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Article Title:
High-Performance Data Acquisition on USB

Article Description:
A common belief among engineers is that, to get the best 
performance, a PC based data acquisition module must plug into the 
PCI bus. Properly implemented, however, a data acquisition module 
can use the PC's USB 2.0 port to pump data into a PC as fast as 
PCI-based cards.

Additional Article Information:
1491 Words; formatted to 65 Characters per Line
Distribution Date and Time: Tue Mar 28 03:27:00 EST 2006

Written By:     Tim Ludy
Copyright:      2006
Contact Email:  mailto:[EMAIL PROTECTED]

Article URL:

For more free-reprint articles by this Author, please visit:


High-Performance Data Acquisition on USB
Copyright © 2006 Tim Ludy
Data Translation

A common belief among engineers is that, to get the best 
performance, a PC based data acquisition module must plug into the 
PCI bus. Properly implemented, however, a data acquisition module 
can use the PC's USB 2.0 port to pump data into a PC as fast as 
PCI-based cards. The keys to achieving this performance are a 
hardware USB interface on the module and optimized driver 
software for the host system.

Because the Universal Serial Bus (USB) was originally developed 
to replace lowspeed peripheral cabling, many engineers fail to 
see its potential as a high performance data acquisition channel. 
The original USB specification did offer only a modest bit rate, 
but USB 2.0 handles 480 Mbits/sec, fast enough to handle 60 
Mbyte/sec data streams. Even with the protocol reserving some 
bandwidth for interrupts and control transfers, and the header 
overhead on data packets, the bus can easily sustain more than 
10 Mbytes/sec of continual data transfer. This is fast enough 
to support extremely high performance data acquisition (DAQ) 

Utilizing the USB as a data portal provides many advantages for 
data acquisition (DAQ). For one, the external connector and 
"plug-and-play" software installation of USB peripherals means 
that users do not have to open their PC, then set-up and 
configure the hardware in order to begin acquiring data. The USB-
based DAQ module sets itself up upon installation. Addressing 
and other potential resource utilization conflicts resolve 

Software for handling the data also becomes simpler to implement. 
The host system USB drivers separate data streams into logical 
channels called "pipes." This means that the host system software 
will automatically form a logical connection from a DAQ channel 
to a destination within the application software, simplifying 
software and hardware set-up. Applications simply need to 
identify the logical pipe they wish to connect with and the 
system software ensures that data travels to the right 
destination. Application software does not need to know the 
details of peripheral addressing, interrupt placement, or other 
installation dependent parameters as it must for PCI plug-in 
cards. The USB also has electronic advantages. The bus can power 
the peripheral (within limits) so that the DAQ system does not 
need its own power source. This further eases system installation 
and use and has the added benefit of removing the sensitive A/D 
converters and amplifiers from the electrically noisy environment 
inside the PC's housing. Unlike PCI plug-in cards, a USB-based 
DAQ module is easily shielded for achieving high bit-level 
resolution. The benefits of USB operation are compelling for a 
DAQ system, but implementing the system does require care in 
order to ensure a high-bandwidth connection. Engineers often 
assume that high data-rate systems should use the USB data 
transfer mode with the highest raw bandwidth and least overhead 
penalty: isochronous transfer. But isochronous transfers are a 
"best-effort" channel. Data that suffers from errors cannot be 
resent. The proper mode to use for DAQ on USB is the bulk 
transfer mode.

1). This mode supports the resending of corrupted packets, 
ensuring data accuracy, and allows fairly large 512-byte blocks, 
keeping overhead effects down.

The drawback is that bulk transfers do not have guaranteed 
timing. The USB host controller assigns bandwidth for bulk 
transfers but reserves priority for interrupt and control 
transfers. Thus, the bandwidth obtained by using bulk transfers 
is an average - not a sustained - data rate while the DAQ module 
needs to send data at a constant speed.

There are two approaches to dealing with the uncertainty in the 
instantaneous bandwidth available for bulk transfers. One is to 
keep the data rate low in order to maximize the likelihood that 
bandwidth available will always exceed the DAQ module's data 
rate. This is the simpler approach, but results in severe 
underutilization of USB's potential.

The alternative approach is for the DAQ module to incorporate 
FIFO buffering to hold data while waiting for the bus to become 
available. Dual buffers are needed in the module - one to fill 
while the other is emptying. The larger the buffers, the more 
tolerant the module becomes of shifting bandwidth availability 
and the closer its data rate can approach the average bandwidth 

In addition to the FIFO buffering on the DAQ module, the software 
drivers should allocate buffer space in the host system at the 
receiving end. This decouples the host system's data processing 
activity from the data acquisition so that neither activity can 
delay or impede the other's performance.

