Data Sheet

AD823

Rev. E | Page 15 of 20

APPLICATION NOTES

INPUT CHARACTERISTICS

In the AD823, 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 < +VS. Driving

the input voltage closer to the positive rail causes a loss of amplifier

bandwidth and increased common-mode voltage error.

The AD823 does not exhibit phase reversal for input voltages up

to and including +VS. Figure 38 shows the response of an AD823

voltage follower to a 0 V to 5 V (+VS) square wave input. The

input and output are superimposed. The output polarity tracks

the input polarity up to +VS, with no 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 AD823’s noninverting input prevents phase

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

illustrated in Figure 39.

GND

100

0%

1V

2µs

1V

90

10

Figure 38. AD823 Input Response: RP = 0, VIN = 0 to +VS

+VS

GND

5V

VIN

RP

VOUT

AD823

1V

10 µs

1V

90

100

10

0%

Figure 39. AD823 Input Response:

VIN = 0 to +VS + 200 mV, VOUT = 0 to +VS, RP = 49.9 kΩ

Because 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 reverses direction as internal

device junctions become forward biased. This is illustrated in

Figure 7.

A current limiting resistor should be used in series with the

input of the AD823 if there is a possibility of the input voltage

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

input voltage is applied to the AD823 when ±VS = 0. The

amplifier becomes damaged if left in that condition for more

than 10 seconds. A 1 kΩ resistor allows the amplifier to

withstand up to 10 V 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 V below

−VS as long as the total voltage from the positive supply to the

input terminal is less than 36 V. In addition, the input stage

typically maintains picoamp level input currents across that

input voltage range.

The AD823 is designed for 16 nV/√Hz wideband input voltage

noise and maintains low noise performance to low frequencies

(see Figure 16). This noise performance, along with the AD823’s

low input current and current noise, means that the AD823

contributes negligible noise for applications with source

resistances greater than 10 kΩ and signal bandwidths greater

than 1 kHz.

OUTPUT CHARACTERISTICS

The AD823’s unique bipolar rail-to-rail output stage swings

within 25 mV of the supplies with no external resistive load.

The AD823’s approximate output saturation resistance is 25 Ω

sourcing and sinking. This can be used to estimate the output

saturation voltage when driving heavier current loads. For

instance, when driving 5 mA, the saturation voltage to the rails

is approximately 125 mV.

If the AD823’s output is driven hard against the output

saturation voltage, it recovers within 250 ns of the input

returning to the amplifier’s linear operating region.

A/D Driver

The rail-to-rail output of the AD823 makes it useful as an A/D

driver in a single-supply system. Because it is a dual op amp, it

can be used to drive both the analog input of the A/D as well as

its reference input. The high impedance FET input of the

AD823 is well suited for minimal loading of high output

impedance devices.