AD8631/AD8632
VCC
VCC
R7
R8
R14
D1 ESD
؊IN
R1
I1
Q5
D3
ESD
R3
R4
Q3 Q1 Q2 Q4
؉IN
D7
R2
R5
R6
D8
D4
ESD
C1
Q8
Q6
I3
Q11
Q10
Q9
D2 ESD
I2
Q7 I7
Q13
I4
I5
R9
R10
Q19
Q20
I8
C4
D9
Q14
C3
C2
R11
Q17
D16
I6
Q15
R12 R13
VOUT
Q18
D6
VEE
VEE
Figure 19. Simplified Schematic
Table I. Typical Battery Life Voltage Range
Battery
Lead-Acid
Lithium
NiMH
NiCd
Carbon-Zinc
Nominal
Voltage (V)
2
2.6–3.6
1.2
1.2
1.5
End-of-Voltage
Discharge (V)
1.8
1.7–2.4
1
1
1.1
RAIL-TO-RAIL INPUT AND OUTPUT
The AD8631 features an extraordinary rail-to-rail input and
output with supply voltages as low as 1.8 V. With the amplifier’s
supply set to 1.8 V, the input can be set to 1.8 V p-p, allowing the
output to swing to both rails without clipping. Figure 20 shows a
scope picture of both input and output taken at unity gain, with a
frequency of 1 kHz, at VS = 1.8 V and VIN = 1.8 V p-p.
VS = 1.8V
VIN = 1.8V p-p
VIN
VOUT
The rail-to-rail feature of the AD8631 can be observed over the
supply voltage range, 1.8 V to 5 V. Traces are shown offset for
clarity.
INPUT BIAS CONSIDERATION
The input bias current (IB) is a non-ideal, real-life parameter that
affects all op amps. IB can generate a somewhat significant offset
voltage. This offset voltage is created by IB when flowing through
the negative feedback resistor RF. If IB is 250 nA (worst case), and
RF is 100 kΩ, the corresponding generated offset voltage is 25 mV
(VOS = IB ϫ RF).
Obviously the lower the RF the lower the generated voltage offset.
Using a compensation resistor, RB, as shown in Figure 21, can
minimize this effect. With the input bias current minimized we
still need to be aware of the input offset current (IOS) which will
generate a slight offset error. Figure 21 shows three different
configurations to minimize IB-induced offset errors.
RF
RI
VI
RB = RIԽԽRF
AD8631
VOUT
INVERTING CONFIGURATION
RF
RI
VI
RB = RIԽԽRF
AD8631
VOUT
NONINVERTING CONFIGURATION
RF = RS
TIME – 200s/Div
Figure 20. Rail-to-Rail Input Output
REV. 0
VI
RS
AD8631
VOUT
UNITY GAIN BUFFER
Figure 21. Input Bias Cancellation Circuits
–9–