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THS6002 “Dual Differential Line Drivers and Receivers”. Output: 15 MHz
I made a PCB with the THS6002 chip "Dual Differential Line Drivers and Receivers". I copied the application example of figure 59 in the datasheet (page: 34), with three differences: the transformer is 1:1, the resistors are 22 Ohm (not 12.5) and the power supply is +-5 VDC (not +-15VDC).
When I turn on the circuit without inputs or with inputs (2 VAC 500Hz), sometimes it works correctly and sometimes the receivers have a sinusoidal output with 400 mVpp and 15 Mhz, which only disappears when I apply a 50 Ohm resistor between any receiver input and ground.
A few things. If the transformer is 1:1, and the secondary impedance is still 100ohms, as shown in the datasheet schematic, the output series resistors of the drivers should be 50ohms, not 22ohms, in order to appropriately match the impedance across the transformer. It would also be helpful to know a bit more about your test setup. Do you have a receive signal generator on the secondary side of the transformer? Scope shots would be useful as well.
Initially it sounds as though your circuit has some sort of oscillation. Please confirm that there are no unintended capacitive connections in the feedback paths of any of the amplifiers. Additionally, you can find recommended layout considerations in the THS6002 EVM User's Guide: http://www.ti.com/lit/ug/slou018/slou018.pdf
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