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

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

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

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

Increased Signal Bandwidth Multiplexed Architecture One A/D Converter One A/D Converter An Instrumentation Amp An Instrumentation Amp A Multiplexer A MultiplexerPerformance: CH Rate per channel Signal Bandwidth 1 150kHz 75kHz 2 75kHz 37.5kHz 3 50kHz 25kHz kHz 18.75kHz 5 30kHz 15kHz 6 25kHz 12.5kHz Simultaneous Architecture A/D Converter per chan A Track-Hold per chan No MultiplexerPerformance: CH Rate per channel Signal Bandwidth 1 150kHz 75kHz 2 150kHz 75kHz 3 150kHz 75kHz 4 150kHz 75kHz 5 150kHz 75kHz 6 150kHz 75kHz

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

Built In Accuracy Simultaneous A/D Converters All inputs sampled at same time All inputs sampled at same time Single clock pulse to acquire all channels Single clock pulse to acquire all channels 35nS max aperture delay 35nS max aperture delay Matched within 5nS across all circuits 1nS jitter (aperture uncertainty) Higher Accuracy at High Speed Higher Accuracy at High Speed Eliminate several sources of error 1. Settling Time 2. Channel-to-channel crosstalk

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 Makes source impedance an issue Can generate erroneous results Can generate erroneous results

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 9 TC’s typical to settle within 0.01% accuracy Assume: 10V on Channel 0 0V on Channel 1 (R = 10kOhm) 0V on Channel 1 (R = 10kOhm) C = 100pF 1TC = (10kOhm * 100pF) = 1uS 0.01% accuracy requires 9 TC’s 9 * 1uS = 9uS (~110kHz)

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

Adding OP Amps to Your Signal Conditioning MUX’d System Slow-speed OP Amp Slow-speed OP Amp Long Settling Times Errors in Measurement (described previously) Added Cost High-speed OP Amp 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 MUX, No Settling No added cost No added cost

How Precise is Your Simultaneous Acquisition Device? Measured in Aperture Jitter (Uncertainty) How to Measure Aperture Jitter: 1. Input a full scale sinusoidal signal on all channels 2. Find the voltage change near the zero-crossing 3. Compare voltage change to A/D resolution Demonstration…

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

1) A sinusoid is determined by the equation: V(t) = pSin(2πft) where: p = peak voltage of sine wave f = frequency of sine wave f = frequency of sine wave V = voltage V = voltage t = time (in seconds) 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 * ) = 628uV (ΔV in 1 nanosecond)

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. 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. Aperture Jitter (nS) Peak Voltage Frequency (Hz) 12-Bit A/D Error ( Number of lsb’s) 16-Bit A/D Error ( Number of lsb’s)

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. 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. 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 Semiconductor device testing Nanotechnology testing Nanotechnology testing Motion Control Motion Control Industrial Applications Industrial Applications Automotive testing Automotive testing

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

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