How To Chose The Right Op Amp
There are two categories of requirements that relate to the performance of an op amp: General and
Application Specific.
General requirements are the degree of mismatch between pairs of circuit elements within an op
amp IC. The best matched op amp will have an offset of zero between all its inputs.
Application Specific requirements are tradeoffs. For example, an op amp with the best input noise
performance has high operating quiescent current. An op amp with the lowest operating quiescent
current has the worse input noise.
Like the laws of quantum reality, the more control you exercise over one aspect of your life the less
control you have over everything else. If you overspecify the General requirements, you restrict the
potential performance of the Application Specific requirements.
The best way to select an op amp is to start by defining the minimum acceptable General
requirements for a circuit application.
Op amps with the best General requirements have the lowest yields and/or require postmanufacture
trimming.
General Category:
Input Offset Voltage
The voltage that must be applied directly between the input measuring terminals, with bias current
supplied by a resistance path, to reduce the output indication to zero. Or the voltage that must be
applied between the two input terminals of an operational amplifier to obtain zero output voltage.
Input Offset Voltage Match Dual or Quad Amps
Offset voltage will tend to introduce slight errors in any opamp circuit. So how do we compensate
for it? Unlike commonmode gain, there are usually provisions made by the manufacturer to trim the
offset of a packaged opamp. Usually, two extra terminals on the op amp package are reserved
for connecting an external "trim" potentiometer. These connection points are labeled offset
null.
Input Offset Voltage Drift
Just because you trimmed out the offset voltage, doesn't mean all is tranquil on paradise island. The
input offset voltage will drift with temperature; you have no control over this. But, knowing your
overall error budget, you can select an op amp with a low enough offset drift for the intended
temperature range.
Voltage Stability
The most stable circuits have the longest response times, lowest bandwidth, highest accuracy, and
least overshoot. The least stable circuits have the fastest response times, highest bandwidth, lowest
accuracy, and some overshoot.
Input Offset Voltage Shift Rail to Rail Amplifiers
The effect of reducing the power supply voltage is a shift in the amplifier's input offset voltage.
The
problem stems from the fact that most conventional op amps, with a typical operating range down
to +4.5 volts, are generally tested, and have their input offset trimmed, at a specific supply voltage,
e.g., +15 volts. Reducing the supply voltage can produce a shift in the input offset voltage. The
shift in the offset voltage can be determined by looking at the
"powersupply rejection ratio" (PSRR), or "powersupply sensitivity" of offset voltage
specification;
it provides a measure of the change in offset for a given change in supply voltage.
Input Offset Current
The difference between the two currents that must be supplied to the input measuring terminals of a
differential instrument to reduce the output indication to zero (with zero input voltage and zero offset
voltage). Or the difference in the currents into the two input terminals of an operational amplifier
when the output is at zero.
Common Mode Rejection Ratio
Common Mode Rejection Ratio Match Dual or Quad Amps
A measure of an instrument's ability to reject an undesired signal that is common to both input
terminals.
Also, CMRR describes the ability of a differential amplifier to reject interfering signals common to
both inputs, and to amplify only the difference between the inputs.
Power Supply Rejection Ratio
Power Supply Rejection Ratio Match Dual or Quad Amps
This specification is the measure of how well the op amp rejects an AC signal riding on a nominal
input DC voltage.
Powersupply rejection ratio is at a maximum at low frequencies, and begins to fall above 1kHz to
10kHz, depending upon the op amp design.
The only way to modify the basic rejection response of the regulator is to add an external network
at the input of the op amp.
There are three methods to choose from:
1. One or more cascades of external RC filters. The additional attenuation adds to the inherent
characteristic of the regulator. Typical values for the single RC and cascade RC filters range from
R=1 to 10, and C=100µF to 10µF respectively. Choose the network 3dB frequency to coincide
with that of the regulator PSRR characteristic.
2. An LC filter. The problem with using this type of filter is the lack of inherent damping at the
output of the network (input of the op amp). The source impedance of the network is low.
However, the op amp's VIN terminal presents a high impedance shunted with a small capacitor.
(When the op amp is operated away from dropout with a constant load, its input current does not
vary, to first order, when VIN is varied.) It is impossible to critically damp the LC network at the
input of the regulator without significant DC loss in the shunt damping resistor. For example, a series
10µH inductor in combination with a shunt 100µF capacitor exhibits a turnover frequency of
5kHz;
this combination requires a critical damping resistor of 0.32ohms between network output (op amp
input) and ground.
