Data Acquisition System
Figure 1 shows a schematic diagram of an ERT data acquisition system (DAS). The DAS
is housed in a portable unit containing four Eurocardsized
printed circuit boards (PCBs) attached to a 64-way digital backplane (the analogue signals are carried
on the front panels via co-axial cables) which perform
the following functions: measurement, de-modulation
and control; waveform generation and synchronization;
multiplexer control; and power supply
The first three of these cards are connected to a remote voltage-controlled current source sub-system mounted in close proximity to the electrodes in the process vessel. This sub-system contains two small PCBs populated with multiplexers, a voltage-controlled current source and voltage buffers to enable current injection and voltage measurement from a set of 16 electrodes on the vessel. Up to four of these sub-systems can be daisy-chained together to collect data from a total of 64 electrodes. The DAS communicates with the host computer via a fast (57.6 kbaud) bi-directional RS232 serial link to receive logging commands and to transfer logged data for subsequent image reconstruction. The following subsections provide a brief structural overview of the data acquisition system.
Figure 1. The electrical resistance tomography data acquisition system. (MK.1 ERT DAS) 
1. Sine wave Generator Module:
For ERT systems (both single and multiple frequencies) a pure sine wave waveform is utilised to reduce the number of harmonic components in the interrogation of material under investigation. Early data acquisition systems which used analogue sine wave generators were replaced by digital systems ‘driven’ by a quartz-crystal to accurately time the waveforms . Direct Digital Synthesisers (DDS) have been commercially available since the mid-nineteen nineties  and for a relatively low cost produce accurate waveforms with good resolution at low frequencies (200 kHz) compared to analogue generators. Within the sine wave generator module the digital signal is converted to analogue voltage by means of a digital-to-analogue converter which is then fed into the VCCS module.
2. Voltage Controlled Current Source Module (VCCS):
Located in close proximity to the electrode array, a VCCS module consists of multiplexes and voltage buffers which enable current injection and voltage measurement from up to 16 electrodes per plane. A design for a data acquisition system (DAS) was proposed by  (Mk.1 ERT DAS) which communicates with a host computer (for image reconstruction purposes) via a high-speed (57.6Kbaud) bi-directional (RS232) serial link. For increased function, up to a total of four of VCCS sub-systems can be linked together, allowing for measurements from four planes of 16 electrode sensors via a 64-way digital backplane. For their DAS (Mk.1c),  report that overall error was below 0.5%. The multiplexes within a VCCS module ‘share’ current source and voltage measurement between the electrodes. The DAS developed  utilised a single-channel system (as opposed to parallel, which has multiple current and voltage measurement stages). This led to improvements in reliability and lower financial outlay, but resulted in longer acquisition times and phase delay due to the higher complexity of circuitry. In ERT, desirable multiplexer qualities include fast switching speed, reliability, low cost and power consumption
3. Measurement and Demodulation:
Following current injection the output voltage produced requires measuring in order to produce an accurate image from within the process vessel or pipeline. It is here that voltage from the VCCS module is fed into a Programmable Gain Instrumentation Amplifier (PGA). Depending upon the current injection approach used (as detailed below), a range of voltages may be encountered (from milli-volts to volts). In their DAS,  employed the use of the Burr–Brown PGA 202 instrumentation Amplifier and a two pole Butterworth active filter to remove unwanted harmonic components originating from the sine wave generator. The filter at the voltage measurement stage is one of the key factors in terms of speed of data acquisition. An Analogue-to-Digital converter (ADC) is then utilised to transform the signal back to digital format. To optimise signal-to-noise ratio (SNR), the amplitude attenuation and phase shift of the sine wave signal are recovered by synchronous de-modulation techniques (as a result of passing through a resistive medium). A digital demodulation technique (based on digital match filter) has been reported to recover signal close to the maximum theoretical SNR .
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