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AD846AR-16-REEL Datasheet(PDF) 10 Page - Analog Devices |
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AD846AR-16-REEL Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page REV. C AD846 –10– Where: RP is the external resistance placed in series with the non- inverting input RF is the feedback resistor RS is the source resistor INN is the noise current in the inverting input INP is the noise current in the noninverting input VN is the input noise voltage. Typical values for these parameters (@ 1 kHz) in pA/ √Hz are: INN = 20, IPN = 6, VN = 2. Or, referring to the signal input, the equivalent mean-square in- put voltage noise is: VIN 2 = R F I NN ()2 + 1+ RS RF 2 V N 2 + R P INP ()2 +4kT R P + 4 kT R S 1 + RS RF Resistor RP is required for both inverting and noninverting (follower) operation, to insure stable operation. The amplifier’s noninverting input current (flowing through RP of 100 Ω) will typically add less than 300 µV to the AD846’s input offset volt- age. This can be trimmed-out using the optional network shown in Figure 44. The following table gives recommended values for RP. Recommended Supply Voltage Gain (RF/RS) Value for RF 6 V to 15 V 1– 10 100 Ω 6 V to 15 V 10 – 20 47 Ω 6 V to 15 V 20 – 200 0 Ω 5 V 1– 10 47 Ω 5 V 10 – 200 0 Ω Figure 42. Op Amp Simplified Noise Model NONINVERTING GAIN OPERATION The AD846 can be used as a noninverting amplifier or voltage follower, operating at gains between 1 and 200. A minimum value of RF equal to 1 k Ω should be employed. For low gains (1 to 2), the input signal should be applied to the AD846’s non- inverting input through a 100 Ω series resistor; this will help reduce peaking. The best transient response will occur when the amplifier’s output level is below 5 V peak to peak. At closed-loop gains of 3 or more, the input resistor is not re- quired unless peak signals greater than 3 V will be applied. The amplifier’s bandwidth can be determined by using the inverting amplifier’s bandwidth equation or from Figure 41. For example, at a gain of + 10 (RF = 1 k Ω, RS = 100 Ω) the bandwidth of the AD846 will be approximately 33 MHz; at a gain of +100, (RF = 1 k Ω, R S = 10 Ω) it will be 4 MHz. At gains of 3 or greater, a small capacitor (2 pF–5 pF) connected across the feedback resistor will help reduce overshoot; but when operating at noninverting gains below 3, this same capacitance will cause instability. Figure 43. AD846 Noninverting Amplifier Configuration USING THE COMPENSATION PIN OF THE AD846 Additional compensation may be provided for the AD846 by applying an external capacitance between Pin 5 and analog ground (Figure 44). The nominal value of the AD846’s internal compensation capacitor is 7 pF. For a given value of feedback resistance (RF), any added external capacitance reduces the amplifier’s slew rate and bandwidth proportionally. Figure 44. AD846 Inverting Amplifier Showing External Compensation Connection, RP and Optional VOS Trim In addition to providing for external compensation, Pin 5 may be used to clamp the output of the amplifier, as shown in Figure 45. The output can be clamped anywhere within the output range (approximately ± 10 V) of the amplifier. The in- put should also be clamped as a precaution against damaging the amplifier’s input transistors. Figure 45. AD846 Used as a Clamped Amplifier |
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