OP777 Datasheet, PDF (10/12 Pages) Analog Devices – Precision Micropower Single Supply Operational Amplifier

OP777
BASIC OPERATION
The OP777 amplifier uses a precision Bipolar PNP input stage
coupled with a high-voltage CMOS output stage. This enables
this amplifier to feature an input voltage range which includes the
negative supply voltage (often ground-in single-supply applications)
and also swing to within 1 mV of the output rails. Additionally, the
input voltage range extends to within 1 V of the positive supply rail.
The epitaxial PNP input structure provides high breakdown voltage,
high gain, and input bias current figure comparable to that obtained
with “Darlington” input stage amplifier but without the drawbacks
(i.e., severe penalties for input voltage range, offset, drift and noise).
PNP input structure also greatly lowers the noise and reduces the dc
input error terms.
Supply Voltage
The amplifiers are fully specified with a single 5 V supply and, due
to design and process innovations, can also operate with a supply
voltage from 2.7 V up to 30 V. This allows operation from most
split supplies used in current industry practice, with the advantage
of substantially increased input and output voltage ranges over
conventional split-supply amplifiers. The OP777 series is specified
with (V_SY = 5 V, V– = 0 V and V_CM = 2.5 V which is most suitable
for single supply application. With PSRR of 130 dB (0.3 µV/V) and
CMRR of 110 dB (3 µV/V) offset is minimally affected by power
supply or common-mode voltages. Dual supply, ±15 V operation
is also fully specified.
Input Common-Mode Voltage Range
The OP777 is rated with an input common-mode voltage which
extends from minus supply to 1 V of the Positive supply. However,
the amplifier can still operate with input voltages slightly below
V_EE. In Figure 43, OP777 is configured as a difference amplifier
with a single supply of 2.7 V and negative dc common-mode volt-
ages applied at the inputs terminals. A 400 mV p-p input is then
applied to the noninverting input. It can be seen from the graph
below that the output does not show any distortion. Micropower
operation is maintained by using large input and feedback resistors.
100k⍀
100k⍀
+3V
؊0.27V
100k⍀
؊0.1V
100k⍀
OP777
V_IN = 1kHz at 400mV p-p
Figure 43. OP777 Configured as a Difference Amplifier
Operating at V_CM < 0 V
Input Over Voltage Protection
When the input of an amplifier is more than a diode drop below
V_EE, large currents will flow from the substrate (V– pin) to the
input pins which can destroy the device. In the case of OP777,
differential voltage equal to the supply voltage will not cause any
problem (see Figure 44). OP777 has built in 500 Ω internal current
limiting resistors, in series with the inputs, to minimize the chances
of damage. It is a good practice to keep the current flowing into the
inputs below 5 mA. In this context it should also be noted that the
high breakdown of the input transistors removes the necessity for
clamp diodes between the inputs of the amplifier; a feature that is
mandatory on many precision op amps. Unfortunately, such
clamp diodes greatly interfere with many application circuits such
as precision rectifiers and comparators. The OP777 series is free
from such limitations.
30V
V p-p = 32 V
OP777
Figure 44a. Unity Gain Follower
V_OUT
0V
V_IN
V_SY = ؎15V
V_IN
V_OUT
TIME – 0.2ms/DIV
Figure 42. Input and Output Signals with V_CM < 0 V
TIME – 400␮s/DIV
Figure 44b. Input Voltage Can Exceed the Supply Voltage
Without Damage
–10–
REV. 0