Data Acquisition FAQ

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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 USBxCH or SERxCH FAQ pages

For a list of questions in other categories, return to the main FAQ Index page
If you still have questions after reading this page, Contact Us 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:

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 Download page.

 
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 hardware/software control.

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 A/D system.

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 useful signals.

 
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 converter.

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 removed.

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 A/D system.

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 +,-,AGND connections?

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 calibration.

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 RF noise.

 
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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