AD820ARMZ-R7 datasheet(16/24 Pages) AD

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AD820

Rev. H | Page 16 of 24

APPLICATIONS INFORMATION

INPUT CHARACTERISTICS

In the AD820, 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 causes a

loss of amplifier bandwidth (as can be seen by comparing the

large signal responses shown in Figure 29 and Figure 32) and

increased common-mode voltage error, as illustrated in

Figure 20.

The AD820 does not exhibit phase reversal for input voltages

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

AD820 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 waveform. For input voltages greater than +VS, a

resistor in series with the AD820 positive input prevents phase

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

illustrated in Figure 38b.

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 AD820 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 AD820 when ±VS = 0 V. The

amplifier can be 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

the negative supply voltage 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 AD820 is designed for 13 nV/√Hz wideband input voltage

noise and maintains low noise performance to low frequencies

(refer to Figure 14). This noise performance, along with the

AD820 low input current and current noise, means that the

AD820 contributes negligible noise for applications with source

resistances greater than 10 kΩ and signal bandwidths greater

than 1 kHz. This is illustrated in Figure 39.

100

10µs

GND

+VS

100

2µs

GND

AD820

VOUT

–

VIN

–

(b)

(a)

Figure 38. (a) Response with RP= 0 Ω; VIN from 0 V to +VS

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

VOUT = 0 V to +VS, RP = 49.9 kΩ

100k

0.1

10k

10G

SOURCE IMPEDANCE (Ω)

10k

100

100k

10M

100M

WHENEVER JOHNSON NOISE IS GREATER THAN

AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE

CONSIDERED NEGLIGIBLE FOR APPLICATION.

RESISTOR JOHNSON

NOISE

1kHz

10Hz

AMPLIFIER-GENERATED

NOISE

Figure 39. Total Noise vs. Source Impedance

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