[SOLVED] Seperating AC from DC and back again

I am working on a circuit where I need to seperate an AC and DC signal and then put it back togther. More specifically, on my input I have a 20-50VDC voltage and a 75mV - 150mV AC voltage. I have designed a highpass filter I want to pass the AC voltage through, but after designing it I realized that I was blocking the DC voltage. I would like to put the 20-50VDC voltage back onto the line after the AC voltages have been filtered. Any ideas? Thanks.

why don't you just start another branch whit a low pass filter in the same point you put the high pass filter?... that way you will have the AC signal through a branch and the DC signal through the other...

Sounds like a splitter. Possible design depends on impedances, currents, load and such. You should specify the problem completely, no need for blind guesses.

I'll try to clarify the problem a little better. I have a two conductor line coming into my circuit. On this line is a digital AC signal with a bit rate of 512kHz with an amplitude of around 75mV (amplitude varies slightly from time to time). My objective is to clean up this line of its lower frequencies, so I created a second order high pass filter with an op amp.

However, in doing so it seems that I cut out the DC component of the signal which is used to power a device at the end of line. I have tried creating a lowpass filter off of the same input in parallel with the highpass to pass DC, but when I apply more DC voltage to the line than the rail voltages of the op amp, everything goes horribly wrong. Right now I'm not using actual components. I am working in PSPICE.

FvM: You said it sounds like a splitter is what I need. I am unfamiliar with this circuit or component. What is it?

Thanks for the replies.

I don't understand, how you could cut out the DC component, but very sure, you should use a passive LC filter when connecting the line. A splitter in general would be a combination of LC high- and low-pass filters.

the splitter is as simple as to branches, one with a high pass filter, where you block the DC signal and get only the AC signal, and the other branch with a low pass filter, where you block the AC and get just the DC...

like this picture:

Hi,
Try this schematic, you need to use High Frequency transformer and choke. The choke should be specified for the DC current in the line.
Regards,
Laktronics

Thanks for the help guys. I used the splitter approach to break out DC and AC, now I just have to put it back together on a single pair of wires. Any suggestions. Thanks again.

For mixing signals again, I guess you could use an analog adder, like the one shown in the picture. It's based on two stages: adder/inverter and inverter.

The first stage just adds Vin1 and Vin2, but inserts a phase delay of 180 degrees, this is: Vout1 = -(Vin1+Vin2)

The second phase is there to correct that phase delay:

Vout2= -Vout1 = Vin1+Vin2


I'm not really sure if this is the optimum solution, but I guess it could do it.