AD8631ART datasheet(9/12 Pages) AD

9 / 12 page

AD8631/AD8632

–9–

REV. 0

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 k

Ω, the corresponding generated offset voltage is 25 mV

(VOS = IB

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.

NONINVERTING CONFIGURATION

AD8631

VOUT

RF = RS

UNITY GAIN BUFFER

VOUT

RB = RI RF

VOUT

RB = RI RF

AD8631

INVERTING CONFIGURATION

AD8631

Figure 21. Input Bias Cancellation Circuits

Q14

VEE

VCC

VOUT

ESD

Q11

Q13

R10

R11

Q15

Q17

R12

Q18

R14

Q19

Q20

VCC

D16

R13

VEE

Q10

Figure 19. Simplified Schematic

Table I. Typical Battery Life Voltage Range

Nominal

End-of-Voltage

Battery

Voltage (V)

Discharge (V)

Lead-Acid

1.8

Lithium

2.6–3.6

1.7–2.4

NiMH

1.2

NiCd

1.2

Carbon-Zinc

1.5

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.

TIME – 200 s/Div

VS = 1.8V

VIN = 1.8V p-p

VIN

VOUT

Figure 20. Rail-to-Rail Input Output

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