Ac coupling to a Buffer

[analogbeginer]

Hi,

I have to ac couple the signal on to a buffer, but have practical problems in implementing it. My signal is @ 32KHz(Differential & having P_P ~700mV) but at different Dc level. So the filter before giving it to the buffer requires some 8M-Ohms res & a 0.25pF load (since it is differential, we require two res & cap). But this cannot be a practical implementation. Can you suggest some practical way of doing this?

 

[keith1200rs]

I don't understand where your 8M ohms & 0.25pF comes from. Maybe a circuit would help explain it.

Keith

 

[analogbeginer]

It basically formed by the High pass filter sitting before the Buffer(To filter out DC & allow 32KHz signal). 8M & 0.25pF value I got keeping the HPF cut off @ 20KHz.

 

[keith1200rs]

What is the source impedance of the signal?

Keith

 

[analogbeginer]

It comes from a CS stage...so the rds ~1-2M

 

[keith1200rs]

I would suggest you use a DC coupled CMOS buffer or instrumentation amplifier then AC couple, or do a DC level shift.

Keith.

PS. Are you actually desiging an IC for this? If so, 0.25pF shouldn't be a problem. The 8M may be a bit more of a problem due to parasitic capacitance depending on the process.

 

[analogbeginer]

If I have understood it correctly, you are suggesting to use skewed kind of buffer. But, my problem is DC level shift doesn't happen always, which in a way avoids us from using skewed buffer.