Two-channel Analog-to-Digital converter board with USB interface

http://www.netams.com/~anton/usb-adc/index.html
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Once I was asked to build an interface board to connect some scientific equipment (for me, a pair of photovoltaic tubes) to a personal computer to acquire measurements results. Signal properties were following: two channels with a common ground, 1 second of acquisition time, max 10V signal level. Desired digitizing properties were about 1000 samples per acquisition, 10 bit depth. The most attractive and cheap solution is to use a PC sound card as an ADC converter. However, most of laptop computers have only mono low-level microphone input onboard. After a careful research, a Texas Instruments PCM2900 chip [1] was chosen as a proper converter (fig.1). It has a stereo I/O (only input is used in this project), USB interface and MS Windows drivers already shipped with an OS.


Fig. 1. USB ADC system outline.

A reference board design from TI was used (fig.2). One can easily found it in the data sheet [2]. Output circuits were removed, and proper capacitors selected. C9 and C10 capacitors were removed as input signal have zero frequency (DC). Instead, a couple of potentiometers (5K and 10K) were placed to do fine board tuning.


Fig. 2. Reference board circuit diagram (larger picture)

PCM2900 sample was obtained through TI's sample program for free, other passive components were ordered through Mouser [3]. List of items is following:

Mouser P.N.

Manufacturer P.N.

Description

Qty.

Each

Total

815-ABL-12-B2

ABL-12.000MHZ-B2

ABRACON HC49/US MHz Range Metal Can Crystals

1

$0.420

$0.42

 

+/-20ppm 12MHZ

140-50N5-180J-TB

CIC180J1HS5T

Xicon Class 1 Ceramic Disc Capacitors - Tape/Box

2

$0.060

$0.12

 

50V,NPO,18PFD,5%

647-UVR1C100MDD

UVR1C100MDD

Nichicon UVR Radial Aluminum Electrolytic Capacitors - 85 Degree

2

$0.180

$0.36

 

16V 10uF 5X11

647-UVR1H010MDD

UVR1H010MDD

Nichicon UVR Radial Aluminum Electrolytic Capacitors - 85 Degree

4

$0.160

$0.64

 

50V 1.0uF 5X11

71-CCF07-G-1M

CCF071M00GKR36

Vishay/Dale 1/4WATT Metal Film Resistors

1

$0.052

$0.05

 

1/4WATT 1MOHMS 2%

647-UVR1H3R3MDD

UVR1H3R3MDD

Nichicon UVR Radial Aluminum Electrolytic Capacitors - 85 Degree

2

$0.160

$0.32

 

50V 3.3uF 5X11

71-CCF07-G-22

CCF0722R0GKR36

Vishay/Dale 1/4WATT Metal Film Resistors

3

$0.052

$0.16

 

1/4WATT 22OHMS 2%

71-CCF07-G-1.5K

CCF071K50GKR36

Vishay/Dale 1/4WATT Metal Film Resistors

2

$0.052

$0.10

 

1/4WATT 1.5KOHMS 2%

71-CCF07-G-22K

CCF0722K0GKR36

Vishay/Dale 1/4WATT Metal Film Resistors

1

$0.052

$0.05

 

1/4WATT 22KOHMS 2%

71-CCF07-G-12K

CCF0712K0GKR36

Vishay/Dale 1/4WATT Metal Film Resistors

1

$0.052

$0.05

 

1/4WATT 12KOHMS 2%

806-ST-3120-3B

ST-3120-3B

Kycon Audio Connectors

1

$0.580

$0.58

 

3.5mm PCB STEREO BLK

317-2080-10K

RV09A-40-20K-A14

Taiwan Alpha PC Mount Black Shaft Potentiometers

1

$0.980

$0.98

 

Audio 9mm 10K

317-2080-5K

RV09A-40-20K-A53

Taiwan Alpha PC Mount Black Shaft Potentiometers

1

$0.980

$0.98

 

Audio 9mm 5K

Prototyping board was supplied by RadioShack (P.N. 276-150) and SSOP-to-DIP converter is from SparkFun [4].

It took about one day to do layout, mounting, soldering and testing of that board. As a source of test signal, a 1.2V battery with a button was used.


Fig. 3. Blank prototype board, converter and a chip.


Fig. 4. Closer look.


Fig. 5. Converter with a chip mounted.


Fig. 6. Assembled board - top view. USB connector is from my old 3.5" bay USB extender enclosure.

After an assembly has been finished, the board was plugged onto my PC (Windows XP): Check the "USB HID", "USB Audio device" and "USB Composite device". Also I had to specify a new source for sound recording.


This is a sample "audio" signal acquired:


References:

[1] http://focus.ti.com/docs/prod/folders/print/pcm2900.html
[2] http://www-s.ti.com/sc/ds/pcm2900.pdf
[3] http://www.mouser.com/
[4] http://www.sparkfun.com/