuit COM R12 R13 R7 Q14 Q3 Q13 Q7 Q4 Q10 Q11 Q12 Q15 Q5 Q6 Q8 Q9 Q2 8A Q1 A R2 R1 R3 R6 C1 R11 R5 R4 R9 R10 R8 –E +E 2.5V OUT Figure 5. Schematic Diagram +E 4.5 ≤ V IN ≤ 30V –E EOUT LOAD AD580 Figure 6. Connection Diagram VOLTAGE VARIATION VERSUS TEMPERATURE Some confusion exists in the area of defining and specifying reference voltage error over temperature. Historically, references are characterized using a maximum deviation per degree Centigrade; i.e., 10 ppm/°C. However, because of the inconsistent nonlinearities in Zener references (butterfly or S type characteristics), most manufacturers use a maximum limit error band approach to characterize their references. This technique measures the output voltage at 3 to 5 different temperatures and guarantees that the output voltage deviation will fall within the guaranteed error band at these discrete temperatures. This approach, of course, makes no mention or guarantee of performance at any other temperature within the operating temperature range of the device. |
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