Higher resolution, faster conversion speeds, better common mode rejection (CMR) were all important objectives to be addressed in a new ADC board design. To achieve the desired performance the analogue input device chosen was a single channel instrumentation amplifier type AD8221. This high performance device has a number of advantages over operational amplifiers (Op Amps) which also includes giving high CMR. The CMR is the ability to cancel common signal such as noise that appears on the differential inputs and as a result only differential signals are amplified.

The next feature to consider was the analogue to digital converter and for this the AD7763 Sigma-Delta converter from Analog Devices was selected. The output of the AD8221 is fed to the A/D converter type AD7763. This device is a 24-bit converter with ‘in-built’ gain and offset registers, as well as a series of low-pass filters. These filters can be configured to optimise the output bandwidth relative to the input clock frequency resulting in the maximum update frequency of 625 kHz or 625kSps (Kilo Samples per second). Furthermore, depending on the number of filters implemented various other data rates can be achieved. The data from all the channels is then synchronised and shifted serially into hardware registers on a micro controller and thereafter into dual ported RAM then onto the UBUS within the UMAC system.
Another useful feature is the optically isolated clock from the micro controller, which allows user-selectable update times. Therefore together with three internal filters the new Acc36UK has improved resolution, reduced noise when operating at 24 bits and will update all 16 channels to the dual RAM port during PMAC’s servo update time.

In addition to all these improvements this innovative design also includes eight hardware interrupts enabling capture of data in real time.

Micromech Systems is an authorised systems integrator for Delta Tau high level, multi-axis motion control products, if you wish to find out more about these exciting products then contact Stirling Morley on 01376 333333

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