Sigma-delta board offers improved A-to-D processing

Higher resolution, faster conversion speeds and better common mode rejection (CMR) were all important objectives in the new analogue-to-digital conversion (ADC) board design. To achieve the desired performance, the analogue input device chosen was a single-channel instrumentation amplifier, the AD8221. This high-performance device has a number of advantages over operational amplifiers (OpAmps), including giving high CMR.

CMR is the ability to cancel common signals - such as noise that appears on the differential inputs - and, as a result, only differential signals are amplified.

The analogue-to-digital converter selected was the AD7763 Sigma-Delta converter from Analogue Devices. The output of the AD8221 is fed to the A-to-D converter. 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. The maximum update frequency of the

AD7763 is 625kHz or 625kSps (kilo samples per second). Depending on the number of filters implemented, various 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 and onto the Delta Tau UBUS within the company's UMAC motion controller. An optically isolated clock from the micro controller allows user-selectable update times.

Together with three internal filters, the new Acc36UK has improved resolution and reduced noise when operating at 24 bits and will update all 16 channels to the dual-port RAM during the UMAC's servo update time.