# PC SOUND-CARD SCOPE INTERFACE FACILITATES DC RESTORATION

To start with I must appreciate acknowledge and thank ‘Christian Zeitnitz’ for the fantastic Sound-card Oscilloscope & signal generator software available at his website  on which this Instructable is based.

While working with the PC sound-card oscilloscope and signal generator I found the following limitations:

1.   A low Input Impedance of the order 10 Kilo Ohms
2.   Input voltage range limited to 2.8 V p-p
3.   AC coupling of the I/O signals
4.   Output voltage limited to 2V p-p
5.   Sampling rate limited to 44kbps

Not much can be done about the sampling rate as it is a limitation of the sound card but the scope interface presented here attempts to improve the other factors.

Most significant is the addition of circuitry to estimate the positive and negative peak DC values of the signal and use this to offset the waveforms providing a realistic DC-coupled scope display.

Let me illustrate this with an two Examples

## Step 1: 555 Timer Example

Waveform before correction

The 555 Timer IC when operated as an astable oscillator from 5V provides a square wave of 0-5V and the waveform at the timing capacitor varies from 1/3 to 2/3 of 5V.

The first screen shot shows the timer IC 555  AC coupled waveforms as captured by the PC sound-card. It can be seen that the zero settles at the average value. (The input has been scaled by 1:10 to remain within the input voltage limits.)

The square wave shows a peak value of 171mV instead of 5V and the capacitor waveform appears to be centered at 0V with a peak value of 66.59mV.

The interface circuit provides two peak-hold circuits which provide the positive and negative peak DC value of the input waveforms. Using a multimeter to measure these values gave 468mV and 283mV as the positive peak values for the square and capacitor waveforms respectively.

We can compute that the square wave needs to be offset by 468mV -171mV = 297mV and the capacitor waveform by 283mV - 66mV = 217mV.

Waveform after DC restoration

The second  figure shows the waveforms after entering the offset values 297mV and 217mV into the offset boxes for CH1 and Ch2.

After DC restoration the square wave varies from 0 to 455mV and the capacitor waveform from 1/3 to 2/3 of 5V.

This would be the display we would see on a scope with DC coupling.

ajoyraman (author) 5 months ago

Yes you can use 2 LM741's instead of the LM747 but you will need to make the appropriate connections.

The 741/747 need +/- 9 V supply typically.

This scheme should comfortably work for signals down to 100mV.

5 months ago

Best part of this scope is that it comes with internal variable gain system. I was planning to make a separate variable gain circuit with another scope instructable and I came across this instructable. A big thank you for that :-)

opampboy6 months ago

Hi,

1-Can I use 2 LM741 instead of single LM747 , I hope the function is same?

2- What is the min Vcc required for these amplifiers? I want to make a scope for low voltage signals(<2V).

stanfk541 year ago

Great Job. Very clear and helpful instructable. You are my hero!!!
I wish you had data file for this PC board, do you?

deserticus3 years ago
how can i get the schematic from here?
ajoyraman (author)  deserticus3 years ago
Thank You for your interest. I am providing a higher resolution version of the schematic file here.
ajoyraman (author) 3 years ago
Thanks for the great response!

I find the DC offset in the X 10 gain is high and have changed the LM747, U1 to the LF353 using an 8Pin to 14Pin converter. The LF353 pins need to be bent upwards delicately and the converter leads soldered on. Plug this into the U1 socket. It works fine now.
Brunomaster3 years ago
Some video of the device working?