Input & Output Connection

The software uses the sound card as the signal input & output device. Signals to be tested should be connected to either the "MIC" jack or the "Line in" jack. Generated signals will be output via the "Speaker" jack or the "Line out" jack.

Typically, the "MIC" jack of a sound card has an input impedance in the range of 600 Ω ~ 50 kΩ (card dependent). Its ADC full-scale input voltage (1/2 Vpp) is in the range of 1 mV ~ 500 mV (card dependent), and is adjustable through the software volume control under Windows Control Panel or the hardware volume control (if any) provided by the sound card. Normally it only allows a single channel input.

The "Line In" jack of a sound card has an input impedance typically around 1 kΩ ~ 50 kΩ (card dependent), and the ADC full-scale input voltage (1/2 Vpp) of this connection is in the range of 500 mV ~ 2 V (card dependent), and is adjustable through the software volume control under Windows Control Panel or the hardware volume control (if any) provided by the sound card. Generally, the "Line In" should be used as the primary input connection as it offers better Signal-to-Noise Ratio (SNR) and bandwidth.

The "Line Out" connection of a sound card has an output impedance typically in the range of 20 Ω ~ 500 Ω (card dependent) and can output signal up to 2 V typically (card dependent). It has better SNR than the "Speaker" connection.

The "Speaker" connection of a sound card has a typical output impedance of 4Ω ~ 8 Ω (card dependent) and output power of 2 W (card dependent). The headpone connection of a sound card has a typical output impedance of 4Ω ~ 100 Ω and output power of 100 mW.

For input connection, the simplest way is to directly connect the signal under test to the sound card "Line In" or "MIC" jack (see the figure below). However, this kind of connection requires the tester to be extremely careful to ensure the input signal is within the allowable range before connecting. Otherwise the sound card or even the PC may be damaged. The maximum allowable input voltage is about 3V (card dependent).

In order to prevent the sound card from excessive input voltage, the following limiter circuit can be added. The two Silicon diodes will clamp the input voltage at about 2 × 0.65 = 1.3 (V). If the sound card ADC full scale is affected, one more Silicon diode can be added in series to clamp the input voltage at about 3 × 0.65 = 1.95 (V) instead. The protection is limited to ± 50 V maximum (also depending on the resister's value and maximum allowable current, and the diode's maximum allowable current). If the amplitude of the signal to be measured exceeds the allowable range of the sound card, it must be attenuated before connecting.

In order to get good measurement result, the output impedance of the circuit under test must be lower than the input impedance of the sound card. Otherwise the signal cannot be properly transferred from the circuit to the sound card. The input impedance of a sound card can be improved by adding a pre-amplifier. The pre-amplifier is responsible for impedance conversion, signal amplification or attenuation and input protection.

The following figure shows the output connection for the Signal Generator. The resister is used to prevent accidental short circuit of the output. It can be omitted if you are careful enough. As the output impedance of a sound card is very low, there should not be any impedance matching problem when connected to external circuits.

It should be noted that for many sound cards (typically the internal sound card of a desktop PC or a laptop PC with built-in AC power supply adapter), the ground line of input and output is connected to the mains earth. This is not a problem if the circuit under test is floating (i.e. isolated from the mains earth). Otherwise, you must make sure that the ground line is connected to a point on the circuit that is also at earth potential.

The above mentioned connection circuits and pre-amplifiers are NOT supplied with the software unless otherwise stated explicitly during purchasing.