B2b - Digital Break-out

Description

The B2b module contains 2 24-bit ADCs (Analog to Digital Converters) which are identical to the ones used in the D4 module. For specifications and more in depth information on the ADCs, see the D4 page. The major difference with the D4 is the addition of an ARM microcontroller which handles all the communication with the ADCs, as seen in the figure below. This allows for exactly timed ADC measurements based on trigger, timers etc. Next to this there is also the addition of 1 Mbit of local storage, which is enough to store twenty thousand samples per ADC channel.

B2b block diagram

Another difference with the D4 modules is the lack of any connectors on the frontpanel. This module is intended to be used with one or two IVVI Rack modules in the neighbouring slots. These modules are then routed to the B2b modules via the backplane, eliminating the need for isolated outputs and external measurement devices like Keithley multimeters. For more information on using IVVI Rack modules in the SPI Rack and in which slots the B2b needs to be placed, check out the IVVI compatibility page.


Triggers

Starting a measurement with the B2b module can happen in one of two ways: a software trigger sent by the Python library, or a hardware trigger on the backplane from either the controller module or a D5b module. 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 ADC starting to measure. It can be set by the user to compensate for any delays necessary. Also notice that the ADC can take more than one sample per trigger input. The amount of samples per trigger can be set by the user. Care should be taken not to set a sample amount per trigger that would take longer to measure than the time between triggers. This will lead to data loss.

B2b trigger timing

Filter Settings

The ADCs inside the module are equipped with two built in digital filters: a Sinc3 filter and a Sinc5+Sinc1 filter. These filters determine to a large extent the performance of the module. By changing the filter setting, the user sets the Data Rate, Resolution, Bandwidth and the 50 Hz rejection. For a complete list of filter types and settings, see the 24b_ADC_Filter.xlsx file below.

  • 24b_ADC_Filter.xlsx (13 ko)
    • Use Case: spectrum analysis

    Thanks to the configurable data rate and the local storage, the B2b module can be used to search for interference frequencies. Running at a high sample rate (83 ksps maximum) and using the maximum amount of storage (20000 samples per channel) allows for an FFT up to roughly 40 kHz with a resolution lower than 10 Hz. If more resolution is required, multiple traces can be used or a lower samples rate can be set. This allows the user to look for interference without changing anything in the measurement setup.

    Use Case: lock-in replacement

    Using the B2b module in conjunction with a D5b module allows for lock-in type measurements: the D5b generates toggling voltages and triggers, and the B2b module uses these trigger to measures synchronously. The user can then do the necessary subtraction and averaging of the data from the B2b 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 D4 module for ADC specifications. The table below only lists specifications that are new or different to the B2b module.

    Parameter Value Units Conditions
    Reference Frequency 10 MHz Both internal and external reference frequency
    Trigger Holdoff Time 400 s Maximum
    100 ns Resolution