Test and Measurement Technical Data

D/A line-up

‘D/A line-up’ (‘Digital / Analogue line-up’) is useful when testing equipment which has both analogue and digital inputs and/or outputs. ‘D/A line-up’ is the analogue level equivalent to digital full scale. For an A-to-D converter, the D/A line-up is the analogue amplitude required at the A to D converter input in order to produce an output amplitude of 0dBFS (digital full scale). Similarly, for a D-to-A converter, the D/A line-up is the analogue output amplitude that would result from a digital input amplitude of 0dBFS.

When we measure such a ‘cross-domain’ device, it can be helpful to think of analogue signal amplitudes in terms of the digital units (or vice versa). If we want to feed an A-to-D converter a voltage which causes the digital output be 1dB below full scale, we would normally have to work out the appropriate voltage and enter that value for the signal generator. With the dScope III, enter ‘-1dBFS’ into the signal generator, and it will output the correct analogue voltage providing that we have set the desired ‘D/A line-up’. Conversely, if we are driving a D-to-A converter, we can set the signal generator’s digital signal amplitude in terms of the output voltage it will produce. Similarly, the signal analyzer can display analogue amplitudes in digital units or vice-versa.

Thus the dScope D/A line-up setting is simply a conversion factor used between the digital and analogue domains.

The D/A line-ups for generator and analyzer are actually settable independently, although they are tied together by default so that if you adjust one, the other follows. This is helpful in most situations, for example when testing converters, or digital equipment with analogue inputs and outputs with unity through-gain. However, for digital equipment with unaligned analogue I/O (i.e. with gain or attenuation), you will need to split the two D/A line-ups and set them separately for the generator and analyzer. An example might be a digital power amplifier with analogue inputs and outputs. The conversion factor (D/A line-up) for the amplifier's inputs will not be the same as for it's outputs. To split the generator and analyzer D/A line-ups, go to the "Utility" menu, select "Options" and then the "Miscellaneous" tag. Un-check the "Lock together generator and analyzer D/A line-up" checkbox. You can then set them independently.

When setting the D/A line-up, there entire range of analogue absolute amplitude units are available. For example, if you are measuring a digital power amplifier you might want to set the D/A line-up(s) in Watts. You could then, for example, set the signal generator in resulting Watts, but have the analyzer display the output amplitude in "dBFS".

Beware of controls which change the input sensitivity or output gain of the equipment-under-test (EUT). It is important that these are not altered after the D/A line-up is set, otherwise the domains will become misaligned, giving incorrect results. Beware also of devices with non-flat frequency responses or frequency-varying clipping amplitudes - for these, D/A line-up may only be correct within certain frequency limits. Amplitude-related inconsistencies may also result if the EUT has any kind of compression or limiting, is driven beyond it's capabilities or is measured in the presence of significant noise.

In summary, D/A line-up is a useful tool, but it is important to remember that it is only a conversion factor. Measurements made using D/A line-up are only as accurate and applicable as this conversion factor.