Download Topic: High Performance Data Acquisition Systems Analog

Topic: High Performance Data Acquisition Systems
Analog Components: Multiplexed Systems
Figure 1 High Performance Data Acquisition System
Figure 1 shows a practical high performance data acquisition system using an analog
multiplexer. Multiplexed systems have the significant advantage of accessing multiple sensor
outputs while minimizing the number of expensive sample and hold and analog to digital
convertors that would be necessary for multi-channel operations. Of course, the challenge is to
time everything in such a way so that the multiplexer can acquire the signal from the sensor,
switch channels, and then settle at the input of the sample and hold/analog to digital convertor
unit to the required accuracy, BEFORE the sample is finalized (all of this must be fast enough to
sample (inclusive of the required number of channels) for the desired system level
performance). In the past, sampling speed limitations in the analog to digital convertor required
designers to parallel multiple channels of ADC’s to sample the output of each sensor. Currently,
there are available fast sampling analog to digital convertors that have high enough sample
rates that when divided by the number of channels being acquired, can still give the desired
overall throughput required in many high performance systems.
Again, the multiplexer must be able to switch channels and the output signal must settle to the
desired level (consistent with the allowable error within the error budget) AND to also be
within the sample rate time interval of the sample and hold/analog to digital convertor AND to
also be able to minimize “cross talk” between channels (channel to channel feed-through).
Therefore, the multiplexer also introduces multiple errors within the budget. DC errors such as
input offset (Rsource x Ibias(off)) and output offset (R(on) x (Id (on) + Ibias (s&h and/or adc)) are introduced.
The dynamic errors of the multiplexer include cumulative time delay between multiplexer
address signal and the settled output to the desired level at the S&H/ADC input(s) (see Figure
2), in addition to channel-to-channel feed-through errors. Care must also be taken when using
multiplexers in terms of overall full scale linearity errors. The inherent linearity of the
multiplexer can be limited at +/- full scale as the output approaches the multiplexer power
supply rails. Also, dynamically changing input bias current conditions into the S&H/ADC (current
glitches) can increase significantly and when multiplied by the multiplexer “Rd(on)” resistance
(which can be varying non-linearly), and can cause huge problems especially in very high
resolution (greater than 12 bit) systems. These errors, which can be significant (and be a
limiting factor in overall performance), but these errors also manifest themselves significantly
over temperature as well. Remember, a multiplexer is the perfect solution to eliminate a
number of expensive parallel analog to digital convertors, but there are subtle performance
errors, or changes in characteristics over time and temperature that can work significantly with
either the input drive impedance and/or the output impedance it is required to drive.
Figure 2 Typical Multiplexer Timing
Another significant usage of a multiplexer in a high performance data acquisition system is in
regards to system level calibration. If several multiplexer channels can be dedicated to drive
stable reference voltages, than the overall data processing system can be continuously
calibrated, effectively removing multiple system errors (including some that vary over
temperature).
Figure 3 Static Crosstalk
As we stated previously, linearity errors within the multiplexer can be quite significant
depending on the input and output loading of the device (both in required source and load
resistance as well as required output voltage swing), but what can even be more troublesome
in an analog multiplexer is crosstalk. Figure 3 shows how signals from one channel can be
coupled into the other channel. Theoretically, Vout consists of Vin1 attenuated by the resistor
divider (not including Rs1) formed by Rd1(on) and Rload. However, the capacitance of switch
number two (C2) does couple some portion of Vin2. This is the simplest form of crosstalk which
can often times be referred to as “static” crosstalk. In high frequency systems, a proportional
amount of signal can easily be coupled through the “OFF” channel, so the designer can simply
affect the static crosstalk level by simply choosing a smaller output load (Rload) that would then
attenuate the fed-through signal. Of course this has negative effects in regards to increased
current flow through the “ON” channel device and can increase static DC errors in the system as
well.
Figure 4 Dynamic Crosstalk
“Dynamic” crosstalk is another issue in an analog multiplexer (see Figure 4). In normal
operation, a multiplexer switches continuously between “ON” and “OFF”. In order to reduce
crosstalk, multiplexers are designed to have break-before-make switching so that no two
channels are addressed at the same time. This causes a non-linear dynamic loading change on
the amplifiers driving the multiplexer as well as dynamically changing source impedance driving
the sample and hold/analog to digital convertor. This make-before-break time interval, and
subsequent dynamic settling time issues that negatively affect the multiplexer drive amplifiers,
multiplexer, and sampling unit, can be quite significant, especially, when these issues couple
between channels as the conditions change depending on the voltage levels at each channel
input at the time of multiplexer addressing and ultimate output channel sampling.
Of course the most confusing of all error signals in an analog multiplexer is “adjacent” channelto-channel crosstalk. This is can actually be the most dominate error component of the device.
While both “static” and “dynamic” crosstalk are capacitive in nature (they vary with frequency
at 6dB/octave), the “adjacent” channel crosstalk is invariant with frequency. The term
“adjacent” refers to adjacent in time. Two channels are adjacent in time when one channel is
sampled after the previous channel. This results in the information of the previous channel to
be “carried forward” to the next channel (in time). This introduces a subtle “sampling” type
error signal that is stored in the various capacitances in the multiplexer network base upon the
previous multiplexer channel output. Adjacent channel crosstalk is a problem in every
application where dynamic crosstalk must be considered, and it must be minimized. This can be
accomplished by shorting the multiplexer output node to ground between address changes.
This will require an additional analog switch. An alternative approach to reducing adjacent
channel crosstalk is to ground every other channel in the multiplexer thereby standardizing the
adjacent channel effects.
Multiplexing multiple sensor outputs into a minimal number of sampling units (sample and
hold/analog to digital convertors) reduces the complexity, expense and many different kinds of
errors that negatively impact a high performance data acquisition system, but it also introduces
other errors that can significantly limit overall performance. Also remember, PCB layout can be
crucial in minimizing all of the error contributors that we have been discussing. Proper layout in
regards to channel-to-channel isolation (and matching), together with power decoupling and
grounding, will be critical to achieve overall desired circuit performance.
Kai ge from CADEKA