Approaching the Ideal Op Amp
Introducing Three Op Amp Families that combine low power, small size, and high
precision
By Brian Black, Product Marketing Manager, Signal Conditioning Products, Linear
Technology Corp.
Op amps are useful because their rules are simple and few. The basic non-inverting gain
equation VOUT=VIN*(1+Rf/Ri), for example, is about as simple a guiding rule as one will find in the
analog realm. This equation, however, is simple only because it makes certain assumptions
including:
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infinite input impedance
zero input current
zero offset voltage
zero noise voltage
zero noise current
In some applications, errors due to these unrealizable assumptions are inconsequential.
However, as the industrial world requires increasingly higher precision, there are many
applications that require amplifiers to push these limits closer and closer to the ideal. For
example, high impedance sensors and photodiode amplifiers are very sensitive to noise and input
bias currents. Low power current sense circuits benefit from ultra-low offset voltage and rail-to-rail
operation.
Although unachievable by definition, the ideal op amp provides a complete collection of
specifications to offer as targets for analog designers. Every application requires a different
combination of specifications and the number of op amps continues to expand to fit those needs.
Using novel techniques, these products get closer to the ideal than ever before, delivering
outstanding performance operating on lower voltages at reduced current consumption. Three
recently introduced amplifier families from Linear Technology illustrate this theme.
Low Supply Current and Low Voltage Operation
For battery-powered applications, size and power consumption are primary concerns. For
precision applications such as portable instrumentation, low input offset voltage and offset voltage
drift are also important. In these situations, there is no choice better than the LT6003 (single),
LT6004 (dual), and LT6005 (quad) micropower op amps from Linear Technology. This family of
amplifiers operates from 1.6V to 16V supply with only 1µA max supply current at 25°C and 1.6µA
max over the full industrial temperature range of –40°C to +85°C. The wide supply voltage range
allows the LT6003 family to work well even with unregulated power supplies such as two AA
batteries. The LT6003 single op amp in a 2mm x 2mm DFN package is the smallest available
and offers 290µV typical offset and excellent drift performance at 2µV/°C.
A challenge for many rail-to-rail op amps is that supply current may increase up to three times
when the outputs are operating near the rails, a condition that occurs during startup. If designers
do not consider this often-unspecified behavior, many micropower amplifiers may demand current
that exceeds the capability of the power supply, preventing a successful power-up sequence. The
LT6003 family employs special design techniques to behave gracefully over the entire operating
range for true micropower operation. Figure 1a shows the behavior of the LT6003 under the most
stressful power up conditions for a micropower op amp, shown in Figure 1b.
Figure 1. LT6003 family behaves well during power up sequences under the most stressful
conditions.
CMOS Op Amps Offer Low Bias Current
For precision amplifiers, bipolar processes have long been preferred over CMOS due to the lower
inherent noise associated with bipolar processes. On the other hand, CMOS processes are
attractive for their inherently lower input bias current (IB). This comes from the fact that bipolar
transistors are current-controlled devices, while MOS transistors are voltage controlled. Bias
current is important in applications with high impedance signal sources where IB can be the
largest single source of error in the signal chain.
Low Noise and Low Input Bias Current
To overcome the higher low-frequency noise associated with CMOS processes, transistor area
can be increased. But the increase in area also increases gate capacitance, which increases the
input capacitance of a CMOS op amp. Non-zero input capacitance implies that at higher
frequencies input impedance decreases. The noise gain of an op amp is governed by
Vout=Vnoise*(1+Zf/Zi), where Zi includes the input impedance of the amplifier as well as the discrete
input resistor. An amplifier with large input resistors may have low noise, but high input
capacitance creates high noise gain at higher frequencies. This only trades a 1/f noise reduction
for an increase in wide band noise content at the output.
The LTC6240/1/2 (single/dual/quad) has very large input devices to reduce the 1/f noise to only
550nVp-p, a figure that rivals a good, low-noise bipolar amplifier. Though one would expect large
input capacitance from such large input structures, the LTC6240 family utilizes an innovative
capacitance cancellation technique resulting in a total input capacitance of only 3.5pF, a third of
competitive CMOS op amps. Figure 2 compares the benefits of this design technique with other
low noise CMOS op amps that do not employ capacitance-canceling circuits. The great
combination of performance of the LTC6241 with noise comparable to good bipolar amplifiers and
1pA bias current substantially improves the performance of low noise, photodiode amplifiers and
high impedance sensor applications.
Figure 2. Input Capacitance Cancellation techniques enable the CMOS LTC6241 to offer
outstanding low frequency noise performance and maintain low noise gain.
The low noise CMOS amplifier family from Linear Technology extends to high frequency
applications with the introduction of the LTC6244. This 50MHz amplifier maintains 5.6pF total
input capacitance for low noise gain and offers outstanding DC input performance. This
combination of performance is important in many wideband sensor conditioning applications.
High impedance sensors such as SONAR receivers and LVDTs require low input bias current
and low noise.
Low Offset and Low Input Bias Current
The LTC6078/9 (dual/quad) employ innovative trimming circuits that yield VOS of only 25µV
maximum with VOS drift of 0.7µV/°C. This VOS performance is as good as some chopper-stabilized
amplifiers and the best bipolar amplifiers while offering 1pA max input bias current at 25°C that
only a CMOS amplifier can deliver. Combining these outstanding input specifications with
maximum supply current of 54uA per amplifier and 2.7V operation, the LTC6078 and LTC6079
extend the capabilities of power-sensitive systems.
Figure 3. LTC6078 low offset, low drift CMOS op amp
In addition to its application as a high accuracy signal conditioner for high-impedance sensors,
the LTC6078 provides significant benefits in handheld applications requiring current sense
capability. The circuit in Figure 4 shows a standard op amp current sense circuit. Selecting the
optimal value for a sense resistor can be a challenge. A larger sense resistor (Rs) dissipates
more power, something that designers of handheld instruments obviously want to avoid. A
smaller sense resistor, however, limits the resolution and accuracy of the current measurement
because the impact of op amp errors increases as the value of the sense resistor decreases. If
the op amp in Figure 4 had a VOS of 1mV the measurement error would be 0.1% with a 1Ω
resistor. Because of the low VOS of the LTC6078 the designer can reduce sense resistor power
dissipation by a factor of 40 by using a 25mΩ sense resistor and maintain the same system
precision.
Figure 4. The precision of the LTC6078 enables a small Rs value reducing power dissipation
without sacrificing DC accuracy.
Summary
While the ideal op amp may never exist, novel design techniques are enabling a new generation
of amplifier products that get closer to the ideal than ever before. These products allow system
designers to offer higher precision, lower power systems in a wide range of applications.