This page contains answers to frequently asked
questions about data acquisition in general
For questions about data acquisition using specific A/D products see the
SERxCH FAQ pages
For a list of questions in other categories,
return to the main
If you still have questions after reading this page,
and we'll be glad to help
Q: What do abbreviations like DAQ, DAS, A/D, ADC, D/A, DAC mean ?
There are a confusing number of abbreviations when it comes to data
acquisition equipment. Adding to the confusion there are no standards.
Usage varies from one manufacturer to another and even within friendly
conversation. Examples you might see are:
|DAQ||= Data Acquisition|
|DAS||= Data Acquisition System|
|A/D||= Analog to Digital|
|ADC||= Analog to Digital Converter|
|D/A||= Digital to Analog|
|DAC||= Digital to Analog Converter|
Does DAS stand for a D/A system, or is it a Data Acquisition system with
perhaps an A/D converter ? Typically, we will use the terms A/D and D/A, and
type out the word "system" when referring to a product.
Q: What is the difference between instrument and audio grade A/D converters ?
A/D converters can be broadly divided into two classes, instrument grade and
audio grade. It is important to pick a converter of the type that is
appropriate for your application.
Instrument grade converters seek to maintain linearity and minimize missing
codes. If you are monitoring a sensor such as a pressure transducer, weigh
scale, or thermocouple, you will probably want an instrument grade converter.
It converts data without the nonlinear transfer function of an audio grade
converter superimposed on the data. Instrument grade converters also
usually have response accuracy down to DC.
Audio grade converters seek to have characteristics appropriate for the human
ear. Since human hearing is not linear, nor does it have a DC response, audio
grade converters can sacrifice these parameters. Instead they emphasize low
cost and wide dynamic range even with missing codes.
Depending on your application either type of converter may be appropriate. SR
products feature instrument grade converters for industrial sensing. Read your
converter spec sheets carefully! You can find links to A/D converter spec
sheets on the Component Specs table on our
Q: Why is memory buffering important for A/D systems ?
Despite the popularity of Windows and Linux, they offer problems for real time
data acquisition. Host computer latencies as long as 1 second caused by task
switching, interrupts, and network activities are to be expected occasionally.
The result can be lost data.
At slow sampling rates, interfaces with the RS232 port are fairly forgiving.
COM ports maintain data buffers on the PC side making up for latencies.
However, at medium and high sampling rates, interfaces with the USB port are
not forgiving. Our USBxCH products are equipped with hardware FIFO buffers,
capable of storing many thousands of samples. This feature helps prevents
data loss even on heavily loaded computers.
Q: How do oversampling and signal averaging affect resolution ?
An A/D converter with a constant input voltage will never return exactly the
same count value on successive conversions. Even though the count should
theoretically be constant, in real life it is not. At best a distribution of
count values around a central value is returned.
This distribution is due to processes within the A/D chip itself. Even with
perfect board design, you will still get the distribution. It is a fact of
life for A/Ds converters of all types.
Can any rule of thumb be given? Well, of course you have to consult the spec
sheet for any particular part, but generally expect about 4 bits of uncertainty
in the count. Thus, for a 16 bit A/D converter this means there are only 12
reliable constant single sample bits! 16 bit systems without signal averaging
rarely return any more than 12-13 bit single sample resolution.
Higher resolutions can be obtained by oversampling and signal averaging. The
input is simply converted many times, and averaged to get a more reliable
central count value. The longer the averaging time, the more accurate the
result. This technique is used in one way or another in virtually every high
resolution A/D converter on the market.
Q: Why does sigma delta A/D resolution vary with sampling rate ?
A reasonable working model for how a sigma delta converter works is that it
oversamples its input and then signal averages down to the final sampling rate.
The extreme signal averaging of the technology results in its high resolution.
On most sigma delta converters, the input oversampling is done at a fixed rate.
The amount of time between output samples defines how much signal averaging can
be done. If output samples are requested at higher rates, then there is less
time for signal averaging and the resolution decreases.
We quote typical information for the tradeoff on our systems, but interested
users may also wish to consult the IC manufacturers' spec sheets. Most of the
IC manufacturers include detailed tables in their spec sheets listing the
sampling rate versus resolution trade off. Our systems closely follow those
results. Follow the links from our site to the respective IC data
sheets if you are interested in these specs.
Q: What is analog multiplexing ?
Multiplexed A/D systems have a single A/D converter and a front end switching
network allowing the A/D to be connected to any desired channel. In effect, it
is as if the test leads of a voltmeter were connected to various inputs under
The advantage of analog multiplexing is it provides a high channel count
at low cost and uses the minimum board space. Only a single (usually expensive)
A/D converter is required, and typically 8 differential channels can be
multiplexed per mux chip.
