Four-Channel Oscilloscope Adaptor

Posted on Feb 5, 2014

This circuit enables you to display four signals simultaneously using only one channel of your oscilloscope. Essentially, it switches each of the inputs through to the output in turn, with some signal massaging in between. As shown, it is suitable only for low-frequency signal measurement and does not include over-voltage protection at the inputs.

Four-Channel Oscilloscope Adaptor
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Each input circuit is identical, utilising one amplifier from a TL084 quad op amp package. Looking at channel 1, the input signal is attenuated by a factor of 10 by the 100k © and 10k © resistors before arriving at the non-inverting input of IC1a. A 15k © resistor in series with the op amp output along with a 10k © resistor to ground provide additional attenuation. Vertical (voltage axis) adjustment is made with VR2, which sets the gain of the amplifier. This is used to calibrate or scale the displayed signal against the actual input voltage level. Using the values shown, the gain can be adjusted from unity to about a factor of 26. Note that the output of the op amp must be limited to ± 10V so that the voltage into the 4-channel multiplexer (IC2) does not exceed ± 5V. Therefore, with a gain of unity, the input voltage can range from ± 100V, whereas with a gain of 26, it must not exceed about ± 3. 85V. VR1 applies a positive or negative DC offset to the output of the op amp. This can be used to compensate for op amp input offset voltage. It can also be used to shift the vertical position of the trace on-screen to provide multiple trace separation. Note, however, that any offset will consume part of the available output swing and therefore limit signal measurement "headroom". Each op amp output is connected to IC2, a 4-channel analog multiplexer. The logic levels on "S0" & "S1" (pins 9 & 10) determine which input channel is...

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