REV. B
OP1177/OP2177/OP4177
–10–
FUNCTIONAL DESCRIPTION
OP1177 is the fourth generation of ADI’s industry standard OP07
amplifier family. OP1177 is a very high-precision, low-noise opera-
tional amplifier with the highly desirable combination of extremely
low offset voltage and very low input bias currents. Unlike JFET
amplifiers, the low bias and offset currents are relatively insensitive
to ambient temperatures, even up to 125°C.
For the first time, Analog Devices’ proprietary process technology
and linear design expertise have produced a high-voltage
amplifier with superior performance to the OP07, OP77, and
OP177 in a tiny MSOP 8-lead package. Despite its small size
the OP1177 offers numerous improvements including low wide-
band noise, very wide input and output voltage range, lower
input bias current, and complete freedom from phase inversion.
OP1177 has the widest specified operating temperature range of
any similar device in a plastic surface-mount package. This is
increasingly important as PC board and overall system sizes
continue to shrink, causing internal system temperatures to rise.
Power consumption is reduced by a factor of four from the OP177
while bandwidth and slew rate increase by a factor of two. The low
power dissipation and very stable performance versus temperature
also act to reduce warm-up drift errors to insignificant levels.
Open-loop gain linearity under heavy loads is superior to competitive
parts like OPA277, improving dc accuracy and reducing distortion
in circuits with high closed-loop gains. Inputs are internally protected
from overvoltage conditions referenced to either supply rail.
Like any high-performance amplifier, maximum performance is
achieved by following appropriate circuit and PC board guidelines.
The following sections provide practical advice on getting the most
out of the OP1177 under a variety of application conditions.
Total Noise Including Source Resistors
The low input current noise and input bias current of the OP1177
make it useful for circuits with substantial input source resistance.
Input offset voltage increases by less than 1 µV max per 500 Ω
of source resistance.
The total noise density of the OP1177 is:
e e i R kTR
nTOTAL nnS S
,=+
()
+
224
Where, e
n
is the input voltage noise density
i
n
is the input current noise density
R
S
is the source resistance at the noninverting terminal
k is Boltzman’s constant (1.38 10
–23
J/K)
T is the ambient temperature in Kelvin (T = 273 + °C)
For R
S
< 3.9 kΩ, e
n
dominates and
ee
n TOTAL n,
≈
For 3.9 kΩ < R
S
< 412 kΩ, voltage noise of the amplifier, current
noise of the amplifier translated through the source resistor, and
thermal noise from the source resistor all contribute to the total
noise.
For R
S
> 412 kΩ, the current noise dominates and
eiR
n TOTAL n S,
≈
The total equivalent rms noise over a specific bandwidth is
expressed as:
Ee BW
n n TOTAL
=
()
,
Where BW is the bandwidth in Hertz.
NOTE: The above analysis is valid for frequencies larger than
50 Hz. When considering lower frequencies, flicker noise (also
known as 1/f noise) must be taken into account.
For a reference on noise calculations refer to Bandpass KRC or
Sallen-Key Filter section.
Gain Linearity
Gain linearity reduces errors in closed-loop configurations. The
straighter the gain curve, the lower the maximum error over the
input signal range will be. This is especially true for circuits with
high closed-loop gains.
The OP1177 has excellent gain linearity even with heavy loads,
shown in Figure 1. Compare its performance to the OPA277,
shown in Figure 2. Both devices were measured under identical
conditions with R
L
= 2 kΩ. The OP2177 (dual) has virtually no
distortion at lower voltages. It was compared to the OPA277 at
several supply voltages and various loads. Its performance exceeded
that of its counterpart by far.
SCALE – V
V
SY
= ⴞ15V
R
L
= 2k⍀
OP1177
SCALE – V
Figure 1. Gain Linearity
SCALE – V
NEED LABEL FOR THIS AXIS
VSY = ⴞ15V
RL = 2k⍀
OPA277
SCALE – V
Figure 2. Gain Linearity
Input Overvoltage Protection
When their input voltage exceeds the positive or negative supply
voltage, most amplifiers require external resistors to protect them
from damage.
The OP1177 has internal protective circuitry that allows volt-
ages as high as 2.5 V beyond the supplies to be applied at the
input of either terminal without any harmful effects.