AD8631 データシートの表示（PDF） - Analog Devices
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AD8631 Datasheet PDF : 12 Pages
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AD8631/AD8632
0
VS = ؎2.5V
0
TA = 25؇C
0
0
0
0
0
0
0
TIME – 1s/DIV
Figure 15. 0.1 Hz to 10 Hz Noise
0
VS = ؎2.5V
AV = 1
VIN = SINE WAVE
0
TA = 25؇C
0
0
0
0
0
0
TIME – 200s/DIV
Figure 16. No Phase Reversal
0
VS = ؎2.5V
AV = +1
TA = 25؇C
0 CL = 33pF
RL = 10k⍀
0
0
0
0
0
0
TIME – 250ns/DIV
Figure 17. Small Signal Transient Response
0
VS = ؎2.5V
AV = +1
TA = 25؇C
0 CL = 100pF
RL = 10k⍀
0
0
0
0
0
0
TIME – 500ns/DIV
Figure 18. Large Signal Transient Response
THEORY OF OPERATION
The AD863x 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 mini-
mize parasitic capacitance, thereby allowing high-speed perfor-
mance. Figure 19 shows a simplified schematic of the AD863x
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 100 µA
is 35 mV maximum 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 kΩ input resistor R1 and R2, together with the diodes D7
and D8, protect the input pairs against avalanche damage.
The AD863x 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 AD8631 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.
–8–
REV. 0
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