Data Sheet

AD823A

Rev. | Page 15 of 20

APPLICATIONS INFORMATION

INPUT CHARACTERISTICS

In the AD823A, N-channel JFETs provide a low offset, low noise,

high impedance input stage. Minimum input common-mode

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

amplifier bandwidth and increased common-mode voltage error.

The AD823A does not exhibit phase reversal for input voltages

input. The input and output are superimposed. The output

reversal. The reduced bandwidth above a 4 V input causes the

rounding of the output waveform. For input voltages greater

input prevents phase reversal, at the expense of greater input

is illustrated in Figure 40.

2.5V

0V

5.0V

1V

2µs

INPUT

OUTPUT

5V

AD823A

3V

0V

6V

1V

10µs

INPUT

OUTPUT

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 − 0.7 V, the input current reverses direction as internal device

junctions become forward biased. This is illustrated in Figure 11.

A current limiting resistor should be used in series with the input

of the AD823A if the input voltage can be driven over 300 mV

more positive than +Vs or 300 mV more negative than –Vs. The

amplifier will be damaged if either condition persists for more than

10 seconds. A 1 kΩ resistor in series with the AD823A input allows

the amplifier to withstand up to 10 V of continuous overvoltage

and increases input voltage noise by a negligible amount.

The AD823A is designed for 14 nV/√Hz wideband input voltage

noise (see Figure 19). This noise performance, along with the

AD823A low input current and current noise, means that the

AD823A contributes negligible noise for applications with high

source resistances. Figure 41 shows that the source resistance

contributes to negligible noise for source impedances lower

than 10 kΩ. The low input capacitance of 0.6 pF also means that

one can use a source impedance up to 13 kΩ without cutting

into the G = +1 small signal bandwidth region.

1

10

10

100

1k

10k

100k

100

SOURCE RESISTANCE (Ω)

TOTAL AMPLIFIER NOISE

AMPLIFIER VOLTAGE AND

CURRENT NOISE

SOURCE RESISTANCE

NOISE

Figure 41. RTI Noise vs. Source Resistance

OUTPUT CHARACTERISTICS

The unique bipolar rail-to-rail output stage of the amplifier swings

within 20 mV of the supplies with no external resistive load.

The approximate output saturation resistance of the AD823A

is 33 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 165 mV.

B