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ADS42JB69EVM: ADS42JB69EVM Tail Pulse Digitization

Christopher Harms
Prodigy 20 points
Part Number: ADS42JB69EVM

Howdy,

I've recently inherited an ADS42JB69EVM board in our lab group and I'm experiencing an issue when attempting to digitize incoming information from an emulator.  The emulator is a CAEN DT5810D providing information via a BNC cable with an adapter to the SMA fitting on the EVM board.  Ultimately, the goal is to digitize tail pulses coming out of a preamplifier hooked to a radiation detector, then pass them through a TSWTSW14J10 to a KC705.

Whenever I try to digitize a tail pulse, the resulting signal exhibits bipolar characteristics (see below).  I have tried inserting a 50-ohm BNC terminator on a T-junction with the input signal and the BNC-to-SMA adaptor in case it was an impedance problem, but that simply reduces the magnitude of the signal and does not change the shape.  The configuration file should also be found below.

I've included examples of the source pulse and the resulting digitization seen through HSDC Pro below; any guidance you can provide as to what might be going wrong would be appreciated.

Digitization of tail pulse with tau_rise = 200ns, tau_decay = 20us:

Source for above:

Digitization of rectangular pulse with width = 20us

Source for above:

Configuration File

LMK04828 
0x00 0x10
0x02 0x00
0x100 0x0C
0x101 0x55
0x103 0x00
0x104 0x20
0x105 0x00
0x106 0x70
0x107 0x11
0x108 0x6C
0x109 0x55
0x10B 0x00
0x10C 0x20
0x10D 0x00
0x10E 0x71
0x10F 0x66
0x110 0x00
0x111 0x00
0x113 0x00
0x114 0x00
0x115 0x00
0x116 0x08
0x117 0x00
0x118 0x6C
0x119 0x55
0x11B 0x00
0x11C 0x20
0x11D 0x00
0x11E 0x78
0x11F 0x61
0x120 0x0C
0x121 0x55
0x123 0x00
0x124 0x00
0x125 0x00
0x126 0x71
0x127 0x01
0x128 0x00
0x129 0x00
0x12B 0x00
0x12C 0x00
0x12D 0x00
0x12E 0x08
0x12F 0x66
0x130 0x0C
0x131 0x00
0x133 0x00
0x134 0x00
0x135 0x00
0x136 0x08
0x137 0x00
0x138 0x24
0x139 0x03
0x13A 0x01
0x13B 0xE0
0x13C 0x00
0x13D 0x08
0x13E 0x00
0x13F 0x00
0x140 0x00
0x141 0x00
0x142 0x00
0x143 0x31
0x144 0xFF
0x145 0x00
0x146 0x18
0x147 0x1A
0x148 0x10
0x149 0x54
0x14A 0x33
0x14B 0x16
0x14C 0x00
0x14D 0x00
0x14E 0x00
0x14F 0x7F
0x150 0x03
0x151 0x02
0x152 0x00
0x153 0x00
0x154 0x78
0x155 0x00
0x156 0x0A
0x157 0x00
0x158 0x96
0x159 0x00
0x15A 0x64
0x15B 0xD4
0x15C 0x20
0x15D 0x00
0x15E 0x00
0x15F 0x0B
0x160 0x00
0x161 0x01
0x162 0x44
0x163 0x00
0x164 0x00
0x165 0x0F
0x166 0x00
0x167 0x00
0x168 0x0F
0x169 0x59
0x16A 0x20
0x16B 0x00
0x16C 0x00
0x16D 0x00
0x16E 0x13
0x17C 0x15
0x17D 0x0F
ADS42JB69 
0x06 0x80
0x07 0x00
0x08 0x18
0x0B 0x00
0x0C 0x00
0x0D 0x90
0x0E 0x90
0x0F 0x00
0x10 0x00
0x11 0x00
0x12 0x00
0x13 0x00
0x1F 0x7F
0x26 0x06
0x27 0x03
0x2B 0x00
0x2C 0x00
0x2D 0x13
0x30 0x20
0x36 0x00
0x37 0x00
0x38 0x00
FPGA 
0x04 0x00
0x05 0x00
0x0E 0x00
0x0F 0x00

  • 0
    TI__Mastermind 18065 points

    Christopher:

    Interesting.  Is the first picture with the decaying pulse accurate? I do not see any "bipolar" abnormalities in that plot.

    In the second set of pictures with the square wave pulse, there is a discrepancy.  Is the "inversion" happening when the input pulse transitions from high to low?

    The digitized data is not an exact replica of the input signal.  I'm not exactly sure what is the expected output.  Is it possible that the sharp high-to-low pulse transition is indeed creating a negative pulse that decays back to "0"?

    -RJH

  • 0 in reply to RJ Hopper
    Prodigy 20 points

    There isn't a way to zoom any further out on the input pulse diagram from the emulator, but that input tail pulse is unipolar (pulse values only above the baseline) and that digitization output exhibits a significant negative lobe below the baseline.

    As for the second unipolar rectangular pulse, yes, it does seem that the zero-crossing happens when the input goes low. That digitization appears like some sort of differentiation is occurring.  I did some back-of-the-envelope math and the decay constant governing that tail is around 0.6-1 microseconds in case that gives you a clue.

    Ideally the ADC should simply digitize the tail pulse and the rectangular pulse without distortion beyond any high-frequency aliasing, should it not?  There are no analog components beyond cable connections between the pulse emulator and the EVM input.  Running the test pulses to oscilloscopes without being connected to the EVM produces the ideal results (the inputs) on the scopes, but the oscilloscopes produce the same waveform as seen in the HSDC pictures when they are connected in parallel with the EVM.

    We've put three heads together on this end and haven't come up with a cause for that discrepancy yet.

    -Chris

    P.S. I have swapped out every connector and cable on the analog side of the equation with uniform results, so the problem doesn't lie there.

  • 0 in reply to Christopher Harms
    TI__Guru* 91947 points

    Chris,

    This might be related to the transformers on the ADC board if this is a very short pulse. Can you provide this input differentially using a diff amplifier? If so, you would then install J11, remove R47, remove T1 and T2 and short the appropriate pads the allow the inputs to get through.

    Regards,

    Jim