Board Varieties

2013jun13 In total there are 5 different varieties of the ADC16x250-8 board as shown below. The main difference is in the type of analog input: differential or single ended.

Of the 5 varieties only 2 are current and available from CASPER's standard supplier Digicom. The 3 other board varieties are deprecated/obsolete and are NOT available from Digicom.

Current Varieties

Differential Analog Inputs (RJ-45)

• ADC16x250-8 RJ45 rev 1 Q2-2013
  • present
    • 16, 8 or 4 differential inputs over a total of 4 RJ45 CAT7 cables
    • run time programmable operating modes :
      • 16 inputs by 250 MSPS by 8 bits (default)
      • 8 inputs by 500 MSPS by 8 bits
      • 4 inputs by 1000 MSPS by 8 bits

Single Ended Analog Inputs (SMB)

• ADC16x250-8 coax rev 2 Q2 2013
  • present
    • 16, 8 or 4 single-ended inputs over a total of 16,8 or 4 50 ohm coax cables.
    • Connector is SMB due to 1U chassis faceplate space limitations.
    • run time programmable operating modes :
      • 16 inputs by 250 MSPS by 8 bits (default)
      • 8 inputs by 500 MSPS by 8 bits
      • 4 inputs by 1000 MSPS by 8 bits

Deprecated/Obsolete Varieties : NOT FOR NEW DESIGNS

Differential Analog Inputs (VPORT) : NOT FOR NEW DESIGNS

• ADC16x250-8 differential rev 1 NOT FOR NEW DESIGNS
  • 16,8 or 4 differential inputs over 1 Samtec VPORT (VPSTP/VRDPC) cable
  • run time programmable operating modes :
    • 16 inputs by 250 MSPS by 8 bits (default)
    • 8 inputs by 500 MSPS by 8 bits
    • 4 inputs by 1000 MSPS by 8 bits

• ADC16x250-8 differential rev 2 NOT FOR NEW DESIGNS: never built.
  • 16,8 or 4 differential inputs over 1 Samtec VPORT (VPSTP/VRDPC) cable
  • run time programmable operating modes :
    • 16 inputs by 250 MSPS by 8 bits (default)
    • 8 inputs by 500 MSPS by 8 bits
    • 4 inputs by 1000 MSPS by 8 bits

Single Ended Analog Inputs (SMB) : NOT FOR NEW DESIGNS

• ADC16x250-8 coax rev 1 NOT FOR NEW DESIGNS
  • 16, 8 or 4 single-ended inputs over a total of 16,8 or 4 50 ohm coax cables.
  • Connector is SMB due to 1U chassis faceplate space limitations.
  • run time programmable operating modes :
    • 16 inputs by 250 MSPS by 8 bits (default)
    • 8 inputs by 500 MSPS by 8 bits
    • 4 inputs by 1000 MSPS by 8 bits

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ADC16 Sample Rate vs Virtex-6 MMCM Limitations

The ADC16 data is sent to the ROACH2's Virtex 6 FPGA over high speed serial links. The timing of these high speed serial data streams can be quite tight at high frequencies. The FPGA provides de-serializer blocks to handle this, but they rely on a high quality clock to properly capture the data bits. The ADC16 provides a clock to the FPGA that is based on the sample clock, which is typically a high quality clock. The ADC line clock (i.e. output clock) drives a Multi-Mode Clock Manager (MMCM) which derives several other clock signals of related frequencies and phases needed by the ADC receive logic and much of the user design as well. The MMCM has two modes of operation: "low bandwidth mode" and "high bandwidth mode". The exact meaning of these terms is not well defined in the Xilinx documentation.

Configuring the ADC16's MMCM such that it runs in "low bandwidth mode" leads to unreliable data capture from the ADC despite extensive probing showing that the input clocks were of very high quality. Configuring the MMCM to run in "high bandwidth mode" resulted in vastly superior performance in terms of data capture reliability.

The MMCM configuration involves several parameters that interact with each other and the input clock frequency and the MMCM's limits. In order to be in high bandwidth mode, the frequency at the MMCM's Phase Frequency Detector (PFD) must be between 135 and 450 MHz for the -1 speed grade FPGAs on the typical ROACH2. Its Voltage Controlled Oscillator (VCO) frequency is limited in the -1 FPGAs to the range 600 to 1200 MHz and the minimum value for the MMCM's CLKFBOUT_MULT_F attribute is 5. Thus, for -1 speed grade Virtex-5 FPGAs, the maximum frequency at the PFD is really limited to 240 MHz (1200/5). To further complicate things, the DIVCLK_DIVIDE attribute cannot be set to 3 or 4 if the input clock frequency is greater than 315 MHz.

For the standard "-1" speed grade FPGAs used on the ROACH2, the MMCM cannot be configured to run in "high bandwidth mode" for all ADC sample clock frequencies, so some sample clock frequency ranges are not usable.

The combined effects of all these constraints for the -1 speed grade FPGAs are summarized in the following table (all frequencies are in MHz). Note that the frequency of the ADC line clock is 0.5x, 1x, or 2x the sample clock frequency depending on the ADC16 input mode chosen by the user.

Line Clock	DIVCLK_DIVIDE	PFD Freq	CLKFBOUT_MULT_F	VCO Freq	MMCM BW
60-100	1	60-100	10.0	600-1000	LOW
100-135	1	100-135	8.0	800-1080	LOW
135-240	1	135-240	5.0	675-1200	HIGH
240-270	2	120-135	8.0	960-1080	LOW
270-480	2	135-240	5.0	675-1200	HIGH
480-500	5	96-100	10.0	960-1000	LOW

The ADC sample clock frequencies corresponding to line clock frequencies are shown here for various numbers of inputs (all frequencies in MHz):

Line Clock	Msps (16 in)	Msps (8 in)	Msps (4 in)	Recommended	Supported
60-100	30-50	60-100	120-200	NO	NO
100-135	50-67.5	100-135	200-270	NO	NO
135-240	67.5-120	135-240	270-480	YES	NOT YET
240-270	120-135	240-270	480-540	NO	NO
270-480	135-240	270-480	540-960	YES	YES
480-500	240-250	480-500	960-1000	NO	NO