Technical Notes: Analogue-to-Digital Converters

When data acquisition hardware receives an analogue signal it converts it to a voltage. It then digitises it with an analogue-to-digital converter, ready for transfer to a computer. This page discusses different types of A-D converters and explains the meaning of A-D hardware specification terms.

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Types of Analogue-to-Digital Converters

Within the Microlink range 5 types of A-D converter are used: successive approximation, dual slope integrating, charge balancing, flash and sigma-delta converter.

Successive Approximation Converter

A successive approximation converter provides a fast conversion of a momentary value of the input signal. It works by first comparing the input with a voltage which is half the input range. If the input is over this level it compares it with three-quarters of the range, and so on. Twelve such steps gives 12-bit resolution. While these comparisons are taking place the signal is frozen in a sample and hold circuit. After A-D conversion the resulting bytes are placed into either a pipeline or buffer store. A pipeline store enables the A-D converter to do another conversion while the previous data is transferred to the computer. Buffered A-D converters place the data into a queue held in buffer memory. The computer can read the converted value immediately, or can allow values to accumulate in the buffer and read them when it is convenient. This frees the computer from having to deal with the samples in real time, allowing them to be processed in convenient batches without losing any data.

Dual Slope Integrating Converter

This converter reduces noise but is slower than the successive approximation type. It lets the input signal charge a capacitor for a fixed period and then measures the time for the capacitor to fully discharge at a fixed rate. This time is a measure of the integrated input voltage, which reduces the effects of noise.

Charge Balancing Converter

The input signal again charges a capacitor for a fixed time, but in this converter the capacitor is simultaneously discharged in units of charge packets: if the capacitor is charged to more than the packet size it will release a packet, if not a packet cannot be released. This creates a pulse train. The input voltage is determined by counting the pulses coming out of the capacitor. Noise is reduced by integrating the input signal over the capacitor charging time.

Flash Converter

A flash converter is the fastest type of converter we use. Like the successive approximation converter it works by comparing the input signal to a reference voltage, but a flash converter has as many comparators as there are steps in the comparison. An 8-bit converter, therefore, has 2 to the power 8, or 256, comparators.

Sigma-Delta Converter

This converter digitises the signal with very low resolution (1-bit) and a very high sampling rate (MHz). By oversampling, and using digital filters, the resolution can be increased to as many as 20 or more bits. Sigma-delta converters are especially useful for high resolution conversion of low-frequency signals as well as low-distortion conversion of signals containing audio frequencies. They have good linearity and high accuracy.

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Specifications of Analogue-to-Digital Converters

Many types of specifications for A-D converters are quoted by hardware manufacturers. Here we try to explain what some of them mean in practice, namely resolution, linearity, offset errors, sample and hold acquisition time, throughput, integration time and re-calibration.

Resolution

The resolution of the A-D converter is the number of steps the input range is divided into. The resolution is usually expressed as bits (n) and the number of steps is 2 to the power n. A converter with 12-bit resolution, for instance, divides the range into 2 ¹², or 4096, steps. In this case a 0-10 V range will be resolved to 0.25 mV, and a 0-100 mV range will be resolved to 0.0025 mV. Although the resolution will be increased when the input range is narrowed, there is no point in trying to resolve signals below the noise level of the system: all you will get is unstable readings.

Linearity

Ideally an A-D converter with n-bit resolution will convert the input range into (2 to the power n)-1 equal steps (4095 steps in the case of a 12-bit converter). In practice the steps are not exactly equal, which leads to non-linearity in a plot of A-D output against input voltage.

Sample and Hold Acquisition Time

A sample and hold circuit freezes the analogue input voltage at the moment the sample is required. This voltage is held constant whilst the A-D converter digitises it. The acquisition time is the time between releasing the hold state and the output of the sample circuit settling to the new input voltage value. Sample and hold circuits are not used with integrating converters.

Throughput

The throughput is the maximum rate at which the A-D converter can output data values. In general it will be the inverse of the (conversion time + the acquisition time) of the A-D converter. Thus a converter that takes 10 microseconds to acquire and convert will be able to generate about 100 000 samples per second. Throughput can be increased by using a pipelined A-D converter, so a second conversion can start while the first is still in progress. Throughput may be slowed down, however, by other factors which prevent data transfer at the full rate.

Integration Time

An integrating A-D converter measures the input voltage by allowing it to charge a capacitor for a defined period. The integration averages the input signal over the integration time, which if chosen appropriately will average over a complete mains cycle thereby helping to reduce mains frequency interference. The throughput of an integrating converter is not the inverse of the integration time, as throughput also depends on the maximum discharge time.

Re-Calibration

Some A-D converters are able to re-calibrate themselves periodically by measuring a reference voltage, and compensating for offset and gain drifts. This is useful for long term monitoring since drifts do not accumulate. If the re-calibrations are set too far apart there may appear to be small discontinuities in the recorded data as the re-calibrations occur. (If you have a reading other than zero for a zero condition, then you have an offset error: every reading will be inaccurate by this amount. When the A-D converter is preceded by signal conditioning circuits offset errors need not normally be considered. Drift occurs because components in the amplifier change over time and with temperature. Drift is usually only significant for people tring to measure low-level signals - a few millivolts - over long periods of time or in difficult environmental conditions.)

You can find these and other technical notes in the
Microlink Measurement and Control Systems Catalogue which is available free from Biodata.

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