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ADR525BRT-REEL7 Fiches technique(PDF) 11 Page - Analog Devices |
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ADR525BRT-REEL7 Fiches technique(HTML) 11 Page - Analog Devices |
11 / 16 page ADR520/ADR525/ADR530/ADR540/ADR550 Rev. E | Page 11 of 1 6 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 (VBE) 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 low temperature coefficient references. Extra- polation of the temperature characteristics 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 ADR520/ADR525/ADR530/ADR540/ADR550 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 (ΔVBE, which has a positive temperature coefficient). The sum of VBE and V1 provides a stable voltage reference over temperature. VBE + – ΔVBE + – 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 remain stable. All shunt voltage references require an external bias resistor (RBIAS) between the supply voltage and the reference (see Figure 19). RBIAS sets the current that flows through the load (IL) and the reference (IIN). Because the load and the supply voltage can vary, RBIAS needs to be chosen based on the following considerations: • RBIAS must be small enough to supply the minimum IIN current to the ADR520/ADR525/ADR530/ADR540/ ADR550, even when the supply voltage is at its minimum value and the load current is at its maximum value. • RBIAS must be large enough so that IIN does not exceed 15 mA when the supply voltage is at its maximum value and the load current is at its minimum value. VOUT VS ADR550 IL IIN R IIN + IL Figure 19. Shunt Reference Given these conditions, RBIAS is determined by the supply voltage (VS), the load and operating currents (IL and IIN) of the ADR520/ADR525/ADR530/ADR540/ADR550, and the output voltage (VOUT) of the ADR520/ADR525/ADR530/ ADR540/ADR550. IN L OUT S BIAS I I V V R + − = (3) Precision Negative Voltage Reference The ADR520/ADR525/ADR530/ADR540/ADR550 are suit- able for applications where a precise negative voltage is desired. Figure 20 shows the ADR525 configured to provide a negative output. VS –2.5V ADR525 R Figure 20. Negative Precision Reference Configuration Output Voltage Trim The trim terminal of the ADR520/ADR525/ADR530/ADR540/ ADR550 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 electrical potentiometer can be used for this adjustment. Figure 21 illustrates how the output voltage can be trimmed using the AD5273, an Analog Devices, Inc., 10 kΩ potentiometer. R1 470kΩ POTENTIOMETER 10kΩ ADR530 AD5273 R VOUT VS Figure 21. Output Voltage Trim |
Numéro de pièce similaire - ADR525BRT-REEL7 |
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Description similaire - ADR525BRT-REEL7 |
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