Functional Description
MEN Mikro Elektronik GmbH
24
5
Functional Description
5.1
Power Supply
Power supply to the logic part is done via the carrier board. The necessary voltage
(only +5V) is supplied to the M-Module from the carrier board. The isolated ±15V
and +5V supply voltages are generated on the M-Module by DC/DC converters.
Even though these DC/DC converters are highly efficient (up to 98 %) they may
develop some heat on the module. It is therefore essential to ensure adequate
ventilation of the assembly. As the DC/DC converters are switched power supplies,
they may produce some noise. Therefore the M36N is meticulously designed for
maximum noise immunity.
5.2
Principle of Data Acquisition
The M36N is an automatic data acquisition system with up to 16 measuring digits. It
is based on ANALOG DEVICES Inc.'s AD7634 chips. These are fast 18-bit A/D
converters. The component uses a charge redistribution successive approximation
register (SAR) architecture to achieve a conversion rate of more than 100kSPS
(8,1µs total conversion time). Overall performance is optimized by digital correction
of internal nonlinearities through gain and offset calibration in the FPGA.
The component provides a serial data output format, which is used on the M-
Module to transfer data across digital isolators, saving components and power.
When the A/D converter shifts out the digital information, the configuration
(channel number, gain etc.) from the FPGA is transferred to the isolated analog
circuit.
Every configuration (channel, gain and mode) is calibrated in production with a gain
and offset value. These values are stored in the on-board Flash memory. During
measurement the serial values from the ADC are converted into parallel 18-bit
words which are then corrected using the stored calibration values. After that the 16
most significant bits (MSB) are stored in a doublebuffer dual-ported RAM array.
The double-buffer mechanism guarantees continuous homogeneity of the values,
also when a new data word is being received while the value is read.
Up to 16 values can be stored in the dual-ported RAM. There are two parameter
bytes for each 16-bit word. One determines the configuration for one measurement
(e.g. gain, channel number of the multiplexer etc.), and the second byte contains a
pointer to the configuration for the next measurement. So it is possible to define a
closed chain - a ring - of measurements.
The measurements will be carried out without any user interventions. After defining
the parameter chain, the user can read the most recent values available in the dual-
ported RAM array. Thus, absolutely no software overhead is necessary in normal
operation to obtain the measuring values.
The time for handling one complete chain is approximately 8.1µs multiplied by the
number of values per chain, i.e. a maximum of 130µs for 16 values. After one
complete measurement cycle an interrupt may be generated if masked.
