Other Parts Discussed in Thread: OPA2677, DRV134
I'm currently in the process of selecting a high current/speed CFA in order to drive a bus with the following specifications :
-characteristic impedance is 50 ohm
-the transmission medium is a coaxial cable that can be up to 100ft
-transmitter voltage range is between 6-9Vpp
-the bus bandwidth is 20Mhz
-the transmitter end must be terminated with a 50 ohm resistor and the bus end must be terminated with 50 ohm terminator to match the bus impedance
The following screenshot shows the configuration I'm using for the THS3491
*Note : transformer is 1:2
I'm using a 1:2 transformer because the headroom on either supply is high and I want to supply it using +/- 7.5V.
Using the above circuit in practice, the output voltage is just fine (9Vpp).
However, when I insert the terminated coaxial cable a the transformer end, the voltage goes down to about 6.5Vpp and about 6Vpp on the terminated end of the cable.
I'm suspecting it is due to a impedance mismatch from the source impedance towards the load impedance.
Here is an explanation about my feedback network :
I'm driving both inputs with a 1.8Vpp signal with a 50% duty cycle for testing purpose
The feedback network is configured with a 2.5 gain, since the desired gain is 1.25 by considering that the output resistor/bus impedance act as a voltage divider
The series resistor (R22) is selected according to transformer impedance ratio : Zp/Zs = (Np/Ns)exp2.
My question goes as follow : Is there any work-around to fix the impedance mismatch or issues I'm currently having, any help would be greatly appreciated.