Sigma-delta board offers faster A-to-D conversion

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Micromech is now supplying Delta Tau's new Acc36UK sigma-delta board that offers higher performance analogue-to-digital processing for the UMAC system.

Higher resolution, faster conversion speeds and better common mode rejection (CMR) were all important objectives to be addressed in the 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 includes giving high CMR. The 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 next feature to consider was the analogue-to-digital converter; 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, 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 625kHz or 625kSps (625 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 microcontroller and thereafter into dual-ported RAM and onto the UBUS within the UMAC system.

Another useful feature is the optically isolated clock from the microcontroller, which allows user-selectable update times. Together with three internal filters, the new Acc36UK therefore benefits from improved resolution, reduced noise when operating at 24 bits, and updates all 16 channels to the dual RAM port during the PMAC's servo update time.

In addition to all these improvements, this innovative design also includes eight hardware interrupts that enable data to be captured in real time.

Micromech Systems is an authorised systems integrator for Delta Tau high-level, multi-axis motion control products.

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