Preamplifier Prototype I

Prototype passed a signal, though not without an oscillation at ~350MHz. Pictured is the result of sending a 500mV square wave with a 1ms period and a 10% duty cycle.

Preamplifier Prototype II

New PCB layout uses a 4-layer board design, and strictly adheres to board layout suggestions in TI datasheets for both the OPA847 and THS3201. This design does not oscillate. A -200mV DC offset exists when the test pulse cable is connected. If undesirable, can be corrected with a simple offset nulling circuit connected to U1-3 (non-inverting input of OPA847).

Update: 9/26/2011 An oscillation has been discovered at 700MHz with a peak to peak voltage of ~120mV. Removal of the .36pF capacitor (C3) in the feedback loop of the OPA847 solves this problem. Smaller oscillations still exist at ~20mV at close to 100MHz, and are currently being addressed.

Update: 9/28/11

  • Oscillation at ~100MHz with 2nd opamp disconnected (R11 removed)


  • R11 changed to 1k
  • 100 ohm resistor was placed in series between the output of the photodiode and input of OPA847 (U1-2).
  • 100pF bypass capacitor added in parallel to both power supplies

10pF input  pic 1  pic 2  pic 3

  • R11 changed to 4k - severely decreased oscillation, added 30mV offset.

CompactPCI Power Board

The CompactPCI Power Board is a two slot wide 6u cPCI backplane that provides ATX power and RS232 breakouts for a single XCalibur 4301 module. Please note that on the version 1 board the ATX 5VSB is connected to ATX 5V. This will prevent the ATX power supply from starting up. Therefore you must not supply the 5VSB to the version 1 board. It is recommended to cut the 5VSB wire ATX pin 9 purple (see This problem is resolved in version 1.1.

The original idea was to design a one-slot backplane-like board which would hold two connectors to mate with the J1 and J2 CompactPCI connectors on the processor board to supply power.

Other Documents

Design Notes

ADC3925 input is 50 ohms, 0dBm full scale (225mV). 10 bits implies 0.2mV or so LSB.