AD820A Analog Devices, AD820A Datasheet - Page 12

• Manufacturer Part Number: AD820A
• Manufacturer: Analog Devices

AD820
APPLICATION NOTES
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 –V
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 5 and 8) and increased
common-mode voltage error as illustrated in TPC 11.
The AD820 does not exhibit phase reversal for input voltages
up to and including +V
AD820 voltage follower to a 0 V to 5 V (+V
input. The input and output are superimposed. The output
polarity tracks the input polarity up to +V
The reduced bandwidth above a 4 V input causes the rounding
of the output wave form. For input voltages greater than +V
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 14b.
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 +V
– 0.4 V, the input current will reverse direction as internal device
junctions become forward biased. This is illustrated in TPC 4.
GND
GND
+V
S
V
IN
R
S
P
. Figure 14a shows the response of an
Ω
AD820
+5V
(a)
(b)
S
S
—no phase reversal.
to 1 V less than +V
S
) square wave
V
OUT
S
, a
S
S
.
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 kΩ 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 –V
The amplifier can safely withstand input voltages 20 V below the
minus 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 TPC 11). 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 kΩ and signal bandwidths greater than 1 kHz.
This is illustrated in Figure 15.
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 approximate 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 be 100 mV.
The amplifier’s open-loop gain characteristic will change as a
function of resistive load, as shown in TPCs 7 through 10. For
load resistances over 20 kΩ, 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.
100k
10k
100
0.1
1k
10
1
10k
RESISTOR JOHNSON
WHENEVER JOHNSON NOISE IS GREATER THAN
AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE
CONSIDERED NEGLIGIBLE FOR APPLICATION.
100k
NOISE
SOURCE IMPEDANCE –
1M
S
are a completely different story.
10M
AMPLIFIER-GENERATED
100M
NOISE
1kHz
1G
10Hz
S
= 0. The
10G