Digitiser front ends need the right inputs
Digitisers must minimise loading of the device under test and provide appropriate coupling. Additionally, filtering may be needed to reduce the impact of broadband noise. All of these features are provided by the instrument’s "front end," which includes all the circuitry between the input and the ADC. Figure 1 shows a block diagram of a modular digitiser. Each input channel has its own front end, shaded in green.
Figure 1. The block diagram of a Spectrum M4i.44xx PCI Express 14/16 bit modular digitiser where the front ends for each channel are shown in green. The front end provides appropriate input coupling and termination along with range selection and bandwidth limit filtering.
Maximising the versatility of a modular digitiser requires that the front end circuits have the following capabilities:
– Multiple input ranges offering the ability to capture a wide variation in input signal levels and at the same time minimising noise and distortion to maintain signal integrity.
– A selection of input termination to offer matching impedances or minimised loading with a high impedance input.
– A choice of coupling modes to offer either AC or DC coupling as needed.
– Filtering to minimise noise and reduce harmonic components if present.
– Internal calibration to maximise accuracy.
Input termination
A measuring instrument should properly terminate the signal source. For most RF measurements, this is generally a 50-Ω termination. A matching termination minimises signal losses due to reflections. The figures of merit for the 50Ω matching can be return loss or VSWR (voltage standing wave ratio). Both indicate the quality of the impedance match.
If the source device has a high output impedance, then it is more properly matched with a 1 MΩ high-impedance termination that minimises circuit loading. The 1 MΩ termination also lets you use high-impedance oscilloscope probes. The probe would increase the input resistance of the digitiser further, decreasing the loading on the circuit. Keep in mind that the probe will also decrease the signal level into the digitiser.
Because there is a tradeoff between convenience and signal integrity in designing with selectable input impedance, some modular digitiser suppliers only offer 50-Ω termination. Thus, if you need a high-impedance termination or both high impedance and 50 Ω you should verify that the manufacturer does offer both.
Input coupling
Input coupling in a measurement instrument offers the ability to AC couple or DC couple the measuring instrument to the source. DC coupling shows the entire signal, including any DC offset (non-zero mean signals). AC coupling eliminates any steady-state DC mean value. AC coupling is useful for measurements such as ripple measurements on the output of a DC power supply. Without the AC coupling, the DC output would require a large signal attenuation that would make the ripple harder to accurately measure. With AC coupling, a higher input sensitivity can be used, which results in a better ripple measurement.
The key specification for AC coupling is its low frequency cutoff (lower -3 dB point) of the AC coupled frequency response. This specifies how much a low-frequency signal will be attenuated by the AC coupling. AC coupling is related to the recovery time, the time needed for the input level to settle after a change in the DC level applied to the instrument. Generally, the lower the cutoff frequency, the larger the coupling capacitor and the longer the settling time.
Some modular digitisers offer only AC or DC coupling, but not both. Again, this is an engineering tradeoff to reduce complexity because a digitiser with fixed coupling doesn’t need relays or switches. Again, your application will determine if a fixed or selectable coupling is acceptable.
Input voltage ranges
The digitiser’s ADC generally has a fixed input range. The simplest interface is to have a single input with a fixed input range matching that of the ADC. While simple, this is not very practical in a measuring instrument unless the single range is exactly the one you need. To bring the input signal swing into the range of the ADC requires either an attenuator or an amplifier.
An attenuator is a simple voltage divider, generally resistive, which reduces the input signal’s amplitude. When designed with quality components, it generally won’t significantly affect signal integrity. One issue that appears with attenuators in the signal path is that the instrument’s internal noise amplitude scales (relative to the input of the attenuator) with the front end attenuation. Thus, your digitiser’s internal noise level is 58 µV rms and you add a 10:1 attenuator, then the noise level, referenced to the input, becomes 580 µV because you’ve reduced signal amplitude but not the digitiser’s internal noise level.