The disadvantage of multiplexing is the A/D will typically see widely
varying inputs as the system switches from one channel to another. For example,
if one channel has a DC +V and the other 0V, the A/D and buffering circuitry
will see a square wave as it switches between them. The system must settle
quickly to the new voltage or else crosstalk will occur. The irony for
a multiplexed system is even though the inputs are DC the downstream hardware
must have excellent AC characteristics to deliver good performance.
Systems like the SER8CH work to minimize settling time, by placing a high
performance op amp between the mux and A/D converter. For the
ultimate in low level crosstalk, consider the USBxCH series
with an "A/D converter per channel" design.
Q: What is the trouble with high impedance analog inputs ?
While many users need high impedance inputs (perhaps even in the 100+ Mohms
range), we have found it is not a good idea to put them directly on the
The reason is this: High impedance inputs run on any length of real world
cabling invite noise problems. Essentially high input impedance means
very small signal currents are flowing. These small currents have to
compete with every other noise source that may be present in your system. And
the higher the input impedance, the smaller the signal currents. It is much
better to run at a reasonable input impedance and keep the high impedance
interface up as close to the physical sensor as possible.
For users of active sensors this has already been done by the sensor
manufacturer. The words "active sensor" usually equate to a high input
impedance op amp being included directly with the sensor. The low impedance
output of the active sensor op amp results in a good match to our A/D inputs.
Our A/D systems generally have op amp buffers capable of 10M ohm input
impedance. We also have a resistor in parallel with the input to reduce that
impedance. The default build uses a 51K ohm value. Other impedances can be
supplied on request. However we recommend users, particularly those with
active sensors, stay with low impedance values whenever possible. It will
improve the noise floor.
Q: What is an active sensor ? What is a passive sensor ?
The analog world is filled with a huge variety of sensors. Yet one generic
classification that can be used is whether they are "passive" or "active".
An active sensor is easy to identify. It usually has some circuitry packaged
with it, has an op amp output, and requires some power in order to function.
The op amp output means the sensor can properly drive its analog signals
down long cables, has a known full scale response, and also has a high
impedance input up very close to the physical sensor mechanism where
sensitivity and signal to noise ratio will be best.
Passive sensors are usually simple raw physical devices, having no associated
circuitry, and requiring no power. Two examples would be thermocouples and
magnetic transducers such as geophones. These kinds of sensors have small
outputs and are easily influenced by their electrical environments. They
invariably require some sort of signal conditioning to turn their outputs into
Q: What are differential inputs ?
Differential inputs probably cause more confusion on the part of new analog
users than any other topic. In an ideal world without noise, most people would
be happier with single ended signaling. Yet in the real world, with noise,
differential signaling provides a significant improvement in noise immunity.
Enough improvement, so it is worth the effort to master it.
The basic idea is simple. Rather than have one wire whose signal value is its
voltage with respect to system ground, have two wires and let the signal be
the difference between the voltages on these two wires. If the two wires are
running through the same environment then any common noise impressed on both of
them will be removed when the difference is performed at the input of the A/D
Common mode noise can come from a variety of sources. It could be EMI
(electromagnetic interference) from 60Hz power lines or a nearby radio station.
It could also be ground loop noise from currents flowing on the system ground.
It can even be leakage currents on the cabling connecting the analog inputs. No
insulator is perfect. At high resolution it is easy to measure the errors
introduced by less than perfect cabling. As long as the signals are running
through similar cable and are differenced, most of the common mode effects are
So, the theory is nice. But troubles set in when theory is put into practice.
First, keep in mind the A/D electronics can only difference the signals
within limits. If the absolute voltage on one of the differential inputs
becomes too large it will pin the input circuitry on that wire and the A/D and
differencing stops. You cannot put 100v on the + wire and 99v
on the - wire and expect the A/D to difference them with a 1v result. Both the
+ and - inputs of the differential pair must be within the
specified allowed input range with respect to the analog ground of the
If a few basic rules are followed, differential signaling is a powerful and
worthwhile noise reduction technique. The next few FAQs cover additional tips
on differential signaling.
Q: What are balanced differential inputs ?
A balanced differential input always drives the + and - input pins on a channel
to be exactly opposite in voltage. So for example, if the + pin is at +1 volt,
then the - pin is at -1 volt. One immediate advantage of this arrangement is
the number of electrons flowing in on the + pin are exactly matched by the same
number flowing out on the - pin. There is no current forced through the analog
ground system. The signal is nicely confined to the +,- input pins and
additionally isolated from common mode noise on the ground system.
