Product Description

The DAC 670 is a single width CAMAC module, which contains two completely separate opto–isolated 18 bit Digital to Analog Converters (DACs) with 18 Bit absolute accuracy. A 17 Bit version is available at reduced cost.

Each converter circuit includes facilities such as references and power supplies so as to make them totally separate. However, the CAMAC power supplies may be selected, if desired, at the loss of isolation.

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, and read via CAMAC Command.

In addition, four optically isolated digital 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 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 to the opto–isolator. This is very useful when testing ADCs to know when the output is ’True’. This takes about 68mS for a 20V step (50mSfor 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 Bit is about 38mV. A 30m ohms wire and connector resistance with 5mA load would give 150mV 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 DACs have four operational signal ranges, which may be selected via CAMAC Command. The first two are Unipolar 0V to 10V and 0–5V, if the output is being externally powered by floating supplies, this may be referenced so as to be 0 to +10V (5V) or 0 to –10V (0–5V), coded in straight binary. The third and fourth range is ±10V or ±5V coded in 2’s Complement binary.

As separate trim components are used for each range, so there is no change in set–up between ranges. However, due to the use of relays, there is a time delay of 10mS between going from one output range to the other.

Accuracy to 18 bits absolute at +25°C ±5°C after a 10 minute warm up period.

Opperating temperature range +5°C to +45°C max.