Why are there no results shown?
The simple answer is that all candidates have been eliminated by the
applied filtering. If you're here looking for help, then you probably
want to know what to do to get some results. If any of the filter
fields have a cyan background, then that field is the one eliminating
all the candidates.
The screenshot shows that the user has asked for a filter to show only opamps
with a CMRR of 150 dB or more. As none of the opamps meets this
criterion, all candidates are filtered out. If the maximum value
is less than the minimum value, the field will also have cyan
background.
If all candidates are eliminated through a combination of the filters
applied, the simplest solution is to undo the last change. Some
permutation of characteristics are difficult to achieve simultaneously,
such as:
High bandwidth and low current consumption
Low voltage noise and low current consumption
Low minimum supply voltage and low voltage noise
What do the different colours of text boxes mean? A white background represents valid fields. An orange background represents an invalid field. A cyan background represents a field whose value eliminates all the candidates.
What do I need to run the chooser?
The opamp chooser is a Java application, and depends on your computer
having a Java Runtime Environment (JRE) installed. Java is available
for Windows, Macs and Linux operating systems. The online applet
version should run on Microsoft Internet Explorer 6 and 7, Mozilla
Firefox, and other Java compatible web browsers.
How do I get Java?
Go to http://java.sun.com and download and install the latest JRE runtime environment.
What computers and operating system will it run on?
Java is available for Windows, Macs and Linux operating systems. The
application was developed on a Windows machine, so testing on other
platforms is less extensive.
What do the letters in the text boxes mean?
The chooser application displays and accepts the SI prefixes for powers
of 1000. This allows values to be entered concisely over a vast range
of values. The SI prefixes are:
P
Peta - 1015
T
Tera - 1012
G
Giga - 109
M
Mega - 106 NB upper case
k
kilo - 103
m
milli - 10-3 NB lower case
u
micro - 10-6
n
nano - 10-9
p
pico - 10-12
f
femto - 10-15
a
atto - 10-18
Note that the prefixes are case sensitive.
Why isn't the slew rate shown in volts per microsecond (V/uS)?
The chooser represents scalar characteristics in SI units, and for slew
rate this is V/sec. MV/sec are the same as V/usec, but very fast opamps
reach slew rates of 1000s of V/usec. The chooser application
shows them as GV/sec, rather than the distinctly ungainly kV/usec.
How accurate are the numbers?
The chooser application uses single precision floating point numbers to
represent the scalar quantities. Floating point numbers can represent
very small numbers and very large numbers to at least 7 decimal places.
This is far more accurate than the data quoted by the manufacturers,
which is rarely accurate to more than 2 significant figures. The use of
floating point numbers can lead to rounding errors, which may affect
the results of comparisons.
Why are obsolete parts included?
One of the useful things you can do with the chooser program is to find suitable candidates to replace obsolete parts.
Why aren't device grouped into categories?
Categories are more of a hindrance than a help. If you are looking for
a video amplifier, then you can restrict yourself to a manufacturer's
list of video amplifiers, but if you want a low noise amplifier that
runs on a low supply voltage, then you don't know whether to look in
the low voltage category or the low noise category.
Why are typical values listed? The use of typical values instead of worst case values has its
pros and cons. A good design will work over the full range of device
characteristics, but the contribution of a device characteristic to the
system behaviour is dependent on many factors. As an example, in a
system which must work over a large temperature range, the worst case
offset voltage will have a contribution from the basic offset voltage
and the offset voltage drift multiplied by the temperature range. In a
choice between two devices, the drift contribution may outweigh a low
initial offset voltage, or it may not. This gap between device
characteristics and system characteristics is the main reason why the
chooser can only recommend a list of candidate devices, rather than a
concrete solution. In many ways typical values are a more useful
representation of the devices, and manufacturers generally 'play by the
rules' and list realistic values. Why are there devices with identical characteristics? Manufacturers sometimes grade devices during test into premium
and standard parts. This is then reflected in the specificatons, and
the price! Although the difference in specification usually shows up in
the columns, this isn't allways the case. As an example, sometimes the
specification of the maximum offset voltage is improved, but the
typical offset voltage specification may be unchanged.
Are duplicates allowed?
Yes. Different authors may interpret data sheets differently, or you
may want multiple entries if the opamp characteristics vary
significantly with conditions. As an example some opamps have a bias
control, which allows the user to trade off speed against power
consumption.
Why doesn't the datasheet hyperlink button work? The hyperlink buttons attempt to open a webpage in a new browser
window. If you have a pop-up blocker installed, you must configure it
to allow pop-ups from this site.
Why is the bias current of the non-inverting opamp used? The inverting and non-inverting input of many opamps is
identical, and the input bias current of the two inputs will therefore
also be the same. In contrast, some devices like current feedback opamps have a high
impedance non-inverting input and a low impedance inverting input. In
practice, current feedback opamps need a relatively low feedback
resistor to work properly, and cannot be used in configurations which
require a low bias current on the inverting input. Voltage feedback and
current feedback opamps can however be compared realistically when
looking at the bias current of the non-inverting input
What do pinout entries like 's8b1p1' mean? The pinout entries for the basic input, output and supply pins
are coded up to make them easier to use. In the example, s8 means that
it's a single device with 8 pins. The b1 means that the basic input and
output pins conform to type 1 and the p1 means that the power pins
conform to type 1. The full description of all the pinout codes is on
the Opamp pinout page.
