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AD680 Datasheet(PDF) 6 Page - Analog Devices |
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AD680 Datasheet(HTML) 6 Page - Analog Devices |
6 / 12 page AD680 Rev. H | Page 6 of 12 THEORY OF OPERATION Band gap references are the high performance solution for low supply voltage operation. A typical precision band gap consists of a reference core and buffer amplifier. Based on a new, pat- ented band gap reference design (Figure 5), the AD680 merges the amplifier and the core band gap function to produce a compact, complete precision reference. Central to the device is a high gain amplifier with an intentionally large proportional to absolute temperature (PTAT) input offset. This offset is controlled by the area ratio of the amplifier input pair, Q1 and Q2, and is developed across Resistor R1. Transistor Q12’s base emitter voltage has a complementary to absolute temperature (CTAT) characteristic. Resistor R2 and the parallel combination of Resistor R3 and Resistor R4 “multiply” the PTAT voltage across the R1 resistor. Trimming the R3 and R4 resistors to the proper ratio produces a temperature invariant of 2.5 V at the output. The result is an accurate, stable output voltage accomplished with a minimum number of components. +VIN Q11 TEMP Q12 R2 R1 Q5 Q4 Q1 1 × Q2 8 × Q9 R5 Q8 Q3 Q7 Q6 Q10 GND C1 R6 R7 R4 R3 VOUT Figure 5. Schematic Diagram APPLYING THE AD680 The AD680 is simple to use in virtually all precision reference applications. When power is applied to +VIN and the GND pin is tied to ground, VOUT provides a 2.5 V output. The AD680 typically requires less than 250 μA of current when operating from a supply of 4.5 V to 36 V. To operate the AD680, the +VIN pin must be bypassed to the GND pin with a 0.1 μF capacitor tied as close to the AD680 as possible. Although the ground current for the AD680 is small, typically 195 μA, a direct connection should be made between the AD680 GND pin and the system ground plane. Reference outputs are frequently required to handle fast transients caused by input switching networks, commonly found in ADCs and measurement instrumentation equipment. Many of the dynamic problems associated with this situation can be minimized with a few simple techniques. Using a series resistor between the reference output and the load tends to “decouple” the reference output from the transient source, or a relatively large capacitor connected from the reference output to ground can serve as a charge storage element to absorb and deliver charge as required by the dynamic load. A 50 nF capaci- tor is recommended for the AD680 in this case; this is large enough to store the required charge, but small enough not to disrupt the stability of the reference. The 8-lead PDIP and 8-lead SOIC packaged versions of the AD680 also provide a temperature output pin. The voltage on this pin is nominally 596 mV at 25°C. This pin provides an output linearly proportional to temperature with a characteristic of 2 mV/°C. NOISE PERFORMANCE The noise generated by the AD680 is typically less than 8 μV p-p over the 0.1 Hz to 10 Hz band. Figure 6 shows the 0.1 Hz to 10 Hz noise of a typical AD680. The noise measurement is made with a band-pass filter made of a 1-pole high-pass filter, with a corner frequency at 0.1 Hz, and a 2-pole low-pass filter, with a corner frequency at 12.6 Hz, to create a filter with a 9.922 Hz bandwidth. 5 μV 1s 100 90 0% 10 Figure 6. 0.1 Hz to 10 Hz Noise Noise in a 300 kHz bandwidth is approximately 800 μV p-p. Figure 7 shows the broadband noise of a typical AD680. |
Número de pieza similar - AD680_15 |
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Descripción similar - AD680_15 |
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