3. An additional linear regulator. This method occupies a small PCB area and requires the least
design time of the other methods. Using two linear regulators in series doubles the PSRR at any
given frequency (assuming identical regulators). The "penalty" for this approach is the doubling
of
the dropout voltage and the need for an additional capacitor. A good design choice is to share the
voltage drop across each op amp. The total assembly requires three capacitors, one each at the
input, output, and the intermediate position.
Application Specific:
Input Bias Current
Opamp inputs usually conduct very small currents, called bias currents, needed to properly bias
the first transistor amplifier stage internal to the opamps' circuitry. Bias currents are small (in
the
microamp range), but large enough to cause problems in some applications.
Bias currents in both inputs must have paths to flow to either one of the power supply "rails"
or to
ground. It is not enough to just have a conductive path from one input to the other.
To cancel any offset voltages caused by bias current flowing through resistances, just add an
equivalent resistance in series with the other opamp input (called a compensating resistor). This
corrective measure is based on the assumption that the two input bias currents will be equal.
Any inequality between bias currents in an opamp constitutes what is called an input offset current.
It is essential for proper opamp operation that there be a ground reference on some terminal of the
power supply, to form complete paths for bias currents, feedback current(s), and load current.
Input Bias Current Shift Also General, Rail to Rail Amplifiers
As the input signal moves from one supply rail to the other, the amplifier shifts from one input pair
to
the other. At the crossover point, this shift can cause changes in the input bias current and offset
voltage that affect both the magnitude and the polarity of these parameters. These offsetvoltage
changes typically worsen the distortion performance and precision specifications of railtorail
amplifiers (in comparison with groundsensing types). To minimize offsetvoltage shifts and smooth
the transition from one input pair to another, Manufactures trims the offset of its railtorail
amplifiers at both the high and the low ends of the commonmode range.
To reduce offset voltages caused by input bias currents, the designer should match impedances at
the op amp's inverting and noninverting nodes. Because input bias currents are typically larger than
input offset currents, this impedance matching is good practice for all types of op amps, not just railtorail
input amplifiers.
Output High Voltage
Output Low Voltage
The output of an op amp is required to swing railtorail or as close as possible. The output swing
is
dependent on the amount of output loading.
The inputtooutput linearity will degrade as the output swing approaches the voltage supply.
If the output is pushed to the rails, it will change the offset voltage of the op amp.
The output swing of the op amp can vary between +V and V supply, and is controlled by the
voltage difference between +In and In. If the voltage at +In is positive with respect to In, then
the
output of the op amp swings positive, toward the +V rail voltage. If the voltage at +In is negative
compared to the voltage at In, the output swings negative. It only takes a small difference in
voltage between the two inputs to create a large change in the output voltage. This is known as the
gain of the op amp.
Open Loop Voltage Gain
This is ratio of the input to the output voltage with no feedback applied. It is the D.C. gain of the
amplifier or the gain at a frequency of 1 Hz. It is dependent on the output voltage swing and output
load.
Common Mode Input Range
The range of input voltage for which the differential pairs behaves as a linear op amp. The upper
limit is determined by one of the two inputs saturating. The lower limit is determined by the input
bias current.
Input Noise Voltage
Input Noise Voltage Density
Input Current Noise
Input Current Noise Density
Op amp input noise specifications are usually given in terms of nV/H.z for noise voltage and pA/H.z
or fA/H.z for noise current and are therefore directly comparable with resistor thermal noise. Due to
the fact that noise density varies at low frequencies, most op amps also specify a typical peaktopeak
noise within a “0.1Hz to 10Hz or 0.01Hz to 1Hz bandwidth.
There are five types of noise in op amps and associated circuitry:
1) Shot noise
2) Thermal noise
3) Flicker noise
4) Burst noise
5) Avalanche noise
Some or all of these noises may be present in a design, presenting a noise spectrum unique to the
system. It is not possible in most cases to separate the effects, but knowing general causes may help
the designer optimize the design, minimizing noise in a particular bandwidth of interest. Proper
design for low noise may involve a “balancing act” between these sources of noise and external
noise sources.
Shot Noise
The name shot noise is short for Schottky noise. Sometimes it is referred to as quantum noise.
It
is caused by random fluctuations in the motion of charge carriers in a conductor.
