Download Topic: High Performance Data Acquisition Systems Analog

Topic: High Performance Data Acquisition Systems
Analog Components: High Performance Amplifiers (Instrumentation)
The front-end amplifier of a high performance data acquisition system receives the signal from
the sensor and amplifies it for further processing through the analog to digital convertor. Thus
far, we have focused our attention on high frequency amplifiers used in high frequency data
acquisition systems; now let’s turn the attention to lower frequency amplifiers used in low
frequency systems. Various types of input sensors, generally, can transmit signals over an
extremely wide frequency range (from DC to well over 1 GHz). Of course systems that measure
signals from the DC to an audio frequency range of less than 100kHz (low frequency) can often
require very high resolution conversion (greater than 14 Bits and usually in the 16 to 24 Bit
range) and therefore need front-end amplifiers with lower bandwidths but very high precision.
Received signals beyond the audio frequency range (of greater than 100kHz) up to several GHz
(high frequency) generally require ADC resolutions of less than 14 Bits (between 6 and 14 Bit
resolutions) and therefore need front-end amplifiers that have much higher bandwidths and
lower precision. High performance data acquisition systems that process lower input
frequencies (DC to audio range) generally use high gain, high precision, low frequency
INSTRUMENTATION amplifiers together with high resolution sigma delta ADC’s to achieve
optimal system performance. This is in contrast to systems that process higher input
frequencies (greater than 100 KHz) that require low gain, lower precision, very wide band, low
AC distortion and noise, and nominal DC performance specifications together with lower
resolution ADC convertors (see Figure 1).
Figure 1 High Performance Data Acquisition Systems (Low and High Frequency)
General Rule of Thumb: High Performance Data Acquisition Systems generally fall into 1 of 2
categories:
1) Low Frequency, High Resolution
-or2) High Frequency, Low Resolution
This seems obvious, but if the designer can keep these two categories in mind, each type of
system has dramatically different error budget constraints and therefore the details in the
front-end amplifiers individual specifications and stability, as well as the PCB design
environment and layout, becomes extremely important to achieve desired system level
performance.
Figure 2 Instrumentation Amplifier Used in a “Bridge” Configuration (with shielding)
Figure 2 shows a typical low frequency instrumentation amplifier used in a “bridge”
configuration for receiving DC input signals (in applications such as a weight scale, etc.).
Instrumentation amplifiers are usually the optimal choice in any precision, high impedance,
noise dominated environment where proper shielding and grounding are necessary to resolve
DC input changes at micro-volt levels. Instrumentation amplifiers are usually specified with high
gains and very accurate DC performance to measure small DC input changes in these types of
“bridge” applications (see typical performance specifications of a CLC1200 in Figure 3).
Figure 3 Typical Performance Specifications for High Precision Instrumentation Amplifier
Instrumentation amplifiers used in high precision systems will usually require gains of up to
1000 (which only gains up a differential input of 1 µV to an output of 1 mV!). This is why proper
layout and shielding is critical in designing with these types of devices as to minimize any
differential input noise levels from being introduced into the system as an error signal. Of
course the specifications in the table of Figure 3 also has input offset levels of less than +/- 125
µV which can be a dominate factor in setting the resolution of the system (unless it is calibrated
out of the measurement), and then the input offset temperature coefficient can then become a
key performance limitation. The designer needs also to pay special attention to input bias
current levels (including temperature coefficient) as well as power supply rejection ratio (PSRR)
and common mode rejection ratios (CMRR) that must significantly be better than 20log of the
ratio of the amplifiers full-scale output range divided by the smallest required input voltage
range fluctuation. For example, if a system is designed using an instrumentation amplifier
output range of 2Vpp at a gain of 1000, that would require an input full scale resolution
fluctuation of 2mV across the bridge application in Figure 2. Well, if your weight scale needs an
accuracy of .1%, then .1% of a 2 mV input range is a 2 µV error signal! Of course a 2 µV error out
of a 2Vpp system, requires PSRR’s and CMRR’s of nearly 120 dB for overall system performance.
Basically, an instrumentation amplifier is essentially a (three amplifier) differential amplifier
that has individual input buffers (see Figure 4) which eliminate the need for input impedance
matching and therefore allows the amplifier to be optimally used in low frequency high
precision measurement systems. Of course this amplifier configuration can allow the circuit to
have very low DC offset, low offset drift, low input bias current, every high common-mode
rejection ratio, high input impedance, etc., but, it comes with problems as well. The
performance specification requirements necessary for a low frequency, high resolution systems
are also very difficult to achieve due to the overall complexity that these additional amplifiers
bring to the circuit in terms of overall stability (we will discuss this more next time). Although an
instrumentation amplifier is usually shown schematically as a standard op-amp, it is best
understood in this three op-amp system. These are arranged so that the (+) and (-) of a
standard op-amp would each be buffered and then each of these amplifier outputs would be
amplified by a third op-amp in a controlled impedance environment for optimal amplification.
Instrumentation amplifiers can be built with individual op-amps and external precision
resistors, but high performance amplifiers generally have integrated resistors that are precisely
matched that allow for maximum common-mode rejection (an instrumentation amplifier
should ideally reject all common mode signals).
Figure 4 Instrumentation Amplifier (Basic 3 Amplifier Configuration)
Of course, when evaluating the overall performance of a high precision instrumentation
amplifier,each of the above parameters should be well specified and understood and accounted
for in the system level error budget. Remember, as we have said before, while the data sheet
specifications are helpful in the selection and specification of an amplifier that will work within
the required error budget, the designer will generally need to individually measure each of the
above parameters- within the real-life circuit/system level environment, for optimal design.
Kai ge from CADEKA