



	Sound card buffer circuit (buff.fig) for use with [x]oscope

	by Tim Witham <twitham@pcocd2.intel.com>


	NO WARRANTY

THIS CIRCUIT IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
THE ENTIRE RISK AS TO THE QUALITY, PERFORMANCE AND SAFETY OF THE
CIRCUIT IS WITH YOU.


	HISTORY

I was probing a circuit with a piece of wire to the line in on my
sound card when I happened to notice that the probing was loading my
circuit down.  This implies my sound card's line input may have less
than ideal input impedance for looking at arbitrary signals.  I
eliminated the loading by using a less intrusive 10x (10 times)
attenuating oscilloscope probe.  But then the signal was too small to
see on [x]oscope.  So, for fun, I actually built a little line in
buffer circuit to solve these problems and more.


	THE BUFFER CIRCUIT

The result is shown in the file buff.fig (xfig source format) or
buff.ps (postscript format).  The purpose of this circuit is to:

1) Look like a real oscilloscope to the circuit under test.  That is,
to have an input impedance of 1 Megohm in parallel with 20 pF.

2) Have a x1 / x10 switchable amplifier to produce the same amplitude
with 1x and 10x probes respectively.

3) Provide a line level trim control for adjusting the amplitude to an
optimum level for the sound card's input.

4) Provide high voltage protection to the sound card and PC for those
times when you accidentally probe your 60Hz line-voltage AC!

The circuit accomplishes these goals.  It is adapted from part of a
circuit shown in figure 4.74 of the excellent book "The Art of
Electronics" by Paul Horowitz and Winfield Hill, 2nd ed., 1989.  Most
of the parts can be purchased at your local electronics store for
under $30.00; see the parts list below.  Two identical circuits are
built, one for each of the sound card's left and right line inputs.


	CIRCUIT EXPLANATION

C1 is for AC coupling to remove any DC first.  R1 and C2 set the input
impedance seen by the circuit.  In retrospect, C1 should probably have
been on the other side of R1 and C2 so as to not affect the input
impedance at all.  But it seems to work fine this way too.

D1, D2 and R2 limit the input voltage to the range -12V to 12.7V for
the high voltage input protection.  R2 must be 1/2 Watt for 150 Volt
protection.  D3 and R3 produce a clamp voltage at -11.3V to keep the
op-amp in common mode range.

The TL082 is a dual JFET input op-amp providing the necessary high
input impedance.  A similar JFET input part may work just as well.
R5, R4 and S1 provide the x1 or x10 amplification.

Finally, R6 is used to trim the output to an acceptable level for the
soundcard. It can be brought all the way down to ground to get the
same effect as the "GND" input coupling of a real oscilloscope.


	ASSEMBLY AND CONNECTION

See buff2.fig or buff2.ps for my circuit layout.  I built two of the
circuits on the back of a full size drive bay cover.  The op-amp and
surrounding components were mounted on a small circuit board (see
pcb.fig / pcb.ps) positioned in the center of the drive bay cover.
The BNC connectors, switches and potentiometers were mounted through
the cover on either side of the circuit board.  The result is under
1/2" thick so an internal drive can even fit behind it.

A -12V power connector may not be readily available in your computer.
The standard disk drive power connectors are +5 and +12.  I added an
alternate power connector in my system by tapping my power supplies'
-12, GND, and +12 wires.  Be careful not to get these mixed up!

Finally, I used a Y adapter cord to connect the two phono jacks to the
1/8" stereo line input on the sound card.


	PARTS LIST

Qty	Part#	Description

1	IC1	TL082 Dual JFET input operational amplifier

2	R1	1M,	1/4 Watt, 5% resistor
2	R2	47k,	1/2 Watt, 5% resistor
2	R3	4.7k,	1/4 Watt, 5% resistor
2	R4	3k	1/4 Watt, 5% resistor
2	R5	27k,	1/4 Watt, 5% resistor
2	R6	100k,	linear potentiometer

2	C1	.01uF, 1kV	ceramic disc capacitor
2	C2	20pF		ceramic disc capacitor
2	C3	100pF		ceramic disc capacitor

6	D1-D3	1N914 or 1N4148 silicon switching diode

2	S1	SPST mini toggle switch

1		circuit board, (1/2 RS 276-159)
1		8-pin DIP socket
2		1/4" knob for R6
2		female BNC connectors, panel mount
2		female RCA connectors, in-line type
		tap-in and power connectors
		connecting wire, power wire


	DISCLAIMER

This circuit was designed and built by me on my own time and
equipment.  My employer has absolutely nothing to do with it.


If you have comments on this thing, please let me know.

	Tim Witham <twitham@pcocd2.intel.com>
