V5-0 - Input clamping diode not clamping
|Assignee:||Tomasz Wlostowski||% Done:||
The I-V function of the Schottky diode D6 (HSMS-2802 - input clamping) is not steep enough to clamp the input to +3.3V. For example, when driving with 5 V pulses from a 50-ohm terminated source, the diode current is approx 30 mA resulting in a dropout of ~1.5 V - so the diode is not clamping the signal at all. On the other hand, Schottky diodes with steeper I-V function have higher junction capacitance.
The previous version of the board used a BAR66 RF PIN diode, which did clipping well but its' long recovery time (0.7 us) was causing spurious TDC events (see issue 515). A workaround in VHDL was provided for that purpose. Therefore, I recommend replacing D6 back with BAR66.
Updated by Erik van der Bij on 01 Oct 2012 at 16:56
- Subject changed from Input clamping diode not clamping to V5 - Input clamping diode not clamping
- Status changed from New to Assigned
- Create in EDMS a V5-1 that includes this change.
This change is not included in V5-1!
V5-1 still contains HSMS-2802, while a BAR66 should be mounted.
V5-2 should solve this.
- V5-2 cards will have a BAR66 diode. DEM will produce a fixed version.
- pre V5-2 cards: the input stage was stressed for 2 weeks with +20 V pulses (100 kHz, 1us). No problems were discovered despite higher clamping voltage of a HSMS-2802 diode.
V5-2 design with BAR66 available in the EDMS. Closing issue.