AD820

REV. B

–13–

APPLICATION NOTES

INPUT CHARACTERISTICS

In the AD820, n-channel JFETs are used to provide a low off-

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

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

comparing the large signal responses shown in Figures 28 and

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

Figure 19.

The AD820 does not exhibit phase reversal for input voltages

input. The input and output are superimposed. The output

The reduced bandwidth above a 4 V input causes the rounding

resistor in series with the AD820’s plus input will prevent phase

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

illustrated in Figure 38b.

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

ing normal operation is negative; the current flows out from the

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

device junctions become forward biased. This is illustrated in

Figure 6.

AD820

+5V

GND

(a)

GND

(b)

A current limiting resistor should be used in series with the

input of the AD820 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 AD820 when

±V

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.

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.

The AD820 is designed for 13 nV/

√Hz wideband input voltage

noise and maintains low noise performance to low frequencies

(refer to Figure 13). This noise performance, along with the

AD820’s low input current and current noise means that the

AD820 contributes negligible noise for applications with source

resistances greater than 10 k

Ω and signal bandwidths greater

than 1 kHz. This is illustrated in Figure 39.

AMPLIFIER-GENERATED

NOISE

RESISTOR JOHNSON

NOISE

WHENEVER JOHNSON NOISE IS GREATER THAN

AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE

CONSIDERED NEGLIGIBLE FOR APPLICATION.

100k

0.1

10G

100

1

100k

10

10k

10k

1k

1G

100M

10M

1M

SOURCE IMPEDANCE –

1kHz

10Hz

Figure 39. Total Noise vs. Source Impedance

OUTPUT CHARACTERISTICS

The AD820’s unique bipolar rail-to-rail output stage swings

within 5 mV of the minus supply and 10 mV of the positive

supply with no external resistive load. The AD820’s approxi-

mate output saturation resistance is 40

Ω sourcing and 20 Ω

sinking. This can be used to estimate output saturation voltage

when driving heavier current loads. For instance, when sourcing

5 mA, the saturation voltage to the positive supply rail will be

200 mV, when sinking 5 mA, the saturation voltage to the

minus rail will he 100 mV.

The amplifier’s open-loop gain characteristic will change as a

function of resistive load, as shown in Figures 9 through 12. For

load resistances over 20 k

Ω, the AD820’s input error voltage is

virtually unchanged until the output voltage is driven to 180 mV

of either supply.

If the AD820’s output is driven hard against the output satura-

tion voltage, it will recover within 2

µs of the input returning to

the amplifier’s linear operating region.