Put another way, current flow is not a continuous effect. Current flow is electrons, charged particles
that move in accordance with an applied potential. When the electrons encounter a barrier, potential
energy builds until they have enough energy to cross that barrier. When they have enough potential
energy, it is abruptly transformed into kinetic energy as they cross the barrier. A good analogy is
stress in an earthquake fault that is suddenly released as an
earthquake.
As each electron randomly crosses a potential barrier, such as a pn junction in a semiconductor,
energy is stored and released as the electron encounters and then shoots across the barrier. Each
electron contributes a little pop
as its stored energy is released when it crosses the barrier.
Thermal Noise
Thermal noise is sometimes referred to as Johnson noise after its discoverer. It is generated by
thermal agitation of electrons in a conductor. Simply put, as a conductor is heated, it will become
noisy. Electrons are never at rest; they are always in motion. Heat disrupts the electrons’ response
to an applied potential. It adds a random component to their motion.
Flicker Noise
Flicker noise is also called 1/f noise. Its origin is one of the oldest unsolved problems in
physics. It
is pervasive in nature and in many human endeavors. It is present in all active and many passive
devices. It may be related to imperfections in crystalline structure of semiconductors, as better
processing can reduce it.
Flicker noise is found in carbon composition resistors, where it is often referred to as excess noise
because it appears in addition to the thermal noise that is there. Other types of resistors also exhibit
flicker noise to varying degrees, with wire wound showing the least. Since flicker noise is
proportional to the dc current in the device, if the current is kept low enough, thermal noise will
predominate and the type of resistor used will not change the noise in the circuit.
Reducing power consumption in an op amp circuit by scaling up resistors may reduce the 1/f noise,
at the expense of increased thermal noise.
Burst Noise
Burst noise, also called popcorn noise, is related to imperfections in semiconductor material and
heavy ion implants. It is characterized by discrete highfrequency pulses. The pulse rates may vary,
but the amplitudes remain constant at several times the thermal noise amplitude. Burst noise makes
a popping sound at rates below 100 Hz when played through a speaker — it sounds like popcorn
popping, hence the name. Low burst noise is achieved by using clean device processing, and
therefore is beyond the control of the designer. Modern processing techniques at Texas Instruments
has all but eliminated its occurrence.
Avalanche Noise
Avalanche noise is created when a pn junction is operated in the reverse breakdown mode. Under
the influence of a strong reverse electric field within the junction’s depletion region, electrons
have
enough kinetic energy that, when they collide with the atoms of the crystal lattice, additional
electronhole pairs are formed (Figure 10–4). These collisions are purely random and produce
random current pulses similar to shot noise, but much more intense.
When electrons and holes in the depletion region of a reversedbiased junction acquire enough
energy to cause the avalanche effect, a random series of large noise spikes will be generated. The
magnitude of the noise is difficult to predict due to its dependence on the materials. Because the
zener breakdown in a pn junction causes avalanche noise, it is an issue with op amp designs that
include zener diodes. The best way of eliminating avalanche noise is to redesign a circuit to use no
zener diodes.
Color Noise
While the noise types are interesting, real op amp noise will appear as the summation of some or all
of them. The various noise types themselves will be difficult to separate. Fortunately, there is an
alternative way to describe noise, which is called color. The colors of noise come from rough
analogies to light, and refer to the frequency content. Many colors are used to describe noise, some
of them having a relationship to the real world, and some of them more attuned to the field of
psychoacoustics.
White noise is in the middle of a spectrum that runs from purple to blue to white to pink and
red/brown. These colors correspond to powers of the frequency to which their spectrum is
proportional, as shown in the table below.
Color Frequency Content
Purple = frequencies squared
Blue = frequencies
White = 1
Pink = 1 / frequencies
Red Brown = 1 / frequencies squared
Gain Bandwidth Product
To determine the maximum gain that can be extracted from the circuit for a given frequency (or
bandwidth) and viceversa.
For example, if the GBW of an opamp is 1 MHz, it means that the gain of the device falls to unity
at 1MHz. Hence when the device is wired for unity gain it will work up to 1MHz (GBW product =
gain x bandwidth, therefore if BW = 1 MHz, gain = 1) without excessively distorting the signal. The
same device when wired for a gain of 10 will work only up to 100 kHz, in accordance with the
GBW product formula. Further, if the maximum frequency of operation is 1 Hz, then the maximum
gain that can be extracted from the device is 1 x 10 to the 6 power.
Slew Rate
This is the rate of change of the op amp's voltage output over time when its gain is set to unity (Gain
=1).