Amplifiers are another story. Even when properly designed, they generally introduce noise into the signal path. This is somewhat compensated for by the fact that the digitiser’s internal noise decreases by the gain of the amplifier when referenced to the input. Amplifiers can also introduce distortion products that further degrade signal integrity. Amplifiers also have a fixed gain-bandwidth product. If you attempt to increase an amplifier’s gain, then the bandwidth falls proportionally. You can see this on high sensitivity ranges where the bandwidth is reduced.
next: high-frequency and buffered ranges
Input voltage range selection is a critical area in digitiser design because it can greatly affect signal integrity. At the same time, it offers greater flexibility to the user in matching the available signal amplitudes to the digitiser input range. Suppliers offer a variety of approaches to handling this tradeoff. They vary from offering a single fixed input range, which shifts design work from the digitiser manufacturer to the end user who needs to care for correct amplification by himself, to offering multiple input paths. The multiple input paths combine a "buffered" path offering the greatest versatility with regard to input ranges and termination, with a "50Ω" high frequency (HF) path, which provides the highest bandwidth and the best signal integrity with a fewer number of input ranges and a fixed 50-Ω termination.
The block diagram in Figure 2 shows the architecture of a modular digitiser, which includes a dual input path.
Figure 2. The block diagram of a Spectrum Instrumentation M4i.44xx Modular Digitiser includes dual input paths, selectable coupling, 50Ω and 1 MΩ termination impedance, filtering, and internal calibration.
Table 1 contains a comparison of the characteristics of each path for the 14 bit, 500 MSample/sec version (Model M4i.445x).
Table 1. A comparison of the HF and Buffered path characteristics in a 14 bit, 500 MSsample/sec modular digitiser.
The HF path is optimised to deliver the greatest bandwidth with the best signal fidelity. The buffered path provides the greatest versatility by offering more and broader choices of input voltage ranges. Users can select the input path which best matches their measurement requirements.
Figure 3 compares the response of the HF and buffered paths to a 256 step ramp on the digitiser’s 500 mV range. The input is a single step in each path (note that adjacent steps have been selected for each path so they do not overlap). The peak-to-peak noise in the buffered path is higher than in the HF path. The HF path design has been optimised to minimise noise and despite having twice the bandwidth of the buffered path, it still shows less noise. The price paid for this performance is a reduction in the number in input ranges available and the necessity to use the 50-Ω termination.
Figure 3. A comparison of differences in the response of the HF and buffered input paths to a 256 step ramp waveform. One step of the ramp is displayed. Note the higher peak-to-peak noise level in the buffered path despite the bandwidth of the buffered path being half that of the HF path.
The advantage of the higher signal integrity of the HF can also be seen in the frequency spectrum of sine wave acquired by the digitiser using both input signal paths. Figure 4 shows the FFT (fast Fourier transform) of the acquired signals through each input.
Figure 4. Comparing the frequency spectrum of the Buffered (left side) and HF (right side) paths. Note that the HF path has a spurious free dynamic range of 80.9 dB compared with 60.7 dB for the Buffered path.
Cursors mark the spectrum peak and the peak of the highest spur. The HF path has a spurious free dynamic range of 80.9 dB compared with 60.7 dB for the buffered path. The noise baseline is lower in the HF signal path case.
Improving signal integrity
No matter which signal path you choose, there are some general rules to help get the best signal integrity. First, use as much of the input range as possible. If the signal’s amplitude is stable, then select an input range that uses at least 90% of that range, but don’t overdrive the ADC. If you exceed the full scale range the result will be distortion or clipping which will produce unwanted harmonics and decrease signal integrity.
Bandwidth limiting filters, if available in your digitiser, can help reduce noise. In the digitisers used in this article, there is an analogue 20 MHz low-pass filter in the front end that can be switched in to limit the digitiser bandwidth. If the input signal has no content above 20 MHz, then using the filter will improve the signal to noise ratio of your acquisition by reducing noise above 20 MHz.
Built-in calibration
Digitiser channels are factory calibrated. Modular digitisers operate in PC environments and may be subject to variations in power-supply voltages and temperature. To compensate for those variations, the software driver can provide routines for an automatic onboard offset and gain (buffered signal path only) calibration of all input ranges of the buffered signal path. Each digitiser card contains a high-precision built-in calibration reference that helps keep the digitiser calibrated despite change in the environment and aging. Good practice is to ensure you perform a calibration once the digitiser is operational and has had sufficient time to reach a stable operating temperature. This is typically reached after 10 to 15 minutes.
The front end of the modular digitiser needs to provide all the features necessary to ensure accurate, repeatable measurements. Multiple input ranges, AC/DC coupling, filtering, and built-in calibration all help to ensure maximum signal integrity and accuracy. A well designed front end will allow the user to condition the input signal appropriately, ensuring it covers as much of the ADC range as possible without over-driving it. Only then can the digitiser achieve the best measurement accuracy and precision.
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