A.2. ADC and the Autoranging Voltmeter

Very often in measuring circuit characteristics, it is necessary to plot some voltage response over a wide range of input-voltage stimulation. An automatic means of adjusting the voltmeter sensitivity through the sweep is thus required. When set for the full measurement range (limit), the 12-bit analog-to-digital conversion (ADC) in the DAQ has either a 2.44 mV (unipolar, 10 V full range) or a 4.88 mV (bipolar, 20 V full range) resolution.

However, the range of the DAQ is software selectable. So, for example, suppose that in the sweep we are measuring 5 mV and only positive voltages. The DAQ can be programmed for a high limit of, for example, 100 mV and a low limit of 0 V (unipolar), such that the resolution is 24.4 μV.

The Project A LabVIEW library, ProjectA.llb, contains a preprogrammed VI for setting automatically the range. The Diagram of the VI, Voltmeter.vi, is shown in Fig. A.12. As indicated, the range is set through the high- and low-limit inputs to AI Acquire Waveform.vi, for example, 10 V (high limit) and 0 V (low limit) in the diagram of our autoranging voltmeter as shown in Fig. A.12.

The autoranging voltmeter is programmed as follows. First, a copy of AI Acquire Waveform.vi is Saved As..., modified for dc only and renamed as AI Acquire Avg.vi. The modification consists of taking the average of the array of samples and providing for a single-point output of the average value of the samples. AI Acquire Avg.vi is programmed to operate in selectable bipolar or unipolar mode. Thus, this feature is included in Voltmeter.vi.

This VI is then used as a subVI in the autoranging dc voltmeter. Upon running Voltmeter.vi, the execution starts with the subVI AI Acquire Avg.vi on the left; the high limit set at 10 V, and with the bipolar mode, the lower limit is –10 V. Thus the voltage being measured is first sampled with minimum sensitivity but this is sufficient to obtain an approximate value of the voltage. The initial measurement value is then processed through a selection scheme to decide which limit should be set into the final measurement subVI that is again AI Acquire Avg.vi. The limit choices are given in Table A.2. These are stored in an Array Digital Control as seen on the left in the diagram. The While Loop compares the measured value with each limit value starting at the top. The loop halts when the comparison finds the smallest possible limit. Bipolar or unipolar are set according to the sign of the initially measured value.

A factor of 2 of improved resolution is available with the unipolar case. As noted, the valid limit settings and associated resolutions are shown in Table A.1. With the autoranging voltmeter, during a sweep, the resolution is maintained, through the sweep, at roughly a fixed percentage of the value of the numbers being measured.

Figure A.13 shows a measurement of a sine-wave from a function generator. This was obtained in LabVIEW using the ac voltmeter that is discussed in a following unit. The voltage waveform is sampled at the two limit settings, 100 mV (a) and 2 V (b). The peak of the sine-wave is 10 mV. It is clear that the resolution is not adequate at the higher limit setting. In an autoranging version of the ac voltmeter, the best limit setting is automatically assigned.