Datasheet AD680 (Analog Devices) - 6

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AD680 THEORY OF OPERATION
Band gap references are the high performance solution for low Reference outputs are frequently required to handle fast supply voltage operation. A typical precision band gap consists transients caused by input switching networks, commonly of a reference core and buffer amplifier. Based on a new, pat- found in ADCs and measurement instrumentation equipment. ented band gap reference design (Figure 5), the AD680 merges Many of the dynamic problems associated with this situation the amplifier and the core band gap function to produce a can be minimized with a few simple techniques. Using a series compact, complete precision reference. resistor between the reference output and the load tends to “decouple” the reference output from the transient source, or a Central to the device is a high gain amplifier with an intentionally relatively large capacitor connected from the reference output to large proportional to absolute temperature (PTAT) input offset. ground can serve as a charge storage element to absorb and This offset is controlled by the area ratio of the amplifier input deliver charge as required by the dynamic load. A 50 nF capaci- pair, Q1 and Q2, and is developed across Resistor R1. Transistor tor is recommended for the AD680 in this case; this is large Q12’s base emitter voltage has a complementary to absolute enough to store the required charge, but small enough not to temperature (CTAT) characteristic. Resistor R2 and the parallel disrupt the stability of the reference. combination of Resistor R3 and Resistor R4 “multiply” the PTAT voltage across the R1 resistor. Trimming the R3 and R4 resistors The 8-lead PDIP and 8-lead SOIC packaged versions of the to the proper ratio produces a temperature invariant of 2.5 V at AD680 also provide a temperature output pin. The voltage on the output. The result is an accurate, stable output voltage this pin is nominally 596 mV at 25°C. This pin provides an accomplished with a minimum number of components. output linearly proportional to temperature with a characteristic of 2 mV/°C.
+VIN NOISE PERFORMANCE Q9 Q8
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
Q11
noise of a typical AD680. The noise measurement is made with a
Q3 Q4 V
band-pass filter made of a 1-pole high-pass filter, with a corner
Q5 OUT Q1
frequency at 0.1 Hz, and a 2-pole low-pass filter, with a corner
1
×
R1
frequency at 12.6 Hz, to create a filter with a 9.922 Hz bandwidth.
Q2 R3 8
×
C1 R2 R5 Q10 1s Q12 100 90 R6 Q6 Q7 TEMP R4 R7 GND
00813-005
5
μ
V
Figure 5. Schematic Diagram
APPLYING THE AD680
The AD680 is simple to use in virtually all precision reference applications. When power is applied to +V
10
IN and the GND pin
0%
is tied to ground, VOUT provides a 2.5 V output. The AD680 typically requires less than 250 μA of current when operating 00813-006 from a supply of 4.5 V to 36 V. Figure 6. 0.1 Hz to 10 Hz Noise To operate the AD680, the +VIN pin must be bypassed to the Noise in a 300 kHz bandwidth is approximately 800 μV p-p. GND pin with a 0.1 μF capacitor tied as close to the AD680 as Figure 7 shows the broadband noise of a typical AD680. 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. Rev. H | Page 6 of 12 Document Outline FEATURES GENERAL DESCRIPTION CONNECTION DIAGRAMS PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS OUTPUT PROTECTION ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS THEORY OF OPERATION APPLYING THE AD680 NOISE PERFORMANCE TURN-ON TIME DYNAMIC PERFORMANCE LOAD REGULATION TEMPERATURE PERFORMANCE TEMPERATURE OUTPUT PIN DIFFERENTIAL TEMPERATURE TRANSDUCER LOW POWER, LOW VOLTAGE REFERENCE FOR DATA CONVERTERS 4.5 V REFERENCE FROM A 5 V SUPPLY VOLTAGE REGULATOR FOR PORTABLE EQUIPMENT OUTLINE DIMENSIONS ORDERING GUIDE