Analog Input Ranges in APx500 Audio Analyzers – What you Should Know
We’ve added a new feature to the APx500 software that I’ll talk about in a future post: As of version 5.0.1 (released April 2, 2019), users can configure the analog generator to stay in a fixed output range. This is especially useful for testing acoustic devices, and some electronic devices, with sensitive input stages .
While preparing to write about this new analog output ranging feature, I realized that during my ten plus years of working with AP customers, there have been many occasions when users were confused about (and sometimes even tripped up by) the behavior of the analog input ranging circuits in APx audio analyzers. Hence, this post on analog input ranging.
Analog Input Ranges
Based on published specs, the residual noise (or noise floor) of APx audio analyzers (Legacy or B Series) varies from about 1.0 to 1.4 microvolts rms, depending on the analyzer model. However, this is a broadband figure that is typically integrated over the entire audio frequency range (20 Hz to 20 kHz). As shown in Figure 1, at any one frequency within that range the residual noise is much lower, so that when doing frequency analysis (e.g. by FFT or narrow band filters), APx analyzers can easily measure levels of 100 nanovolts (10-7 V) rms and lower.
Figure 1. FFT spectrum of typical APx555 analog input noise (320 mV input range, input shorted, FFT bin width = 1 Hz, 100 averages).
In APx audio analyzers, the upper limit for voltage measurements varies from 80 to 300 Vrms, depending on analyzer model and analog input type (balanced or unbalanced). This means that voltage measurements must span a dynamic range of 180 to 190 dB. It’s physically impossible for an analog circuit to have a dynamic range of 180 dB, and even the best analog to digital converters (ADCs) have a dynamic range of less than 120 dB. So, to enable audio analyzers to measure voltages over this wide dynamic range from tens of nanovolts to hundreds of volts, the analog inputs are divided into a series of input ranges. Each range has a specified nominal upper limit. The range values or steps are chosen to optimize the performance of the system by ensuring as much dynamic range as possible without allowing noise and harmonic distortion to get too high.
Most of the audio analyzers in the APx500 family have input range steps which are 10 dB (a ratio of about 3.2 times) apart. For example, consider the APx525: its nominal input ranges (specified in terms of rms voltage) are 0.32, 1, 3.2, 10, 32, 100 and 300 V. An exception to the 10 dB range steps is the flagship APx555 audio analyzer which has input ranges that are 6 dB apart, for higher performance.
By default, APx audio analyzers are configured to change input ranges automatically using a feature called Auto Range. When Auto Range is enabled, a detector at the first input stage of each channel senses the instantaneous voltage and automatically sets the input range of that channel to the lowest range that will accommodate the input signal. The ranging behavior is programmed into the instrument firmware and range changes occur more or less instantly. As such, another important feature of the ranging system is that it protects internal input circuitry in lower ranges from exposure to potentially damaging high voltages.
When Auto Range is enabled, the measurement ranges overlap as illustrated by the darker blue shaded bars in Figure 2. Note that each range has a small amount of “headroom”, and hysteresis is used to ensure that the ranges truly overlap. For example, with respect to Figure 2, consider what happens when an input signal is ramped from say 1 mV to 2 V rms and back to 1 mV. Initially, the system is in the lowest (320 mV) range and it stays in that range until the input signal exceeds 320 mV by about 10 %. It then stays in the 1 V range until the signal exceeds that nominal voltage by about 10 %, at which point it jumps to the 3.2 V range. As the signal decreases from 2 V rms, the system stays in the 3.2 V range until the input signal falls below about 0.9 V rms (slightly lower than the 1.0 V nominal voltage of the 1 V range). As the voltage continues to drop, the system will switch to the 320 mV range when the voltage falls below about 300 mV. This hysteresis – switching to a higher range at voltage slightly above the range step and switching to a lower voltage at slightly lower than the range step – ensures that the ranges overlap sufficiently to provide complete coverage.
In each input range, the system applies a gain stage to the signal that amplifies or attenuates it to optimally match the input range of the 24-bit ADCs. This minimizes quantization errors as the signal is digitized.
Figure 2. APx525 analog input ranges. (Note: approximately 5 decades removed from horizontal axis below 10-1 V).
Input Range Indicator on the Status Bar
There is an indicator on the status bar in the lower right corner of the APx500 UI that shows the current analog input range(s), as shown in Figure 3. This indicator is updated continuously, even when a measurement or sequence is in progress. This is handy for observing what input range(s) the system is currently using. The example shown is for a 2-channel APx555 audio analyzer. Channel 1 is in the 310 mV range and Channel 2 is in the 620 mV range.
Figure 3. The APx500 status bar, showing the analog input range indicator.
