Download Topic: High Performance Data Acquisition Systems Analog

Topic: High Performance Data Acquisition Systems
Analog Components: High Performance Amplifiers Settling Time
Figure 1 High Performance Data Acquisition System
Figure 1 shows a high performance data acquisition system (with storage array). As we have
discussed in the past, each component within the system contributes to an overall “error
budget.” Ideally, the system should convert the signal from the transducer/sensor to an
accuracy and resolution that is set by the analog to digital convertor. The design engineer must
simply specify and select each component (in its specified circuit configuration) within the
system to have cumulative errors (RSS’d) that are sufficiently below the contributed error levels
of the ADC.
Generally, a high frequency data acquisition systems output response (including errors) is
measured in the frequency domain using FFT’s (Fast Fourier Transform algorithms). FFT’s
operate on finite data sequences-sets of data with each point discretely and evenly spaced in
the time domain. However, the signal that is received and transmitted by the
transducer/sensor that is to be transformed (real-life waveforms) are analog in nature. They are
continuous in the time domain and therefore must be “sampled” at discrete points (and
digitized) before the FFT algorithm can be used to process the data (see Figure 2). In short, the
sampled waveform array corresponds to a table of values you might construct from a point-topoint measurement of the waveforms amplitude. The majority of measured time-domain data
is real-valued data. This means it has no “complex” (imaginary) component. Therefore, you only
need one waveform array to store real time-domain data (usually required for low
frequency/high accuracy systems which will be analyzed in later articles). However, in higher
frequency systems, usually two waveform arrays are required for the storage of the
corresponding frequency domain data. One of these arrays is for the “real” part (or magnitude),
and the other is for the imaginary part (or phase). Later, we will discuss the two basic concepts
of “windowing” and “sampling” in regards to analog to digital conversion in order to better
understand FFT results. But this “sampling” of a continuous waveform at “discrete” time points
and then processing the output signal in the frequency domain means that careful
consideration must be taken in analyzing the error contributions in BOTH time and frequency
domains. Remember the frequency domain is simply the inverse of the time domain (f=1/τ).
Therefore both types of errors (f & τ) must taken into consideration when compiling the overall
error budget.
Figure 2 Finite Data Sequence Discretely and Evenly Space in the Time Domain
So far, in regards to the front-end high performance amplifier within the data conversion
system, we have thus far looked at noise and distortion in regards to impacting the system level
error budget. Now, let’s look into the time domain or “settling” response of the amplifier.
Simply speaking, amplifier distortion in frequency domain is just settling errors in the time
domain. This is good to observe, because troubleshooting (and trying to minimize the errors
within a system) is often times easier to do when measuring responses using an oscilloscope
(and DVM), and making settling time/DC measurements. Let’s go over some important issues
when determining the time domain (settling time) responses of a high performance amplifier.
Amplifier “settling time” is simply the time required for an amplifier to accurately respond to an
ideal instantaneous step input to the time at which the closed loop amplifier output has
entered and remained within a specified error band (usually symmetrical about the final value.)
Of course this “final” value would also include any DC errors of the amplifier, slew rate
limitations, overdrive recovery, propagation delay, etc. (see Figure 3). Remember, settling time
is influenced by a combination of amplifier factors (linear and nonlinear, internal and external)
and it cannot be predicted by individual amplifier specifications such as slew rate, and small
signal bandwidth, etc., but all errors must be accounted for. One word of caution in specifying
and selecting a high performance amplifier for your system, an amplifiers output can never
settle within a given error band if its output noise (or distortion) is comparable to the
magnitude of the error band defined for settling! This also includes interference signals that the
amplifier receives through the circuit environment such as power supply and ground noise that
is not rejected.
Figure 3 Settling Time Factors to Consider
Let’s look at the time domain settling errors that must be accounted for:
-DC Gain: To ensure the fundamental accuracy of the amplifiers required final value within a
specified sampling time frame, the “DC gain” of the amplifier must be within the desired error
band. For instance, if the amplifier is to settle to within a +/- .01% (1 part in 10,000), this means
the DC gain error has to be below this value within the sampling time interval, over the whole
input voltage range amplitude, at whatever specified input frequency!
-DC Offset & Drift: For high precision applications, again the DC offset and drift (over
temperature) must be within the given error band. If the designer again uses a +/-.01% error
band with a +/-1Vpp signal, the amplifier would be required to have an offset and drift of less
than 100 µV error! This would be a very challenging specification to meet, but fortunately
within a data acquisition system, offset errors are easily calibrated and adjustable.
-Dynamic Stability: Obviously, amplifiers stability greatly affects the amplifiers settling time.
Depending on the amplifiers gain, required output load, and input drive impedance, a high
frequency amplifier can be either over-damped (band-limited), under-damped (potentially
unstable and ringing), or critically-damped (in-between over and under-damping). Measuring
the frequency response of the amplifier (Vout/Vin) and watching for gain “peaking” or “early
roll-off” will greatly help the designer assess the amplifiers dynamic stability.
-Non-linear Effects: Other factors that impact amplifier settling time include: Slew rate (the
ability for the amplifier to drive large signals usually during an open-loop state), Recovery (how
the amplifier transitions from an overdriven (saturated) state to a closed-loop final value state),
and Propagation Delay (the real time delay for the amplifier to respond to a change at its input).
Of course, when evaluating the overall performance of a high speed data acquisition system,
each of the above parameters should be well specified and understood and accounted for in
the system level error budget. Remember, 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