D5b - 8 Channel DAC + uC

Description

The D5b module contains 8 18-bit DACs (Digital to Analog Converters) which are identical to the ones used in the D5a module. For specifications and more in depth information on the DACs, see the D5a page. The big difference with the D5a is the addition of an ARM microcontroller which handles all the communication with the DACs, as seen in the figure below. This allows for exactly timed DAC updates based on triggers, timers etc.

D5b block diagram

DAC Output Modes

At the moment of writing, each DAC can be set in one of two modes: DC mode or toggling mode. In DC mode it behaves as in the D5a: the moment the user sets the DAC from the PC the output gets updated and stays that way until the next update. In toggling mode the DAC output will toggle between two specified output voltages $n$ times, where $n$ is set by the user. The counting starts after the module receives a trigger, which can be generated by the controller module or via software. On each toggle, the D5b module will generate its own trigger on the SPI Rack backplane. Other units can listen for this trigger and use it for synchronous operations. The figure below shows all of the signal in a timing diagram. Important to notice is time $t_1$, this is the trigger holdoff time: the time between receiving the trigger and the DAC output starting to toggle. It can be set by the user to compensate for any delays necessary. Outside of the toggling, the DAC will be set to a third voltage which the user also has to set.

D5b trigger timing

Use Case: lock-in replacement

Using the D5b module in conjunction with one or more B2b/D4b modules allows for lock-in type measurements: the D5b generates toggling voltages and triggers, and the ADC module uses these trigger to measures synchronously. The user can then do the necessary subtraction and averaging of the data from the ADC modules. Two advantages of using this setup over a traditional lock-in are: multiple measurement units can easily be used with one source, and the start of the measurement can be perfectly synchronised with an external trigger. This was the use case this module was designed for: trigger on the switching of samples using a cryogenic multiplexer controlled by the U2 module. For a traditional lock-in the switching of the sample would always happen somewhere randomly in the measurement cycle. This means that the measurement has to be discarded, and time is wasted. By synchronising the measurement with the switching, this is no longer an issue.

Reference Clock

The module has to possibility to run on either an internal or an external 10 MHz reference. If multiple modules are placed in one SPI Rack and they need to be synchronized, it is recommended to set them to an external reference. The connector for the external reference is found at the back of the SPI Rack.

External clock: Never turn off or disconnect the external clock when modules are using it. This will halt the microcontrollers completely.

Specifications

See the D5a module for DAC specifications. The table below only lists specifications that are new or different to the D5a module.

Parameter Value Units Conditions
Reference Frequency 10 MHz Both internal and external reference frequency.
Trigger Holdoff Time 30 μs Minimum.
400 s Maximum.
100 ns Resolution.
Toggle Time 5 μs Minimum. Output frequency limited by output filters at 100 kHz
400 s Maximum.
100 ns Resolution.