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Multiplexed vs. Simultaneous Data Acquisition Using USB Devices

Multiplexed vs. Simultaneous Data Acquisition Using USB Devices. Presented by: Rene Messier Company: Data Translation. DAQ Criteria. Resolution Number of Channels Speed Range In addition, the particular analog input architecture chosen will affect the sampling and

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Multiplexed vs. Simultaneous Data Acquisition Using USB Devices

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  1. Multiplexed vs. Simultaneous Data Acquisition Using USB Devices Presented by: Rene Messier Company: Data Translation

  2. DAQ Criteria • Resolution • Number of Channels • Speed • Range In addition, the particular analog input architecture chosen will affect the sampling and accuracy of your results.

  3. Architectures Multiplexed Simultaneous • Multiplexed systems use one A/D converter • Simultaneous systems use an individual A/D converter for each channel

  4. Channel to Channel Skew Eliminated • Simultaneous Sampling • Eliminates time skew between channels • Simplifies both time and frequency based analysis techniques • Multiplexed Sampling • Channels are sampled sequentially • May require software correction for detecting certain patterns

  5. Multiplexed Architecture One A/D Converter An Instrumentation Amp A Multiplexer Performance: CH Rate per channel Signal Bandwidth 150kHz 75kHz 75kHz 37.5kHz 50kHz 25kHz 37.5kHz 18.75kHz 30kHz 15kHz 25kHz 12.5kHz Simultaneous Architecture A/D Converter per chan A Track-Hold per chan No Multiplexer Performance: CH Rate per channel Signal Bandwidth 150kHz 75kHz 150kHz 75kHz 150kHz 75kHz 150kHz 75kHz 150kHz 75kHz 150kHz 75kHz Increased Signal Bandwidth

  6. Higher Signal Bandwidth • Bandwidth is the area of all frequencies up to the 70% roll-off point • Data Translation products offer a front end bandwidth that is ten times the Nyquist Limit • Minimizes roll-off and phase errors

  7. Built In Accuracy • Simultaneous A/D Converters • All inputs sampled at same time • Single clock pulse to acquire all channels • 35nS max aperture delay • Matched within 5nS across all circuits • 1nS jitter (aperture uncertainty) • Higher Accuracy at High Speed • Eliminate several sources of error • Settling Time • Channel-to-channel crosstalk

  8. Settling Time For Mux’d Systems • Each Channel is tied to the same A/D • Minimum settling time is required for the switched voltage to reach the actual input signal level • Some portion of signal from previous channel can “cross over” to next channel • Makes source impedance an issue • Can generate erroneous results

  9. Source Impedance and Settling Time • Source Impedance (R) • Capacitance (C) • R * C = 1 Time Constant • 9 TC’s typical to settle within 0.01% accuracy Assume: 10V on Channel 0 0V on Channel 1 (R = 10kOhm) C = 100pF 1TC = (10kOhm * 100pF) = 1uS 0.01% accuracy requires 9 TC’s 9 * 1uS = 9uS (~110kHz)

  10. Channel-to-Channel Crosstalk • Signal on a channel couples with the signal on another channel • Occurs because of parasitic capacitance across each open switch Example: • Assume an 8 channel MUX’d system: • Each 5pF capacitor can cause crosstalk between channels • (5pF * 7ch) = 35pF

  11. Adding OP Amps to Your Signal Conditioning • MUX’d System • Slow-speed OP Amp • Long Settling Times • Errors in Measurement (described previously) • Added Cost • High-speed OP Amp • Will Ring When Hit with the 100pF Switch Transients From the MUX • Added Cost • Simultaneous System • No MUX, No Settling • No added cost

  12. How Precise is Your Simultaneous Acquisition Device? • Measured in Aperture Jitter (Uncertainty) • How to Measure Aperture Jitter: • Input a full scale sinusoidal signal on all channels • Find the voltage change near the zero-crossing • Compare voltage change to A/D resolution Demonstration…

  13. 1) Input full scale sine wave defined by: V(t) = pSin(2πft)

  14. 1) A sinusoid is determined by the equation: V(t) = pSin(2πft) where: p = peak voltage of sine wave f = frequency of sine wave V = voltage t = time (in seconds) 2) In order to find the voltage change near the zero-crossing, take the derivative: (6.283 * 10,000Hz * 10V) = 628,300 dV / dt (ΔV in 1 second) (628,000 * .000000001) = 628uV (ΔV in 1 nanosecond)

  15. 3) Compare voltage change to A/D resolution: ΔV = 628uV in 1nanosecond Given a 16bit A/D the resolution is: (20V / 65536) = 305uV So, with an aperture uncertainty of 1nS, a 10kHz signal should yield a voltage change near the origin of 628uV or ~2 bits of error for a 16-bit A/D converter.

  16. Cost vs. Value For many applications, a simultaneous acquisition device is certainly the architecture of choice due to its inherent speed and accuracy but, until recently, the cost of these devices was somewhat prohibitive. Times have changed and simultaneous devices are now as cost effective as multiplexed devices. Data Translation has designed a series of simultaneous data acquisition modules for USB 2.0. The Simultaneous Series provide high speed, highly accurate measurements, useful in many applications. • Semiconductor device testing • Nanotechnology testing • Motion Control • Industrial Applications • Automotive testing

  17. Exceptional Quality and Value • DT9836 • Features • Simultaneous Sampling • High 225kHz per Channel throughput • 6 or 12 channel versions • 0 or 2 channel D/A output • 32 Digital IO bits • 5 Counter / Timers • 3 Quad Decoders • Competitive pricing

  18. Questions?Applications?

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