Oscilloscope with USB audio adapter 2

The article about my first oscilloscope made with USB audio adapter can be found on link [1]. After that, I built a new hardware with additional features:

-AC/DC signal input capability
-selector for input voltage domain
-two channel signal generator with AC, DC and TTL output

With these modifications, I made a data acquisition module with maximum 48 kHz sampling frequency and 20 KHz bandwidth

The USB audio adapter we used is recognized by Windows 7 and a generic driver is installed automatically (the driver is included in Windows 7). Usually we need to install a driver provided by the manufacturer of the USB module/chipset before connecting it to the computer.

Figure 1
Figure 2
Figure 3

The USB audio adapter included in our project contains one A/D converter used as input for oscilloscope and two D/A converters used for signal generator outputs. Looking at the images included bellow in our article, you will see the signals we used in our project:
- MICIN - microphone input, will be used for analog input of the A/D converter;
- VREF - reference voltage, used as reference for analog input. According to datasheet is a 2.25V output of a band gap circuit (actually 2.16V measured on my board);
- LOL - output left, will be used as output of the D/A converter, channel A;
- LOR - output right, will be used as output of the D/A converter, channel B;
- VBIAS - output voltage for microphone bias. This signal must be disconected from the input.

Figure 4
Figure 5
Figure 6
Figure 7

There are some modifications made to the original printed circuit board - PCB (see above Figures 4 and 5 for initial PCB and Figures 6 and 7 for modifications). First, we identify all the above mentioned signals on USB adapter PCB, then we removed all the components not needed for the project:
- two audio connectors were removed (J1 and J2 in Figure 7);
- two capacitors from audio outputs were removed (C22 and C23 in Figures 4 and 6);
- one resistor and two capacitors were removed from the input circuit (R7, C7 and C9 in Figures 4 and 6);
- strap (connection) in place of the capacitor C9 (Figures 4 and 6);
- strap (connection) between VREF line and C23 pad (see Figures 4 and 6), after C23 was removed;
- red LED was removed (not used in this project);
- USB connector was removed (other connector was used on the base PCB);
- the modified module was mounted on a larger base PCB with the rest of the additional components.

Figure 8
Schematic
drawing (PDF)
Figure 9
Figure 10
Figure 11

The schematic drawing is in Figure 8. It was designed by me. Images of the prototype are in figures 9, 10 and 11.

The input circuit includes:
- AC/DC selector made with C1 and SW1;
- input attenuator with 1:1, 10:1 or 100:1 attenuation made with R1, R2, R3 (metal-film resistors with 1% tolerance) and SW2;
- voltage follower made with U1A (TLC 274 CN - high performance CMOS quad operational amplifiers -[7]);
- two inverting stages made with U1B and U1D (same TLC 274 CN) which act together as a voltage adder (adds the reference voltage to the input voltage);
- protection circuit made with D1, D2 and D3 which restrict the MICIN input to positive signals.
The reference circuit consists of:
- voltage follower made with U1C (same TLC 274 CN).
The negative power supply (necessary for input circuit):
- made with U4.
The output circuit (two channels) includes:
- voltage followers U3A and U3B (TLC 272 CN - high performance CMOS dual operational amplifiers) for output of AC/DC generated signals;
- comparators U2A and U2B (TLC 372 CP - dual low power CMOS voltage comparators) for digital TTL outputs;
- output selector AC/DC/TTL made with SW3 and SW4
(actually SW1, SW2, SW3 and SW4 had been implemented using one DIP switch with 10 positions).

As software application for displaying the measured signals, I used Visual analyzer [2] (it is freeware and includes oscilloscope, voltmeter, signal generator, spectrum analyzer and so on).

The oscilloscope can be calibrated using an external DC reference voltage. On the computer, go to Control Panel - Sound, select Recording - Microphone Generic USB Audio Device - Properties - Custom and there clear AGC selection (this will disable the +20dB boost). Put SW1 on DC position and SW2 on 1V position.
- First step: Put the input to the ground. Adjust P2 - ZERO to get zero on the computer (volts DC).
- Second step: put 1.000 V to the input. Then adjust P1 - SPAN and input source level from OS software mixer to get the maximum level on screen without limitation. For doing this, begin with input source level at zero position, then increase level from P1 and when you get the limitation, go back a little with P1 and increase software input source level slowly until you reach the maximum on screen. Save the calibration.
- Repeat the two above steps again, this time with fine adjustments. Save again the calibration.

Specification:

- Analog inputs: 1 channel
- Input voltage ranges: ±1 / ±10 / ±100 Vpp, selectable
- Input impedance: >10 Mohm
- Resolution: 16 bit
- Bandwidth: 20 kHz
- Maximum sampling frequency: 48 kHz
- Output channels: 2
- Maximum output voltage - AC and DC positions: 2.8 Vpp
- Output voltage range - TTL position: 0-5 V
- Maximum output current: 20mA
- Powered from USB, maximum supply current: 100mA

Figure 12
Sine wave
Figure13
Square wave
Figure 14
Spectrum

Warning: This oscilloscope has no galvanic isolation between inputs/outputs and USB interface. Therefore you have to pay attention when computer and/or devices you measure have power supplies connected to the mains AC sockets. Due to some particularities of mains power supplies design, lack of protective earth connection or inside electronic circuit design of the measured devices, there may be hazardous voltages between grounds or other parts of different devices. In some circumstances, this can cause electrical injuries to user or damages to the oscilloscope, computer or other devices. A good idea is to check first with a multimeter the AC and DC components of the voltage between oscilloscope ground and measured device ground or other parts.

Disclaimer: The information on this web site is provided "AS IS", without warranty of any kind. The author has made the best efforts to ensure the design and the information provided are reliable. Under no circumstances shall the author be liable for any direct, indirect, incidental, special or consequential loss, damage, expense or injury incurred or suffered which is claimed to resulted from use of this site, even if expressly advised of the possibility of such loss, damage, expense or injury, including, without limitation, any fault, error, omission, interruption or delay with respect thereto.

Links:
[1] - Oscilloscope with USB audio adapter 1
[2] - Visual analyzer website