3dB Bandwidth
3dB bandwidth is defined as the frequency at which the signal intensity (or gain) has fallen to 0.5
of
the zero frequency value.
The bandwidth is the difference between the upper 3dB frequency and lower 3dB frequency of a
bandpass filter (High F3dB – Low F3dB).
Phase Margin
The phase margin is the difference between the phase of the response and 180° when the loop
gain is 1.0. The frequency at which the magnitude is 1.0 is called the unitygain frequency or
crossover frequency. It is generally found that gain margins of three or more combined with phase
margins between 30 and 60 degrees result in reasonable tradeoffs between bandwidth and
stability. The phase margin is the additional phase required to bring the phase of the loop gain to
180 degrees.
The system is unstable when the loop gain equals 1. That is, gain has a magnitude of one and a
phase of 180 degrees. An unstable system oscillates. A system close to being unstable has a large
ringing overshoot in response to a step input.
Full Power Bandwidth
Full Power Bandwidth (FPBW) is the frequency at which the reconstructed output fundamental
drops 3 dB below its low frequency value for a full scale input.
Total Harmonic Distortion + Noise
THD is simply a measurement of how much RMS harmonic current is present in comparison to the
amount of fundamental current. THD does NOT take into account frequency. It is the RMS value
of ONLY the harmonics divided by the RMS value of the fundamental and is expressed in percent.
The RMS value of a sine wave regardless of frequency is simply the peak value of the sine wave
divided by the square root of 2. (V means voltage)
Linear loads on a power system draw a load current that is a pure sine wave at the power
frequency of 60Hz. Most everyone has seen the typical sine wave of 60Hz voltage that can be
displayed on an oscilloscope. A linear load draws a current waveform of the same shape.
When a non linear load draws current from the power system (i.e. from the transformer), the
current waveform is distorted from the mathematically perfect shape of a sine wave. A square wave
or saw tooth waveform is an example of a distorted waveform.
The distorted waveform is actually a summation of the fundamental frequency sine wave and a
variety of harmonics. Harmonics are actually pure sine waves themselves but each has a frequency
that oscillates at a multiple of 60Hz (i.e. 3rd harmonic = 3 x 60 = 180Hz, 5th harmonic = 5 x 60 =
300Hz).
To find the total RMS value of any distorted wave, you have to take "the square root of the sum
of
the squares" of the RMS value of the fundamental and the series of harmonics. (admittedly it's
easier
to see this demonstrated than to explain it!!)
In this example the fundamental current is 10 amps. The harmonic RMS value is 5 amps.
5 / 10 x 100% = 0.5 x 100% = 50%
Therefore the THD is 50% regardless of the harmonic frequency.
In truth, THD is limited in its application. It is really only good for expressing harmonic limits on
a
feeder or that is produced by a load in a general way. It doesn't tell you which harmonics are
present and how much of each harmonic (i.e. their RMS values) is present.
For that you need to perform a Harmonic Spectrum Analysis which is easily done with clamp on
meters that are readily available. (Fluke has some hand held models that cost around $1500) These
meters will tell you which harmonics are present, their respective RMS values, and even the phase
angle between the harmonic and the fundamental. (The phase angle is seldom used in any
calculation or harmonic study.)
You must have a Spectrum Analysis performed at different points on your power system (such as at
a non linear load, at the transformer, at capacitor banks, at the panelboard, etc.) before any solution
to any harmonic problem can be undertaken.
Once the analysis is complete, and if the harmonics are causing you trouble (such as overheating
transformers, blowing cap bank fuses, tripping breakers, causing telephone interference, etc.), the
information can be used to apply harmonic filters, to size K rated transformers, or to install
harmonic cancellation equipment.
Settling Time
This is the length of time for the output voltage of an operational amplifier to approach, and remain
within, a certain tolerance of its final value. This is usually specified for a fast fullscale input
step.
Operating Quiescent Current
Quiescent current is the current that flows in an electrical circuit when no load is present.
Differential Gain
The differential gain is the ratio of the difference of the outputs over the difference of the inputs.
In video it is the amount of change in the color saturation (amplitude of the color modulation) for
a
change in lowfrequency luma (brightness) amplitude.
In fiber optics it's a type of distortion in a video signal that causes the brightness information to
be
incorrectly interpreted.
Differential Phase
In video it is the change in hue (phase of the color modulation) for a change in lowfrequency luma
(brightness) amplitude.
