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Analog InputsThe dB4 module supports four ana-log input channels and a tachometerinput. Gains of 1 and 10 are supportedfor effective input ranges of +/- 10V and+ /- 1V. The module uses 24-bit Delta-Sigma analog-to-digital converters(ADCs) that provide anti-aliasing filtersbased on the clock rate. These filterseliminate aliasing, which is a condition

where high frequency input componentserroneously appear as lower frequenciesafter sampling.

You can read data from one or moreanalog input channels using an analoginput channel list. You can enter up to5 entries in the channel list, includingfour analog input channels and thetachometer input.

Analog Inputs with IEPE FunctionsApplications requiring accelerometers,vibration, noise, and sonar measure-ments often use IEPE sensors. ThedB4 module supports the following

software programmable IEPE functionsfor each of its four analog inputs:

Current source - Enable or disable

the use of a 4mA current source to

drive the IEPE sensors.

AC/DC coupling - Select whether AC

coupling or DC coupling is used.

Delta-Sigma A/D ConvertersThese modules have built-in anti-alias-ing filters for superior AC performancein noise and vibration testing applica-tions.

Internally, the dB4 module uses aDelta-Sigma converter for each analog

input. Delta-Sigma converters offer thefollowing advantages for analog inputoperations, making them ideal for noiseand vibration testing applications.

Reduce noise and improve accuracy

by oversampling each input.

Eliminate errors that result from

aliasing and high frequency noise.

Provide excellent low-level signal-to-

noise performance, which improves

dynamic accuracy on low-level sig-

nals.

Provide excellent differential linearity,

which ensures consistently accurate

data conversion across the full input

range.

Group Delay and Data

SynchronizationBecause of the inherent filtering algo-rithms, Delta-Sigma converters have aninitial delay of 38 clock pulses after thesample clock is first started and beforethe first conversion is completed, due tothe group delay of converters.

The tachometer data (which does nothave the 38 sample group delay) is syn-chronized with the analog data stream.This is done through the firmware anddevice driver by caching the tachometerdata and aligning it in time with theanalog data in the users data buffers.

Tachometer InputThe dB4 includes support for a

tachometer input in the analog inputdata stream for synchronous measure-ments. The measurement edge for thetachometer input is rising. The moduleaccepts one 30V, 31-bit tachometerinput signal with a maximum frequencyof 380kHz and a minimum pulse widthof 1.3 microseconds. The threshold volt-age is fixed at 2V with 0.5V of hystere-sis. Measurements are based on twoconsecutive rising edges of the tachome-ter input signals.

The tachometer input is treated likeany other channel in the analog inputchannel list; therefore, all the clocking,triggering, and conversion modes sup-ported for analog input channels aresupported for the tachometer input.

Waveform Quality Analog OutputThe dB4 supports one 24-bit D/Aconverter with an output range of+/- 10V. You can output a single valuefrom the analog output channel, or youcan use a software trigger to start awaveform operation. A two-pole,10kHz Butterworth filter is applied toremove clocking noise and smooth sig-nal output.

A standard waveform such as a sine,triangle, or square wave is easily pro-

duced by loading the onboard FIFO with8,192 samples. The specific waveform isoutput at a fixed rate of 46.875 kHz.Great care has been used in design tominimize the glitch energy for any majoror minor carry. This results in extreme-ly smooth waveforms.

You can update the analog outputchannel as you are acquiring analoginput data for gap-free simultaneousstimulus and response. Note that sincethe module uses a Delta-Sigma D/Aconverter, 34 clock pulses are requiredbefore the first D/A conversion is com-plete.

Analog In

Analog In

Simultaneous A/DUp to 52.734 kHz per Channel

4 Delta-Sigma

Converters

24-bitA/D

24-bitA/D

TachometerInput

Figure4. Simultaneous Delta-Sigma A/D converters and thetachometer input are synchronizedthrough software to accommodatefor the group delay of the A/D

converters.

Wide Band Pass

0.5Hz 25.8kHz

-3dB@ .49 x SamplingFrequency(Eliminate

Nyquist Aliases)

FREQUENCY

- 3dB

0dB

AMPLITUDE

- 3dB single pole- 6dB per octave rolloff

Near 0 Ripple in Pass Band

(Linear Phase)

-100dB @.55 xSampling

Frequency

Built-In DigitalFilter ProvidesBrick Wall

Roll Off

Figure5. dB4 provides software selectable AC and DC coupling. WhenAC coupling is selected, thedB4 provides zero ripple in the wide passband and excellent brick wall anti-alias filter eliminates unwantedhigh frequency interference.

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USB 2.0 CompatibilityThe dB4 module uses a high-speedUSB 2.0 interface, which provides trans-fer rates between the module and the hostat up to 480 Mbits/s. This means that allacquired signals stream to and from thehost at full acquisition speeds. The dB4can also be used with USB 1.1

ports, but at USB 1.1 performance (12Mbits/s).

Easy Signal ConnectionsThe dB4 provides BNCs for the ana-log input, tachometer input, analog out-put, and external trigger signals.

EMI and ESD Design CriteriaThe dB4 has been designed to per-form with the lowest noise characteris-tics. Damping resistors in series withevery I/O line minimize ringing and EMIand provide current limits that protectagainst transient signals.

Ordering Summary

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