Product Description

The VDD 2670 is a single width, dual height VME slave board, which contains two completely separate opto–isolated 18 bit Digital to Analog Converters (DACs) with 18 Bit absolute accuracy.

Each converter requires a 21–24V stable smoothed voltage, which should be provided by an external linear power supply. This ensures full isolation from the VME bus.

The data for the DAC is stored into counting registers formed from parallel data entry up/down synchronous counters.

These counting registers may be loaded, incremented or decremented by software, and read via VMEbus Commands.

In addition, four digital non-isolated inputs allow either register to be incremented or decremented via the front panel.

Each time the data stored in the DAC data register is altered an automatic micro sequence is started that transfers the data from the register into the main DAC in such a way as to minimise transmission glitches from the analog output.

This circuit also triggers a digital monostable multivibrator to provide a digital signal for each DAC for the whole time the Analog value from the DAC is in transition, changing from one value to the next, a second monostable is triggered to form an output ’True’ pulse. This signal is not isolated. This is very useful when testing ADCs to know when the output is ’True’. This takes about 68uS for a 20V step (50uS for 10Vstep).

The data from the data registers is passed through opto–isolators and fed into the two separate hybrid laser trimmed DAC chips which contain data latches and current–to–voltage amplifiers.

The Analog output is passed in voltage form from the DAC outputs into a precision power amplifier working at unity gain. This has been included so that a fully sensing four wire output system can be provided, together with allowing high output currents without self–heating effects on the DACs.

As can readily be seen to get near 18 Bit performance over any reasonable length of output lines, a fully sensing circuit is necessary. Consider the case, when used on +10V full scale 1 L.S.B. is about 38uV. A 30m ohms wire and connector resistance with 5mA load would give 150uV error, (4 bits).

To help get over these problems, the output amplifier has two stages. The first is a very temperature stable and low noise high gain operational amplifier with a +10mA drive capability. the second is a high power unity gain power driver. A wire link allows the internal amplifier alone to be used for low current work and the power buffer for high power work. The Power Output Drivers are capable of driving 50mA. The noise performance is better without the buffer amplifier.

The DACs have four operational signal ranges, which may be selected via VMEbus command. Unipolar 0V to 10V, if the output is being externally powered by floating supplies, this may be referenced so as to be 0 to +10V or 0 to –10V; +/- 10V;  0-5V;  +5V to -5V.