Design & Operating Notes:
Normally, this extremely high input impedance of greater than 1014Ω
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.
1. The ALD1736A/ALD1736 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 ALD1736A/ALD1736 is internally com-
pensated 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.
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.
2. The ALD1736A/ALD1736 has complementary p-channel and n-
channel input differential stages connected in parallel to accomplish
rail to rail common mode input voltage ranges. 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
switchingpointhasbeenselectedtobeabout1.5Vbelowthepositive
supply voltage. Since offset voltage trimming on the ALD1736A/
ALD1736ismadewhentheinputvoltageissymmetricaltothesupply
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 below 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 provisions in the design to allow for input offset voltage
variations.
5. The ALD1736A/ALD1736 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 ALD1736A/ALD1736, with its ultra 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 less than 0.1°C above ambient temperature
under most operating conditions.
3. The input bias and offset currents are essentially input protection
diodereversebiasleakagecurrents,andaretypically 0.01pAatroom
temperature. This low input bias current assures that the analog
signal from the source will not be distorted by input bias currents.
7. The ALD1736A/ALD1736 has an internal design architecture that
provides robust high temperature operation. Contact factory for
custom screening versions.
TYPICAL PERFORMANCE CHARACTERISTICS
SUPPLY CURRENT AS A FUNCTION
OF SUPPLY VOLTAGE
COMMON MODE INPUT VOLTAGE RANGE
AS A FUNCTION OF SUPPLY VOLTAGE
±6
100
80
INPUTS GROUNDED
OUTPUT UNLOADED
±5
+25°C
T
A
= 25°C
-25°C
±4
T
A
= -55°C
60
±3
±2
±1
40
20
+125°C
±5
+70°C
0
0
0
±1
±2
±3
±4
±6
0
±1
±2
±3
±4
±5
±6
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
INPUT BIAS CURRENT AS A FUNCTION
OF AMBIENT TEMPERATURE
OPEN LOOP VOLTAGE GAIN AS A
FUNCTION OF LOAD RESISTANCE
1000
100
1000
100
10
V
= ±2.5V
S
10
1.0
V
T
= ±2.5V
= 25°C
S
A
0.1
0.01
1
10K
-50 -25
0
+25
+50
+75
+100 +125
100K
1M
10M
AMBIENT TEMPERATURE (°C)
LOAD RESISTANCE (Ω)
ALD1736A/ALD1736
Advanced Linear Devices
4 of 9
ALD [ ADVANCED LINEAR DEVICES ]
