AD823A Analog Devices, AD823A Datasheet - Page 13

• Manufacturer Part Number: AD823A
• Description: Dual, 16 Mhz, Rail-to-rail Fet Input Amplifier
• Manufacturer: Analog Devices

REV. 0
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
+V
device junctions become forward biased. This is illustrated in
Figure 6.
A current limiting resistor should be used in series with the in-
put of the AD823 if there is a possibility of the input voltage ex-
ceeding the positive supply by more than 300 mV, or if an input
voltage will be applied to the AD823 when V
fier will be damaged if left in that condition for more than 10
seconds. A 1 k resistor allows the amplifier to withstand up to
10 volts of continuous overvoltage, and increases the input volt-
age noise by a negligible amount.
Input voltages less than –V
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 AD823 is designed for 16 nV/ Hz wideband input voltage
noise and maintains low noise performance to low frequencies
(refer to Figure 15). 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 k 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
sourcing and sinking. This can be used to estimate output satu-
ration voltage when driving heavier current loads. For instance,
when driving 5 mA, the saturation voltage to the rails will be ap-
proximately 125 mV.
If the AD823’s output is driven hard against the output satura-
tion voltage, it will recover 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 along with
its reference input. The high impedance FET input of the
AD823 is well suited for minimally loading of high output im-
pedance devices.
S
– 0.4 V, the input current will reverse direction as internal
S
are a completely different story.
S
= 0. The ampli-
–13–
Figure 38 shows a schematic of an AD823 being used to drive
both the input and reference input of an AD1672, a 12-bit
3 MSPS single supply A/D converter. One amplifier is config-
ured as a unity gain follower to drive the analog input of the
AD1672 which is configured to accept an input voltage that
ranges from 0 to 2.5 V.
The other amplifier is configured as a gain of two to drive the
reference input from a 1.25 V reference. Although the AD1672
has its own internal reference, there are systems that require
greater accuracy than the internal reference provides. On the
other hand, if the AD1672 internal reference is used, the second
AD823 amplifier can be used to buffer the reference voltage for
driving other circuitry while minimally loading the reference
source.
The circuit was tested with a 500 kHz sine wave input that was
heavily low pass filtered (60 dB) to minimize the harmonic con-
tent at the input to the AD823. The digital output of the
AD1672 was analyzed by performing an FFT.
During the testing, it was observed that at 500 kHz, the output
of the AD823 cannot go below about 350 mV (operating with
negative supply at ground) without seriously degrading the sec-
ond harmonic distortion. Another test was performed with a
200
go as low as 200 mV without seriously affecting the second har-
monic distortion. There was, however, a slight increase in the
third harmonic term with the resistor added, but it was still less
than the second harmonic.
(1.25V)
Figure 38. AD823 Driving Input and Reference of the
AD1672, a 12-Bit 3 MSPS A/D Converter
V
REF
V
IN
pull-down resistor to ground that allowed the output to
1k
5
6
2
3
AD823
+5VA
4
8
1k
1
7
0.1µF
CLOCK
49.9
10µF
10µF
20
21
22
23
24
25
26
27
16
0.1
+5VA
µF
19
+V
COM
REFOUT
AIN1
AIN2
REFIN
IN COM
NCOMP2
NCOMP1
ACOM
REF
AD1672
28
CC
+V
COM
+5VD
19
18
D
DD
0.1
µF
15
13
14
12
11
10
9
8
7
6
5
4
3
2
1
AD823
OTR
BIT1 (MSB)
BIT2
BIT3
BIT4
BIT5
BIT6
BIT7
BIT8
BIT9
BIT10
BIT11
BIT12 (LSB)
+5VD
10
µF
0.1µF