But there is another important advantage. Suppose you have +/-10 volt analog
inputs each driven by op amps powered with +/-10 power supplies ( and we ignore
op amp headroom for this example ). The A/D will go to positive full counts
when the (+,-) inputs are at (+10,-10) volts, a difference of +20 volts. And
to negative full counts at (-10,+10) volts, a difference of -20 volts. You have a
total voltage span of 40 volts between max and min full counts. This span of
40 volts has been achieved with only +/-10 supplies (!) and it gives additional
signal to noise ratio, essentially for free. A big advantage, pushing your
signal up further from the noise floor by at least another A/D bit.
Many active sensors automatically have this type of balanced signaling with
equal and opposite voltages on the +,- wires. It is also not hard to set up if
you are constructing your own amplifiers. It can be as simple as having a non
inverting buffer on the + signal and an inverting buffer on the - signal, both
driven from the same single ended sensor output.
Q: My sensor only has two pins, what do I do for differential AGND ?
Suppose you have a passive sensor with only two output pins and no pin marked
analog ground. How should this be connected to a differential A/D input with
You might think you could connect the two pins from the sensor to the
differential +,- inputs and not make any connection to the A/D AGND. But, this
will cause trouble. Differential inputs must be referenced to the A/D AGND, and
you have no guarantees the sensor's pins are within +/-10v of the A/D
AGND. In fact, many sensors will float around if not referenced to AGND,
creating the confusing situation where things appear to work for awhile and then
stop after the sensor has floated to the rails of the A/D.
A proper connection with a two pin sensor connects a 100K ohm resistor from the
+ terminal of the sensor to AGND and another 100K ohm resistor from the -
sensor terminal also to AGND. Do this along with connecting to the +,- A/D
input pins. The 100K resistors make sure the sensor stays referenced to the
AGND of the A/D converter, and using a balanced pair makes sure the common
mode rejection of the A/D is not compromised.
Some SR A/D products have these resistors included in their front end signal
conditioning. See the circuit diagrams for each product.
Q: How do I calibrate the A/D converters ?
Even though an input range may be quoted for an A/D converter, it is only
approximate. Input signal conditioning always involves resistors, capacitors,
and op amps, each with their own tolerances and imperfections. Precise
counts/volt calibration always requires a per system and per channel
The SR software usually includes a calibrating program called "CAL".
Refer to that program for specific calibration steps.
Q: Is twisted pair good for analog inputs ?
Yes! Twised pair can significantly help prevent 50/60Hz and other noise from
ever even entering the system in the first place. The open loops created by
loose input cabling can easily pick up power line noise by magnetic coupling of
flux lines. Twisted pairs will reduce this coupling significantly from all
such noice sources.
Foil and braid shielding does not do much good against 60/50Hz noise. It is
the "twist" in twisted pair that is effective. However, foil and braid
shielding does significantly help protect against static discharge damage and
Q: How much crosstalk is there between channels on your A/D products ?
Crosstalk is a common performance spec to worry about. You don't want a valid
signal on one channel to be erroneously corrupting and appearing on other
channels even at a small level.
Of course how much crosstalk a system has depends on the system architecture.
Our 24 bit products with an individual A/D per channel basically have crosstalk
that cannot be measured. The channel to channel isolation is so complete
it doesn't exist. Undeniably, multiplexed systems, are more susceptible to
crosstalk. Consult the spec sheets for the numbers.
Measuring crosstalk is simple. First, set up a full scale signal ( often a
sine wave ) and short out the remaining channels. Then use a program like
SCOPE and crank up the bit resolution being displayed. When the shorted traces
start to show signs of the sine wave signal, you're seeing crosstalk. The bit
resolution at which crosstalk starts to appear is the same as the ratio between
the signal generator amplitude and the signal on the quiet channel. If you are
doing this experiment, be sure to adhere to the following:
1) Do not drive the input sine wave beyond the maximum input amplitude of the
system. If the amplitude is too great, input amplifiers may saturate and you
won't get valid results.
2) Drive all inputs with low impedance sources or short them. On multiplexed
systems this is critical. Charge from a signal channel cannot be drained by a floating
channel when the multiplexer switches and will appear as a fake signal. Short
unused channels and use low impedance inputs!
Q: What is the format of your data files ?
Our products use a variety of file ( and pipeline data ) formats, both binary
and ASCII. All have one common unifying theme. Data is presented
in rows and columns. Each row represents the samples taken across all channels
at a particular time point, a database "record". The columns are the record
"fields" with the values from the analog channels and parameters like time
stamps and system temperature also recorded at that time point.
Refer to the documentation with a particular product for details on the
formats. Most customers will find the ASCII formats particularly easy to use
as they are readable in any text editor and easily imported into downstream
programs like spreadsheets.
INI parameters in the acquisition programs and utilities specify details about
the various formats.
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