[SI-LIST] Trace Capacitance
- From: johny leon <leonjony@xxxxxxxxx>
- To: SI LIST <si-list@xxxxxxxxxxxxx>
- Date: Mon, 14 Nov 2011 09:19:15 +0530 (IST)
Gurus,
I am designing a system with two Analog Boards and one Digital Board. Digital
board is on the backplane and the two analog boards connect to it through
Samtec SEAM + SEAF-RA connectors.
Analog board is having data acquisition system(DAS) and the ADC is working at
100MHz. Its 12 Bit PARALLEL data, which is going through the SEAF-RA+ SEAM
connector to Digital board's Spartan-6. Analog Board trace length is 1.5" and
the digital board trace is having 6" length.
Now comes my question: As per ADC data sheet (LTC2252), Output capacitance
shouldn't increase 10pF to avoid digital interference to the sensitive analog
circuitry. Some online tools shows that this trace length (6"+1.5" + Conn) will
give more than 10pF with my board stackup. Now I'm forced to use Buffers(6pF
Cin) at the output of the ADC on the Analog Board to avoid this. I have 8
ADCs(100MSPS) on the analog board in addition to 4 DACs(100MSPS) and don't have
enough space to add the buffer and series resistors(source termination). ADCs
output structure is very good with enough drive strength and in-built 43E
series resistor. But the buffers with in-built series resistor which I
found(74LVC162244A) are having less drive strength at 1.8V of -2/-4mA(ADC to
FPGA interface voltage). Simulation (Allegro PCB SI) shows good results for
both circuits (with buffer and without buffer).
1) Should I worry about the output capacitance of the traces along with conn
capacitance and add buffer?
2) Digital board is having two connectors to connect two Analog Boards. Analog
board are similar and connects to two Spartan-6 FPGAs. These board are close to
FPGAs and DDR3 memories on the Digital Board. ADCs and DACs on the analog
boards will come to close to the FPGAs and DDR3s once the two boards are mated.
Should I provide some shield connection option(GND-SLOT footprint to solder
shield) on the Analog board to avoid potential Digital interference?
Thanks in advance!
Johny.
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