Quick design explanation
Here is a quick explanation of the design where each different part is described in a few words. For more information, read the paper 'Realisation of a low noise acquisition board : design choices'. The schematics PDF can be found here: document:"ADC board design PDF". All the design has been done on KiCAD. For a better experience, it is advised to download the last KiCAD project from the repository. For example, this would give the reader the possibility to open the components'datasheets with three clicks (double click on the component and then on the "Documentation" tab).
The whole board
WholeBoard.jpg
On this card will be connected all the future time measurement elements. So, the goal of this board is only to convert the analog data in numerical ones. As we want the best possible resolution for the ADC, we pay extremely attention to the noise generated by other components. This board is a carrier card for the MicroZed board which will be used to process incoming data.
The design schematics are divided into 5 different parts :
> # the first ADC,
> # the second one,
> # the clock splitter circuitry,
> # the MicroZed Board connections,
> # the power supply
This schematic is only to show the different connections between these different parts. In addition to the power lines and data lines, we can see the SPI bus between the MicroZed and both the ADC.
The two ADC
As the schematics for the two ADC are the same, we only put one of them here.
ADC1.jpg
We have two inputs for each ADC. On each input are one SMA connector for ground referenced signals and two others for differential ones. All inputs have an impedance around 50 ohm on a two decades bandwidth (200kHz to 20MHz).
A particular attention is given to the power supply decoupling. Multiple capacitors with multiple values are placed to reduce the impedance on a large bandwidth and then avoid the noise to disturb the measurements. This has been done as well for the analog power lines as for the digital ones. To have the lowest ESR and the highest SRF, we only use ceramic capacitors (C0G/NP0 when possible, otherwise we use X7R).
The clock signal arrives from the splitter on a differential line.
The clock splitter
clockSplitter.jpg
This part is used to synchronise both the ADC by giving them the same
clock signal. This clock splitter allows us to use different clock
frequencies in a range from 5 MHz to 125 MHz. This component can also
filter the input clock signal. The parameters of that filter can be set
using the FILTA
and FILTB
inputs. To be able to change the filter
and to adjust it to our needs, we put these inputs on jumpers.
The power Supply
PowerSupply.jpg
We need enough power to supply all the components on the board but also the MicroZed board. Furthermore, these components needs different voltage levels.
The first stage is a buck converter. It is used to convert the voltage from 12 V to 5V DC. These 5 V are needed by the MicroZed board. We then converter these 5 V in a 3.3 V one. This last is used to supply the clock splitter. Finally, two LDO are used to regulate the digital and analog power supplies of the ADC.
We put two different connectors for the power : a DC barrel jack and a MINI DIN 4. To be sure to have the right voltage polarity, we put a double rectifier bridge using Schottky diodes. These diodes are chosen to avoid a too high voltage drop at the input. A diode is also placed next the barrel jack to protect the circuit in case the polarity is inverted.
The LM43603 has been chose because its switching frequency can be set up to 2.2 MHz. In addition, this frequency can be changed using an external clock. These high frequencies are easier to shield than lower ones. A further reason for choosing this buck converter is that its switching MOSFETS are inside the chip package, reducing considerably the "hot loop".
Instead of using big capacitors at the input and the output of the converter, we decided to use different values to catch the high harmonics and then improve the noise rejection.
External connections
FCI.jpg
The connections to the MicroZed board are done via its FCI connectors. All these inputs can be configured to work in LVDS. As the output of our ADC are already in LVDS, it fits ideally with our project.
In addition to these connections to the ADC, we also provide the board other connections to be able to use the other MicroZed GPIO.