NCS7101, NCV7101
http://onsemi.com
10
APPLICATION INFORMATION AND OPERATING DESCRIPTION
GENERAL INFORMATION
The NCS7101 is a rail−to−rail input, rail−to−rail output
operational amplifier that features guaranteed 1.8 volt
operation. This feature is achieved with the use of a modified
analog CMOS process that allows the implementation of
depletion MOSFET devices. The amplifier has a 1.0 MHz
gain bandwidth product, 1.2 V/ms slew rate and is
operational over a power supply range less than 1.8 V to as
high as 10 V.
Inputs
The input topology of this device series is unconventional
when compared to most low voltage operational amplifiers.
It consists of an N−channel depletion mode differential
transistor pair that drives a folded cascode stage and current
mirror. This configuration extends the input common mode
voltage range to encompass the V
EE
and V
CC
power supply
rails, even when powered from a combined total of less than
1.8 volts. Figures 27 and 28 show the input common mode
voltage range versus power supply voltage.
The differential input stage is laser trimmed in order to
minimize offset voltage. The N−channel depletion mode
MOSFET input stage exhibits an extremely low input bias
current of less than 40 pA. The input bias current versus
temperature is shown in Figure 4. Either one or both inputs
can be biased as low as V
EE
minus 300 mV to as high as 10 V
without causing damage to the device. If the input common
mode voltage range is exceeded, the output will not display
a phase reversal but it may latch in the appropriate high or
low state. The device can then be reset by removing and
reapplying power. If the maximum input positive or
negative voltage ratings are to be exceeded, a series resistor
must be used to limit the input current to less than 2.0 mA.
The ultra low input bias current of the NCS7101 allows
the use of extremely high value source and feedback resistor
without reducing the amplifier’s gain accuracy. These high
value resistors, in conjunction with the device input and
printed circuit board parasitic capacitances C
in
, will add an
additional pole to the single pole amplifier shown in
Figure 30. If low enough in frequency, this additional pole
can reduce the phase margin and significantly increase the
output settling time. The effects of C
in
, can be canceled by
placing a zero into the feedback loop. This is accomplished
with the addition of capacitor C
fb
. An approximate value for
C
fb
can be calculated by:
C
fb
+
R
in
C
in
R
fb
Figure 30. Input Capacitance Pole Cancellation
+
-
Output
R
fb
C
in
R
in
C
fb
C
in
= Input and printed circuit board capacitance
Input
Output
The output stage consists of complementary P and N
channel devices connected to provide rail−to−rail output
drive. With a 2.0 k load, the output can swing within 100 mV
of either rail. It is also capable of supplying over 95 mA
when powered from 10 V and 3.0 mA when powered from
1.8 V.
When connected as a unity gain follower, the NCS7101
can directly drive capacitive loads in excess of 390 pF at
room temperature without oscillating but with significantly
reduced phase margin. The unity gain follower
configuration exhibits the highest bandwidth and is most
prone to oscillations when driving a high value capacitive
load. The capacitive load in combination with the
amplifier’s output impedance, creates a phase lag that can
result in an under−damped pulse response or a continuous
oscillation. Figure 32 shows the effect of driving a large
capacitive load in a voltage follower type of setup. When
driving capacitive loads exceeding 390 pF, it is
recommended to place a low value isolation resistor
between the output of the op amp and the load, as shown in
Figure 31. The series resistor isolates the capacitive load
from the output and enhances the phase margin. Refer to
Figure 33. Larger values of R will result in a cleaner output
waveform but excessively large values will degrade the
large signal rise and fall time and reduce the output’s
amplitude. Depending upon the capacitor characteristics,
the isolation resistor value will typically be between 50 to
500 ohms. The output drive capability for resistive and
capacitive loads is shown in Figures 2, 3, and 23.
Figure 31. Capacitance Load Isolation
+
-
Output
R
Isolation resistor R = 50 to 500
C
L
Input
Note that the lowest phase margin is observed at cold
temperature and low supply voltage.
