Datasheet ADR1500 (Analog Devices) - 6

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ADR1500 THEORY OF OPERATION V +5V (+3V) ±10%
The ADR1500 uses the band gap concept to produce a stable
S
voltage reference suitable for high accuracy data acquisition
2.94kΩ RS IR + IL RS (1.30kΩ)
components and systems. This device makes use of the underlying
IL
physical nature of the silicon transistor base emitter voltage in
VR + VR +
the forward-biased operating region. All such transistors have
IR VOUT VOUT
an approximate −2 mV/°C temperature coefficient, which is not
– –
suitable for use as a low TC reference; however, extrapolation of 9 -00 the temperature characteristic of any one of these devices to 9 74
(A) (B)
05 absolute zero (with collector current proportional to absolute Figure 8. Typical Connection Diagram temperature) reveals that VBE goes to approximately the silicon band gap voltage. Therefore, if a voltage could be developed Figure 8 shows a typical connection of the ADR1500 operating with an opposing temperature coefficient to the sum with the at a minimum of 100 μA. This connection can provide ±1 mA V to the load, while accommodating ±10% power supply BE, than a zero TC reference would result. The ADR1500 circuit in Figure 7 provides such a compensating voltage, V1, variations. by deriving two transistors at different current densities and
TURN-ON TIME
amplifying the resultant VBE difference (ΔVBE, which has a positive TC). The sum of the V The turn-on time is a critical parameter for applications BE and V1 provides a stable voltage reference. demanding a large amount of processing. Figure 9 shows the turn-on characteristics of the ADR1500.
+ V+ 2.4V 0V VIN V1 C – L = 200pF + ΔVBE –
8
+
-00 49
VBE V–
057
–
Figure 7. Schematic Diagram 10 0 9-
250mV/DIV 5µs/DIV APPLYING THE ADR1500
574 0 Figure 9. Response Time The ADR1500 is simple to use in virtually all applications. To operate the ADR1500 as a conventional shunt reference, see Upon application of power (cold start), the time required for Figure 8. An external series resistor is connected between the the output voltage to reach its final value within a specified supply voltage and the ADR1500. error is the turn-on settling time. Tow components are normally associated with the time for active circuits to settle and the time For a given supply voltage, the series resistor, RS, determines the for the thermal gradients on the chip to stabilize. This characteristic reverse current flowing through the ADR1500. The value of RS is generated from cold start operation and represents the true must be chosen to accommodate the expected variations of the turn-on waveform after power up. Figure 10 shows both the supply voltage, VS, load current, IL, and the ADR1500 reverse course and fine turn-on settling characteristics of the device; voltage, VR, while maintaining an acceptable reverse current, IR, the total settling time to within 1.0 mV is about 6 μs, and there through the ADR1500. is no long thermal tail when the horizontal scale is expanded to The minimum value for R 2 μs/DIV. The output turn-on time is modified when an S should be enough to limit IR to 10 mA when V external noise reduction filter is used. When present, the time S is at its maximum, and IL and VR are at their minimum. The equation for selecting R constant of the filter dominates the overall settling. S is (V − V ) S R R = S (I + I ) R L Rev. 0 | Page 6 of 8 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLYING THE ADR1500 TURN-ON TIME TRANSIENT RESPONSE OUTLINE DIMENSIONS ORDERING GUIDE