Datasheet AD822 (Analog Devices) - 18

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AD822 Data Sheet APPLICATIONS INFORMATION INPUT CHARACTERISTICS 100k WHENEVER JOHNSON NOISE IS GREATER THAN
In the AD822, N-channel JFETs are used to provide a low offset,
AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE 10k CONSIDERED NEGLIGIBLE FOR APPLICATION.
low noise, high impedance input stage. Minimum input common-
1kHz V)
mode voltage extends from 0.2 V below −VS to 1 V less than +VS.
1k
Driving the input voltage closer to the positive rail causes a loss
ISE (µ O RESISTOR JOHNSON
of amplifier bandwidth (as can be seen by comparing the large
E N NOISE G 100
signal responses shown in Figure 34 and Figure 37) and increased
A LT O
common-mode voltage error as il ustrated in Figure 20.
V 10 UT
The AD822 does not exhibit phase reversal for input voltages up
10Hz INP
to and including +VS. Figure 42 shows the response of an AD822
1
voltage follower to a 0 V to 5 V (+V
AMPLIFIER-GENERATED
S) square wave input. The
NOISE
input and output are superimposed. The output tracks the input
0.1 10k 100k 1M 10M 100M 1G 10G
043 up to +VS without phase reversal. The reduced bandwidth above a
SOURCE IMPEDANCE (Ω)
00874- 4 V input causes the rounding of the output waveform. For input Figure 43. Total Noise vs. Source Impedance voltages greater than +VS, a resistor in series with the AD822
OUTPUT CHARACTERISTICS
noninverting input prevents phase reversal, at the expense of greater input voltage noise. This is illustrated in Figure 42. The AD822 unique bipolar rail-to-rail output stage swings within 5 mV of the negative supply and 10 mV of the positive supply with Because the input stage uses N-channel JFETs, input current no external resistive load. The approximate output saturation during normal operation is negative; the current flows out from resistance of the AD822 is 40 Ω sourcing and 20 Ω sinking, which the input terminals. If the input voltage is driven more positive can be used to estimate output saturation voltage when driving than +VS − 0.4 V, then the input current reverses direction as heavier current loads. For instance, when sourcing 5 mA, the internal device junctions become forward biased. This is illustrated saturation voltage to the positive supply rail is 200 mV; when in Figure 7. sinking 5 mA, the saturation voltage to the negative rail is 100 mV. A current-limiting resistor should be used in series with the input The open-loop gain characteristic of the amplifier changes as a of the AD822 if there is a possibility of the input voltage exceeding function of resistive load, as shown in Figure 10 to Figure 13. For the positive supply by more than 300 mV, or if an input voltage is load resistances over 20 kΩ, the AD822 input error voltage is applied to the AD822 when +VS or −VS = 0 V. The amplifier is virtual y unchanged until the output voltage is driven to 180 mV of damaged if left in that condition for more than 10 seconds. A 1 kΩ either supply. resistor al ows the amplifier to withstand up to 10 V of continuous overvoltage and increases the input voltage noise by a negligible If the AD822 output is overdriven so that either of the output amount. devices are saturated, the amplifier recovers within 2 μs of the input returning to the linear operating region of the amplifier. Input voltages less than −VS are different. The amplifier can safely withstand input voltages 20 V below the negative supply voltage Direct capacitive loads interact with the effective output if the total voltage from the positive supply to the input terminal impedance of the amplifier to form an additional pole in the is less than 36 V. In addition, the input stage typically maintains amplifier feedback loop, which can cause excessive peaking on picoampere (pA) level input currents across that input the pulse response or loss of stability. The worst case occurs voltage range. when the amplifier is used as a unity-gain follower. Figure 44 shows the AD822 pulse response as a unity-gain follower The AD822 is designed for 13 nV/√Hz wideband input voltage driving 350 pF. This amount of overshoot indicates approximately noise and maintains low noise performance to low frequencies 20° of phase margin—the system is stable, but nearing the edge. (refer to Figure 14). This noise performance, along with the AD822 Configurations with less loop gain, and as a result less loop low input current and current noise, means that the AD822 bandwidth, are much less sensitive to capacitance load effects. contributes negligible noise for applications with source resistances greater than 10 kΩ and signal bandwidths greater than 1 kHz. This is illustrated in Figure 43. Rev. J | Page 18 of 24 Document Outline FEATURES APPLICATIONS CONNECTION DIAGRAM GENERAL DESCRIPTION REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE MAXIMUM POWER DISSIPATION ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION INPUT CHARACTERISTICS OUTPUT CHARACTERISTICS SINGLE-SUPPLY VOLTAGE TO FREQUENCY CONVERTER LOW DROPOUT BIPOLAR BRIDGE DRIVER OUTLINE DIMENSIONS ORDERING GUIDE