S
Steve
- Jan 1, 1970
- 0
Hi,
I'm trying to supply a clock to one of the 3.3V GCLK inputs on a Xilinx
Virtex-II FPGA. Due to other requirements I need to use a 5V VCO (TI
TLC2932) to drive this input, but I'm having some problems.
I thought about using a voltage divider to convert the 5V VCO output to
3.3V, but the PCB trace between the VCO and the FPGA is reasonably long,
so I need to include some termination. This is where I run into
difficulty.
I was going to terminate the line with a 33Ohm series resistor near the
VCO. This would be fine if the VCO had a low output resistance, as I could
just use something like a (33 - VCutput_resistance)Ohm resistor in
series and a 75 Ohm to ground, which should give me 3V with 30Ohm looking
back into the source from the transmission line.
However, the VCO provides a 5V square wave at 2mA, so would I be right in
saying that it's got a 2.5k source impedance?
If that is correct then this is where I get stuck. I can divide the
signal down to 3V with a 5k pull down at the output, but the equivalent
resistance of this will be 1.7k. Alternatively, I could use a 33Ohm
pulldown on the output which would give me my 33 Ohm terminating
resistance, but the signal would only be 65mV.
Does anyone have any hints on how I can source terminate this VCO to
33Ohms whilst level translating from 5V to 3.3V?
Steve
I'm trying to supply a clock to one of the 3.3V GCLK inputs on a Xilinx
Virtex-II FPGA. Due to other requirements I need to use a 5V VCO (TI
TLC2932) to drive this input, but I'm having some problems.
I thought about using a voltage divider to convert the 5V VCO output to
3.3V, but the PCB trace between the VCO and the FPGA is reasonably long,
so I need to include some termination. This is where I run into
difficulty.
I was going to terminate the line with a 33Ohm series resistor near the
VCO. This would be fine if the VCO had a low output resistance, as I could
just use something like a (33 - VCutput_resistance)Ohm resistor in
series and a 75 Ohm to ground, which should give me 3V with 30Ohm looking
back into the source from the transmission line.
However, the VCO provides a 5V square wave at 2mA, so would I be right in
saying that it's got a 2.5k source impedance?
If that is correct then this is where I get stuck. I can divide the
signal down to 3V with a 5k pull down at the output, but the equivalent
resistance of this will be 1.7k. Alternatively, I could use a 33Ohm
pulldown on the output which would give me my 33 Ohm terminating
resistance, but the signal would only be 65mV.
Does anyone have any hints on how I can source terminate this VCO to
33Ohms whilst level translating from 5V to 3.3V?
Steve