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AD8517 Datasheet(PDF) 8 Page - Analog Devices |
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AD8517 Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD8517/AD8527 –8– REV. B TIME – 1s/Div VS = 2.5V AV = 120k TA = 25 C Figure 15. 0.1 Hz to 10 Hz Noise TIME – 200 s/Div 0 0 0 0 0 0 0 0 VS = 2.5V AV = + 1 VIN = SINEWAVE TA = 25 C Figure 16. No Phase Reversal THEORY OF OPERATION The AD85x7 is a rail-to-rail operational amplifier that can operate at supply voltages as low as 1.8 V. This family is fabricated using Analog Devices’ high-speed complementary bipolar process, also called XFCB. The process trench isolates each transistor to minimize parasitic capacitance thereby allowing high-speed performance. Figure 19 shows a simplified schematic of the AD85x7 family. The input stage consists of two parallel complementary differen- tial pair: one NPN pair (Q1 and Q2) and one PNP pair (Q3 and Q4). The voltage drops across R7. R8, R9, and R10 are kept low for rail-to-rail operation. The major gain stage of the op amp is a double-folded cascode consisting of transistors Q5, Q6, Q8, and Q9. The output stage, which also operates rail-to-rail, is driven by Q14. The transistors Q13 and Q10 act as level-shifters to give more headroom during 1.8 V operation. As the voltage at the base of Q13 increases, Q18 starts to sink current. When the voltage at the base of Q13 decreases, I8 flows through D16 and Q15 increasing the VBE of Q17, then Q20 sources current. The output stage also furnishes gain, which depends on the load resistance, since the output transistors are in common emitter configuration. The output swing when sinking or sourcing 250 µA is 35 mV from each rail. The input bias current characteristics depend on the common- mode voltage, see Figure 4. As the input voltage reaches about 1 V below VCC, the PNP pair (Q3 and Q4) turns off. The 1 k Ω input resistor R1 and R2, together with the diodes D7 and D8, protect the input pairs against avalanche damage. The AD85x7 family exhibits no phase reversal as the input signal exceeds the supply by more than 0.6 V. Excessive current can flow through the input pins via the ESD diodes D1–D2 or D3–D4, in the event their ~0.6 V thresholds are exceeded. Such fault currents must be limited to 5 mA or less by the use of external series resistance(s). LOW VOLTAGE OPERATION Battery Voltage Discharge The AD8517 operates at supply voltages as low as 1.8 V. This amplifier is ideal for battery-powered applications since it can operate at the end of discharge voltage of most popular batteries. Table I lists the Nominal and End of Discharge Voltages of several typical batteries. TIME – 500ns/Div VS = 2.5V AV = 1 TA = 25 C CL = 100pF RL = 10k Figure 17. Small Signal Transient Response TIME – 200ns/Div VS = 2.5V AV = 1 RL = 10k TA = 25 C Figure 18. Large Signal Transient Response |
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