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ADR520BRT-REEL7 Datenblatt(PDF) 11 Page - Analog Devices |
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ADR520BRT-REEL7 Datenblatt(HTML) 11 Page - Analog Devices |
11 / 16 page ADR520/ADR525/ADR530/ADR540/ADR550 Rev. A | Page 11 of 16 THEORY OF OPERATION The ADR520/ADR525/ADR530/ADR540/ADR550 use the band gap concept to produce a stable, low temperature coefficient voltage reference suitable for high accuracy data acquisition components and systems. The devices use the physical nature of a silicon transistor base-emitter voltage in the forward-biased operating region. All such transistors have approximately a –2 mV/°C temperature coefficient (TC), making them unsuitable for direct use as a low temperature coefficient reference. Extrapolation of the temperature characteristic of any one of these devices to absolute zero (with the collector current proportional to the absolute temperature), however, reveals that its VBE approaches approximately the silicon band gap voltage. Thus, if a voltage develops with an opposing temperature coefficient to sum the VBE, a zero temperature coefficient reference results. The ADR5xx circuit shown in Figure 18 provides such a compensating voltage (V1) by driving two transistors at different current densities and amplifying the resultant VBE difference (∆VBE, which has a positive temperature coefficient). The sum of VBE and V1 provides a stable voltage reference over temperature. VBE + – ∆V BE + – V1 V– V+ + – Figure 18. Circuit Schematic APPLICATIONS The ADR520/ADR525/ADR530/ADR540/ADR550 are a series of precision shunt voltage references. They are designed to operate without an external capacitor between the positive and negative terminals. If a bypass capacitor is used to filter the supply, the references remains stable. All shunt voltage references require an external bias resistor (RBIAS) between the supply voltage and the reference (see Figure 19). The RBIAS sets the current that flows through the load (IL) and the reference (IIN). Because the load and the supply voltage can vary, the RBIAS needs to be chosen based on the following considerations: • The RBIAS must be small enough to supply the minimum IIN current to the ADR5xx, even when the supply voltage is at its minimum value and the load current is at its maximum value. • The RBIAS must be large enough so that IIN does not exceed 10 mA when the supply voltage is at its maximum value and the load current is at its minimum value. Given these conditions, the RBIAS is determined by the supply voltage (VCC), the ADR5xx load and operating current (IL and IQ), and the ADR5xx output voltage (VOUT). IN L OUT CC BIAS I I V V R − − = (3) VOUT VS ADR550 IL IIN RBIAS IIN + IL Figure 19. Shunt Reference Precision Negative Voltage Reference The ADR5xx is suitable for applications where a precise negative voltage is desired. Figure 20 shows the ADR5xx configured to provide a negative output. VCC –2.5V ADR525 RBIAS Figure 20. Negative Precision Reference Configuration Output Voltage Trim The ADR5xx TRIM terminal can be used to adjust the output voltage over a range of ±0.5%. This allows systems designers to trim system errors by setting the reference to a voltage other than the preset output voltage. An external mechanical or elec- trical potentiometer can be used for this adjustment. Figure 21 illustrates how the output voltage can be trimmed by using the AD5273, an Analog Devices 10 kΩ potentiometer. R1 470k Ω POTENTIOMETER 10k Ω ADR530 AD5273 RBIAS VCC VOUT Figure 21. Output Voltage Trim |
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Ähnliche Beschreibung - ADR520BRT-REEL7 |
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