OP1177_07 Datasheet, PDF (14/24 Pages) Analog Devices – Precision Low Noise, Low Input Bias Current Operational Amplifiers

OP1177/OP2177/OP4177
FUNCTIONAL DESCRIPTION
The OPx177 series is the fourth generation of Analog Devices,
Inc., industry-standard OP07 amplifier family. OPx177 is a high
precision, low noise operational amplifier with a 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.
Analog Devices proprietary process technology and linear design
expertise has produced a high voltage amplifier with superior
performance to the OP07, OP77, and OP177 in a tiny MSOP
8­lead package. Despite its small size, the OPx177 offers numerous
improvements, including low wideband noise, very wide input
and output voltage range, lower input bias current, and complete
freedom from phase inversion.
OPx177 has a specified operating temperature range as wide as
any similar device in a plastic surface-mount package. This is
increasingly important as PCB 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, and
bandwidth and slew rate increase by a factor of two. The low
power dissipation and very stable performance vs. temperature
also act to reduce warmup drift errors to insignificant levels.
Open-loop gain linearity under heavy loads is superior to compet-
itive parts, such as the 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 PCB guidelines.
The following sections provide practical advice on getting the
most out of the OPx177 under a variety of application conditions.
TOTAL NOISE-INCLUDING SOURCE RESISTORS
The low input current noise and input bias current of the OPx177
make it useful for circuits with substantial input source resistance.
Input offset voltage increases by less than 1 μV maximum per
500 Ω of source resistance.
The total noise density of the OPx177 is
( ) e_n,TOTAL = e_n² + i_nR_S ² + 4kTR_S
where:
en is the input voltage noise density.
in is the input current noise density.
RS is the source resistance at the noninverting terminal.
k is Boltzmann’s constant (1.38 × 10^−23 J/K).
T is the ambient temperature in Kelvin (T = 273 + temperature
in degrees Celsius).
For RS < 3.9 kΩ, en dominates and
en,TOTAL ≈ en
For 3.9 kΩ < RS < 412 kΩ, voltage noise of the amplifier, the
current noise of the amplifier translated through the source
resistor, and the thermal noise from the source resistor all
contribute to the total noise.
For RS > 412 kΩ, the current noise dominates and
en,TOTAL ≈ inRS
The total equivalent rms noise over a specific bandwidth is
expressed as
( ) e _n = e _{n , TOTAL}
BW
where BW is the bandwidth in hertz.
The preceding 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 the Band-Pass
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. This is especially true for circuits with high
closed-loop gains.
The OP1177 has excellent gain linearity even with heavy loads,
as shown in Figure 51. Compare its performance to the OPA277,
shown in Figure 52. Both devices are measured under identical
conditions, with RL = 2 kΩ. The OP2177 (dual) has virtually no
distortion at lower voltages. Compared to the OPA277 at several
supply voltages and various loads, OP1177 performance far
exceeds that of its counterpart.
V_SY = ±15V
R_L = 2kΩ
OP1177
(5V/DIV)
Figure 51. Gain Linearity
Rev. E | Page 14 of 24