While FIFO buffering maximizes the DAQ module's ability to 
utilize the available bulk transfer bandwidth, however, it is not 
the only way to improve the achievable speed on the USB. The 
module's USB interface hardware and latency in the host 
controller software can seriously constrain achievable USB data 
rates if not implemented with high performance in mind. For the 
interface hardware, using a state-machine hardware controller 
optimized for 512-byte transfers (the largest bulk transfer 
packet) produces a much faster interface than the use of a 
software-dependent USB microcontroller.

On the host side, careful driver design can reduce latency by 
speeding the host system's response to incoming USB data. The 
traditional Windows drivers allocate buffer space in response to 
an incoming USB transfer, taking many milliseconds getting ready 
to receive data once the DAQ module declares data ready to send. 
Drivers that are proactive instead of reactive, pre-allocating 
buffer spaces of the right size can decrease host latency by an 
order of magnitude. These are not simply theoretical suggestions. 
Data Translation has achieved a data rate of 250k samples/sec on 
12 simultaneous channels with its DT9836 USB data acquisition 
module and 2M samples/sec on 2 simultaneous channels at with the 
DT9832A. Experience has shown that the USB can provide reliable, 
sustainable data rates as great as 10.9 Mbytes/sec, corresponding 
to sample rates of 5.45M samples/sec. The potential exists to 
take USB-based data acquisition to even higher levels with 
additional buffering and driver optimization.

But a high data rate is not all that is required of a high-
performance DAQ module. Issues such as accuracy, aperture 
uncertainty, and noise levels are equally important. Further, the 
inclusion of additional capabilities such as digital I/O lines, 
counter/timers, analog outputs, and quadrature encoders can 
greatly increase the utility of a DAQ module. The DT9832 and 
DT9836 series offer all of these features.

Accuracy in a high-performance DAQ module has many facets. The 
most obvious is the A/D converter's resolution. The Simultaneous 
Series offer 16-bit A/D conversion: 2 channels at 2.0 MHz for 
the DT9832A, 4 channels at 1.25 MHz for the DT9832, and 6 or 12 
channels at 225 kHz for the DT9836. The modules are fabricated 
with 12-layer boards, keeping the signal integrity intact and 
typically achieving an ENOB (effective number of bits) rating 
of greater then 14-bits.

Accuracy also involves timing, however, especially in a multi-
channel DAQ module. In order to properly compare and correlate 
sampled signals it is important to know the relative timing of 
samples from one channel to the next. Most DAQ systems use a 
single A/D converter with a multiplexer front-end to handle 
multiple channels. This results in each channel's signal being 
sampled at a different time, forcing the use of interpolation to 
bring the signal data into temporal alignment and resulting in 
relative timing errors or phase noise. Ideally, the samples for 
all channels should be made simultaneously to eliminate phase 

The DT9836 and DT9832 series modules do not use a multiplexer. 
Each channel has its own track-and-hold stage with an independent 
successive-approximation A/D converter (see Figure 4). The 
converters use a common clock and the track-and-hold stages have 
a common trigger, so that the modules offer true simultaneous 
sampling. The channels have an aperture delay of 35 nsec with 
uncertainty (aperture jitter) of 1 nsec, and channels are well 
matched so that there is less than 5 nsec difference between 
channels. This virtually eliminates phase noise in data.

With sample rate, bit accuracy, and timing accuracy at high 
levels, the availability of additional features is simply a 
performance bonus. The Simultaneous Series offer these extras, as 
shown in Figure 2. Each module includes two 16-bit D/A channels, 
16 digital output channels, 16 digital input channels, two 32-bit 
counter timers, and 3 quadrature encoders. The additional digital 
I/O lines provide considerable flexibility for incorporating 
functions such as time stamping, pattern recognition, and 
synchronization with external events. The counter/timers offer a 
convenient means of triggering test events while the quadrature 
decoders simplify the use of the module with X/Y positioning and 

Supporting these additional signal lines means that the data 
channel must be fast enough to handle the additional signal and 
control bits without impacting the module's sampling rate. The 
USB is fast enough. It has proven that it can provide the needed 
capacity, and offers significant ease-of-use benefits. There is 
no longer a need to pry open a PC to assemble a high-performance 
DAQ system. Simply plug one in to the USB port.

For related images and graphs, see: 

Tim Ludy is a Product Marketing Manager with <a href=>Data 
Mr. Ludy graduated from Northeastern University with a degree 
in Computer Science. email: [EMAIL PROTECTED]



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