Digitiser front ends need the right inputs

December 01, 2014 // By Arthur Pini , Greg Tate & Oliver Rovini
Digitisers used for capturing both low-speed and high-speed signals need to match their inputs to the fixed input range of their ADC (analogue to digital converter). To best use these instruments, you need to understand the tradeoffs.

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