AD8517/AD8527

–8–

REV. B

TIME – 1s/Div

Figure 15. 0.1 Hz to 10 Hz Noise

TIME – 200 s/Div

0

0

0

0

0

0

0

0

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

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

Figure 17. Small Signal Transient Response

TIME – 200ns/Div

Figure 18. Large Signal Transient Response