Transcript Multiplexed vs Simultaneous Data Acquisition Using USB

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
accuracy of your results.
Architectures
Multiplexed
Simultaneous
Multiplexed systems use one A/D converter
Simultaneous systems use an individual A/D
converter for each channel
Channel to Channel Skew Eliminated
Simultaneous Sampling
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Eliminates time skew between
channels
Simplifies both time and
frequency based analysis
techniques
Multiplexed Sampling

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Channels are sampled
sequentially
May require software
correction for detecting certain
patterns
Increased Signal Bandwidth
Multiplexed Architecture
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Simultaneous Architecture
One A/D Converter
An Instrumentation Amp
A Multiplexer
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Performance:
CH
1
2
3
4
5
6
Rate per channel
150kHz
75kHz
50kHz
37.5kHz
30kHz
25kHz
A/D Converter per chan
A Track-Hold per chan
No Multiplexer
Performance:
Signal Bandwidth
75kHz
37.5kHz
25kHz
18.75kHz
15kHz
12.5kHz
CH
1
2
3
4
5
6
Rate per channel
150kHz
150kHz
150kHz
150kHz
150kHz
150kHz
Signal Bandwidth
75kHz
75kHz
75kHz
75kHz
75kHz
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
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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
1.
2.
Settling Time
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
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Makes source impedance an issue
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
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)
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
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

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No MUX, No Settling
No added cost
How Precise is Your Simultaneous
Acquisition Device?
Measured in Aperture Jitter (Uncertainty)
How to Measure Aperture Jitter:
1.
2.
3.
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…
1) Input full scale sine wave defined by: V(t) = pSin(2πft)
1) A sinusoid is determined by the equation:
where:
V(t) = pSin(2πft)
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)
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.
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)
1
10
100
.001
.021
1
10
1000
.013
.206
1
10
10000
.13
2.06
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.
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Semiconductor device testing
Nanotechnology testing
Motion Control
Industrial Applications
Automotive testing
Exceptional Quality and Value
DT9836
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Features
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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
Questions?
Applications?