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ADR1500BKSZ-REEL Datasheet(PDF) 6 Page - Analog Devices |
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ADR1500BKSZ-REEL Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page ADR1500 Rev. 0 | Page 6 of 8 THEORY OF OPERATION The ADR1500 uses the band gap concept to produce a stable voltage reference suitable for high accuracy data acquisition components and systems. This device makes use of the underlying physical nature of the silicon transistor base emitter voltage in the forward-biased operating region. All such transistors have an approximate −2 mV/°C temperature coefficient, which is not suitable for use as a low TC reference; however, extrapolation of the temperature characteristic of any one of these devices to absolute zero (with collector current proportional to absolute temperature) reveals that VBE goes to approximately the silicon band gap voltage. Therefore, if a voltage could be developed with an opposing temperature coefficient to the sum with the VBE, than a zero TC reference would result. The ADR1500 circuit in Figure 7 provides such a compensating voltage, V1, by deriving two transistors at different current densities and amplifying the resultant VBE difference (ΔVBE, which has a positive TC). The sum of the VBE and V1 provides a stable voltage reference. V+ V– + ΔVBE VBE – + – V1 + – Figure 7. Schematic Diagram APPLYING THE ADR1500 The ADR1500 is simple to use in virtually all applications. To operate the ADR1500 as a conventional shunt reference, see Figure 8. An external series resistor is connected between the supply voltage and the ADR1500. For a given supply voltage, the series resistor, RS, determines the reverse current flowing through the ADR1500. The value of RS must be chosen to accommodate the expected variations of the supply voltage, VS, load current, IL, and the ADR1500 reverse voltage, VR, while maintaining an acceptable reverse current, IR, through the ADR1500. The minimum value for RS should be enough to limit IR to 10 mA when VS is at its maximum, and IL and VR are at their minimum. The equation for selecting RS is ) ( ) ( L R R S S I I V V R + − = (A) RS VS VR IR + IL VOUT IR + – IL (B) RS +5V (+3V) ±10% VR 2.94kΩ (1.30kΩ) VOUT + – Figure 8. Typical Connection Diagram Figure 8 shows a typical connection of the ADR1500 operating at a minimum of 100 μA. This connection can provide ±1 mA to the load, while accommodating ±10% power supply variations. TURN-ON TIME The turn-on time is a critical parameter for applications demanding a large amount of processing. Figure 9 shows the turn-on characteristics of the ADR1500. 250mV/DIV 5µs/DIV 2.4V 0V VIN CL = 200pF Figure 9. Response Time Upon application of power (cold start), the time required for the output voltage to reach its final value within a specified error is the turn-on settling time. Tow components are normally associated with the time for active circuits to settle and the time for the thermal gradients on the chip to stabilize. This characteristic is generated from cold start operation and represents the true turn-on waveform after power up. Figure 10 shows both the course and fine turn-on settling characteristics of the device; the total settling time to within 1.0 mV is about 6 μs, and there is no long thermal tail when the horizontal scale is expanded to 2 μs/DIV. The output turn-on time is modified when an external noise reduction filter is used. When present, the time constant of the filter dominates the overall settling. |
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