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AD822

REV. A

–13–

100

GND

2µs

Figure 37. VS = +5 V, 0 V; Gain of Two Inverter Response

to 2.5 V Step Centered –1.25 V Below Ground, RL = 10 kΩ

100

10µs

500mV

GND

Figure 38. VS = 3 V, 0 V; Gain of Two Inverter, VIN = 1.25 V,

25 kHz, Sine Wave Centered at –0.75 V, RL = 600 Ω

100

2µs

100

10µs

GND

+5V

OUT

Figure 39. (a) Response with RP = 0; VIN from 0 to +VS

(b) VIN = 0 to +VS + 200 mV

VOUT = 0 to +VS

RP = 49.9 kΩ

APPLICATION NOTES

INPUT CHARACTERISTICS

In the AD822, n-channel JFETs are used to provide a low

offset, low noise, high impedance input stage. Minimum input

common-mode voltage extends from 0.2 V below –VS to 1 V

less than +VS. Driving the input voltage closer to the positive

rail will cause a loss of amplifier bandwidth (as can be seen by

comparing the large signal responses shown in Figures 31 and

34) and increased common-mode voltage error as illustrated in

Figure 17.

The AD822 does not exhibit phase reversal for input voltages

up to and including +VS. Figure 39a shows the response of an

AD822 voltage follower to a 0 V to +5 V (+VS) square wave

input. The input and output arc superimposed. The output

tracks the input up to +VS without phase reversal. The reduced

bandwidth above a 4 V input causes the rounding of the output

wave form. For input voltages greater than +VS, a resistor in

series with the AD822’s noninverting input will prevent phase

reversal, at the expense of greater input voltage noise. This is

illustrated in Figure 39b.

Since the input stage uses n-channel JFETs, input current

during normal operation is negative; the current flows out from

the input terminals. If the input voltage is driven more positive

than +VS – 0.4 V, the input current will reverse direction as

internal device junctions become forward biased. This is

illustrated in Figure 4.

A current limiting resistor should be used in series with the

input of the AD822 if there is a possibility of the input voltage

exceeding the positive supply by more than 300 mV, or if an

input voltage will be applied to the AD822 when

±V

S = 0. The

amplifier will be damaged if left in that condition for more than

10 seconds. A 1 k

Ω resistor allows the amplifier to withstand up

to 10 volts of continuous overvoltage, and increases the input

voltage noise by a negligible amount.

Input voltages less than –VS are a completely different story.

The amplifier can safely withstand input voltages 20 volts below

the minus supply voltage as long as the total voltage from the

positive supply to the input terminal is less than 36 volts. In

addition, the input stage typically maintains picoamp level input

currents across that input voltage range.

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