xoscope buffer hardware questions

[Ben]

Hello,

I was looking at the buffer circuit designed to go with xoscope.

Circuit:
http://xoscope.sourceforge.net/hardware/hardware.html

Description:
http://xoscope.cvs.sourceforge.net/xoscope/xoscope/HARDWARE?view=markup

He has an explanation of the circuit, but I was wondering if someone
could explain how it works a bit more to me.

For example
Does it need to be AC coupled? Could I just remove C1 to measure DC?
How does the impedance coupling work? The input protection?

I hope these aren't too broad or too simple of questions. It vaguely
makes sense to me, but I couldn't explain it to anyone, so I
definitely don't understand.

Thanks for any and all help!
-Ben

 

[Phil Allison]

"Ben"

I was looking at the buffer circuit designed to go with xoscope.

Circuit:
http://xoscope.sourceforge.net/hardware/hardware.html

Description:
http://xoscope.cvs.sourceforge.net/xoscope/xoscope/HARDWARE?view=markup

He has an explanation of the circuit, but I was wondering if someone
could explain how it works a bit more to me.

For example
Does it need to be AC coupled? Could I just remove C1 to measure DC?

** Yes. But be aware that small ( sub 1 uA) leakage currents in the
protection diodes may produce a small DC offset at the input and hence
output of the op-amp.

Low leakage type diode are preferred here.

How does the impedance coupling work? The input protection?

** The 47kohm limits current flow in the 1N914 diodes to a safe value,
even with very large input voltages.

The parallel 100pF cap eliminates any high frequency attenuation due to
op-amp input, diode & stray capacitances coming after the 47 kohm.




....... Phil

 

[Bob Masta]

Hello,

I was looking at the buffer circuit designed to go with xoscope.

Circuit:
http://xoscope.sourceforge.net/hardware/hardware.html

Description:
http://xoscope.cvs.sourceforge.net/xoscope/xoscope/HARDWARE?view=markup

He has an explanation of the circuit, but I was wondering if someone
could explain how it works a bit more to me.

For example
Does it need to be AC coupled? Could I just remove C1 to measure DC?

This amp doesn't *need* to be AC coupled, but the sound card that it's
driving is almost certainly AC coupled... so removing C1 won't help.

How does the impedance coupling work?

C1 and R1 form a high-pass filter with a corner frequency of
about 16 Hz. Many sound cards will go quite a bit lower than
this, so you might want to increase C1 to 0.1 or even larger.

The input protection?

I'm not sure what they have in mind here. I'd be inclined to
leave out R3 and D3 and connect the bottom of D2 right to
-12. That will clamp the inputs to +/-12V max.

I hope these aren't too broad or too simple of questions. It vaguely
makes sense to me, but I couldn't explain it to anyone, so I
definitely don't understand.

Thanks for any and all help!
-Ben

Best regards,


Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!