Digital data acquisition system block diagram

A data acquisition system (DAS) collects information about one or more physical quantities, converts it to digital form, and then processes, displays and stores it. A digital DAS converts the signals to digital form early, so the data can be handled by a computer.

Block diagram of a multi-channel digital DAS

 Transducer 1 -> [Signal cond.] -+
 Transducer 2 -> [Signal cond.] -+-> [Analog MUX] <------+
 Transducer n -> [Signal cond.] -+         |             |
                                           v             |
                                      [Amplifier]        |
                                           |       [Control logic]
                                           v       [  and clock  ]
                                   [Sample & hold] <-----+
                                           |             |
                                           v             |
                                        [ ADC ] <--------+
                                           |
                                           v
                              [Digital processor / computer]
                                           |
                          +----------------+--------------+
                          v                v              v
                      [Display]   [Recorder/printer]  [Storage]

Function of each block

1. Transducers: convert physical variables (temperature, pressure, strain) into electrical signals.
2. Signal conditioning: amplifies, filters (including the anti-aliasing low-pass filter), linearises, provides excitation to the transducers, and isolates.
3. Multiplexer: connects the channels one at a time to a single ADC through analog switches, which reduces cost.
4. Sample-and-hold (S/H): freezes the signal level during A/D conversion so that the value does not change.
5. A/D converter: converts the held analog value into a binary number (e.g. a successive-approximation type).
6. Control logic and clock: generate the channel address, the S/H command and the conversion-start signal, and synchronise them.
7. Digital processor / computer: scales, linearises, analyses, checks limits, stores and controls.
8. Display, recorder, printer, storage: present the output to the operator; the data can also be sent over a network.

Other points

• The sampling rate per channel is fs = total ADC rate / number of channels, and it must satisfy the Nyquist condition for each signal.
• A single-channel DAS needs no multiplexer.

Uses: industrial process monitoring, laboratory testing, medical monitoring, weather stations.