Design & Operating Notes:
1. The ALD2711A/ALD2711B/ALD2711 CMOS operational amplifier
uses a 3 gain stage architecture and an improved frequency
compensation scheme to achieve large voltage gain, high output
driving capability, and better frequency stability. In a conventional
CMOS operational amplifier design, compensation is achieved with
a pole splitting capacitor together with a nulling resistor. This method
is, however, very bias dependent and thus cannot accommodate the
large range of supply voltage operation as is required from a stand
alone CMOS operational amplifier. The ALD2711A/ALD2711B/
ALD2711 is internally compensated for unity gain stability using a
novel scheme that does not use a nulling resistor. This scheme
produces a clean single pole roll off in the gain characteristics while
providing for more than 70 degrees of phase margin at the unity gain
frequency.
2. The ALD2711A/ALD2711B/ALD2711 has complementary p-channel
and n-channel input differential stages connected in parallel to
accomplish rail to rail input common mode voltage range. This means
that with the ranges of common mode input voltage close to the power
supplies, one of the two differential stages is switched off internally.
To maintain compatibility with other operational amplifiers, this
switching point has been selected to be about 1.5V below the positive
supply voltage. Since offset voltage trimming on the ALD2711A/
ALD2711B/ALD2711 is made when the input voltage is symmetrical
to the supply voltages, this internal switching does not affect a large
variety of applications such as an inverting amplifier or non-inverting
amplifier with a gain larger than 2.5 (5V operation), where the common
mode voltage does not make excursions above this switching point.
The user should however, be aware that this switching does take
place if the operational amplifier is connected as a unity gain buffer
and should make provision in his design to allow for input offset voltage
variations.
3. The input bias and offset currents are essentially input protection
diode reverse bias leakage currents, and are typically less than 1pA
at room temperature. This low input bias current assures that the
analog signal from the source will not be distorted by input bias
currents. Normally, this extremely high input impedance of greater
than 10
12
Ω
would not be a problem as the source impedance would
limit the node impedance. However, for applications where source
impedance is very high, it may be necessary to limit noise and hum
pickup through proper shielding.
4. The output stage consists of class AB complementary output drivers,
capable of driving a low resistance load. The output voltage swing is
limited by the drain to source on-resistance of the output transistors
as determined by the bias circuitry, and the value of the load resistor.
When connected in the voltage follower configuration, the oscillation
resistant feature, combined with the rail to rail input and output feature,
makes an effective analog signal buffer for medium to high source
impedance sensors, transducers, and other circuit networks.
5. The ALD2711A/ALD2711B/ALD2711 operational amplifier has been
designed to provide full static discharge protection. Internally, the
design has been carefully implemented to minimize latch up. However,
care must be exercised when handling the device to avoid strong
static fields that may degrade a diode junction, causing increased
input leakage currents. In using the operational amplifier, the user is
advised to power up the circuit before, or simultaneously with, any
input voltages applied and to limit input voltages to not exceed 0.3V
of the power supply voltage levels.
6. The ALD2711A/ALD2711B/ALD2711, with its micropower operation,
offers numerous benefits in reduced power supply requirements, less
noise coupling and current spikes, less thermally induced drift, better
overall reliability due to lower self heating, and lower input bias current.
It requires practically no warm up time as the chip junction heats up
to only 0.2°C above ambient temperature under most operating
conditions.
TYPICAL PERFORMANCE CHARACTERISTICS
SUPPLY CURRENT AS A FUNCTION
OF SUPPLY VOLTAGE
INPUTS GROUNDED
OUTPUT UNLOADED
±7
COMMON MODE INPUT VOLTAGE RANGE
AS A FUNCTION OF SUPPLY VOLTAGE
±6
±5
±4
±3
±2
±1
0
T
A
= 25°C
+25°C
-25°C
COMMON MODE INPUT
VOLTAGE RANGE (V)
SUPPLY CURRENT (µA)
500
400
T
A
= -55°C
300
200
0
0
±1
±2
±3
±4
±5
±6
SUPPLY VOLTAGE (V)
+125°C
+70°C
0
±1
±2
±3
±4
±5
±6
±7
SUPPLY VOLTAGE (V)
OPEN LOOP VOLTAGE GAIN AS A
FUNCTION OF LOAD RESISTANCE
1000
INPUT BIAS CURRENT AS A FUNCTION
OF AMBIENT TEMPERATURE
1000
INPUT BIAS CURRENT (pA)
100
10
V
S
=
±2.5V
OPEN LOOP VOLTAGE
GAIN (V/mV)
100
1.0
10
V
S
=
±2.5V
T
A
= 25°C
1
10K
100K
1M
10M
0.1
0.01
-50
-25
0
25
50
75
100
125
LOAD RESISTANCE (Ω)
AMBIENT TEMPERATURE (°C)
ALD2711A/ALD2711B
ALD2711
Advanced Linear Devices
4 of 9
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