THS6002 "Dual Differential Line Drivers and Receivers". Output: 15 MHz

by Juanma   Last Updated August 09, 2019 22:25 PM

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).

enter image description here

link

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.



Answers 1


You have scaled your resistors by 22/12 = 1.8:1 but by halving the turns ratio you have changed your impedance by a factor of 4. (N^2). Arguably your resistors should be 50 ohms if you are driving a standard impedance line.

Note the droll comment at the bottom of page 36. Viz

  • A common error for the first-time CFB user is to create a unity gain buffer amplifier by shorting the output directly to the inverting input. A CFB amplifier in this configuration is now commonly referred to as an oscillator.

This does not apply to you directly BUT any changes away from the designed impedances of the example make you more liable to experience oscillations such as they mention.

The combination of 12.5 ohm resistors, line impedance reflected vi a 2:1 transformer and the 1l &2k resistors form a balanced 4 wire to 2 wire "hybrid" intended to minimise transmit signal seen on the receive ports.

Hybrids:

This looks useful - ADSL Line Driver/Receiver Design Guide, Part 1
and Part 2 also looks very useful

Wikipedia

Elliott sound tutorial

About 2002 from TI. May be informative. An ADSL Integrated Active Hybrid Circuit


It would be useful for you to explain what you are doing and why in the overall context of your application. It is not obvious what you are trying to achieve or in what context you are doing this. I assume that anyone playing with something this complex has a reasonably good grasp of what they are wanting to do. You have also reduced the line drive voltage possible by a factor of 2:1 by changing the transformer turns ratio but have also reduced the drivers' peak to peak capability by a factor of more than 3:1 so overall it seems at first glance like you could not drive anything like a normal length of line. Is this consistent with what you are trying to do?.

Russell McMahon
Russell McMahon
March 05, 2012 11:21 AM

Related Questions



Regulating power on load

Updated August 28, 2019 15:25 PM


Single Supply in Op-amps

Updated September 17, 2019 11:25 AM

PCB Layout validation

Updated March 11, 2019 16:25 PM