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AD8631ART Datasheet(PDF) 9 Page - Analog Devices |
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AD8631ART Datasheet(HTML) 9 Page - Analog Devices |
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 RS VI VOUT RF = RS UNITY GAIN BUFFER VOUT RF RI RB = RI RF VI VOUT RB = RI RF VI RF RI AD8631 INVERTING CONFIGURATION AD8631 Figure 21. Input Bias Cancellation Circuits Q14 IN VEE VCC VOUT Q1 Q3 R5 R6 I2 R1 IN R2 R4 R3 I1 R7 R8 D2 ESD D7 D8 Q2 Q4 D1 ESD D3 ESD D4 ESD Q5 Q6 Q7 C1 Q8 Q11 Q9 I4 I3 I5 Q13 I7 I6 C3 R10 R9 R11 Q15 Q17 D6 R12 Q18 I8 C2 R14 Q19 Q20 C4 D9 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 2 1.8 Lithium 2.6–3.6 1.7–2.4 NiMH 1.2 1 NiCd 1.2 1 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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