Fixed Input Ranges
The Auto Range feature can be disabled to accommodate measurement applications where auto-ranging is undesirable (more on this later). The feature applies individually to each measurement in Sequence Mode. It can be accessed from the Advanced Settings button available in the Analyzer control group of each measurement (Figure 4). In Bench Mode, the Auto Range feature is common to all measurements. It is accessible from the Input Range button in the Input Configuration control group.
Figure 4. Accessing the Input Range controls from a Sequence Mode measurement.
As shown in Figure 4, when Auto Range is unchecked, the input range controls are enabled. The Minimum Range field is used to specify a fixed range for each input channel, or for all channels when Ranges Track Ch1 is checked. You can specify a fixed range by typing a value in the Minimum Range field. When you do, the system will choose the next range higher than the value you type. For example, entering 1.0 Vrms in the Minimum Range field for an APx555 will set the input range to 1.25 Vrms.
The term Minimum Range is used to reflect the way that fixed ranging works; in this mode, the system will still move to a higher range if the input voltage exceeds the specified minimum range, but it will never move to a lower range. Some users may find this confusing, but the design ensures that input circuitry in lower ranges is still protected from potentially damaging higher voltages.
The lighter blue shaded bars with dashed outline in Figure 2 illustrate how fixed ranging works. For example, when the Minimum range is set to 320 Vrms on an APx555, the system will use that range to measure all voltages from the noise floor up to 320 Vrms. Of course, the residual noise floor will be higher in this range due to dynamic range limitations, etc.
When Not to Use Auto Range
As mentioned above, Auto Range is enabled by default. This is convenient for most users in most situations, because the system will automatically choose the optimum range for each channel, even during a long measurement involving several range changes. However, there are some situations where auto ranging is not the best choice. These include:
1. Production test applications that expose the audio analyzer to a high number of range change cycles. The ranging circuits use relays to switch ranges and these relays have a finite life. Therefore, in production test applications, the recommended practice is to fix the input range of each measurement to the lowest range that can accommodate the measurement. This is also a best practice to minimize test time, because measurements will stabilize faster when range changes do not occur.
2. Acoustic measurements in an area with fluctuating ambient noise levels. In this situation, if Auto Range is enabled, fluctuations in sound pressure level can cause the microphone channel(s) to repeatedly range up and down, causing the ranging relays to “chatter”. This is not only annoying, but more importantly it can subject the relays to unnecessary wear. One way to circumvent this is to “park” the analyzer in a measurement which is set to a fixed range well above the maximum voltage output by the microphones when it is not is use.
3. Log-swept sine chirp measurements (Frequency Response, Continuous Sweep and Acoustic Response) when a range change occurs after the measurement has started. This happens frequently when driving speakers. For accurate results, the acquired waveform must not include a range-change event. If one does occur, the system will automatically repeat the measurement in the higher range. As a result, the user will observe two chirp signals being generated when they have only requested one.
4. Measurements where the user has elected to save the acquired waveform to a file (e.g., Measurement Recorder, Signal Analyzer, etc.). If a range change occurs while writing an acquired waveform file to disk, the system will abort the measurement and notify the user. Otherwise, the scaling of the acquired waveform file would be incorrect.
Another consideration with respect to input ranging is the presence of out-of-band noise in the input signal. For protection purposes, the ranging circuits in APx analyzers are located outboard of more sensitive input stages, including filters. Therefore, if the input signal contains significant levels of high-frequency noise, the noise voltage is factored in to the selected input range, which may cause the input range to be set much higher than required to measure the signals within the audio band. This can be a consideration when testing, for example, Class D power amplifiers. In this case, a passive low-pass filter like the AUX-0025 or AUX-0040 can be used to reduce the high-frequency noise to a point where a lower input range can be used for better measurement accuracy.
When audio analyzer input channels are DC coupled, DC voltages are also factored into input range selection. This is particularly important for APx audio analyzers that have one or two 8-channel analog input modules (APx582, 585 and 586). The AC coupling in these analyzers is implemented with digital filters, not analog circuitry. As a result, the ranging circuits in these analyzers are always exposed to any DC voltage present, resulting in range selections high enough to accommodate the combined DC and AC level of the input signals. If a high-level DC voltage is present and a lower input range is required to analyze the AC component of a signal, an external AC coupling circuit can be used. One such alternative is the AUX-0100 8-channel pre-analyzer filter, which, in addition to reducing high-frequency out-of-band noise, is AC coupled.
That’s it for this discussion on APx analog input ranges. Hopefully we’ve shed some light on this great feature, or at least made it a little more discoverable. In my next post, I’ll talk about analog output ranging and the new fixed output range feature added in version 5.0.1 of the APx500 software.