One of the best ways to learn electronics is to see how someone else, more experienced designed something. Whether you’ll disassemble some device or will see some schematic or design – it gives you invaluable experience. In many cases you don’t need to reinvent the wheel, just use already known design cliche and only if needed add your own pieces of design. The more design cliche (building blocks) you know – the less time you’ll spend to make some project and the better design you’ll create.
It’s a three board construction. Very nice modular design. First is a USB connector board:
Which is made double layer to put decoupling capacitor between GND and +5V.
Then goes the main board, where all the magic is happening:
And finally the microphone board:
Which connected with three wires to the main board. The wires is GND, +5V, MIC_OUT.
Since they’ve used ordinary screws instead of self-tappers, some sort of adhesive were applied to each screw to prevent it’s loosening under vibration:
Also, it’s a pretty good close-up photo of SMD inductors.
Now let’s look at the main board:
Bottom side of the board was mirrored, to make it easier to track where the signals go. It’s like to view board layout in some CAD software package with the difference that it’s a photo.
USB connector on the left and microphone board connects on the right.
Now let’s see what we got on this board and what all that things do:
- Power filtering inductor connected between USB +5V and bypass capacitors.
- +5V bypass capacitor.
- +5V bypass cpacitors. In this case the advantage of connecting multiple capacitors in parallel is the reduction of ESR (Equivalent Series Resistance). Means less ripple and noise.
- +3.3V bypass capacitors that connected to output of #6 – 3.3V voltage regulator.
- Two inductors one on the left is connected between USB GND and this board digital ground, between USB GND and analog ground. Inductor in the middle is connected between digital and analog ground.
- 3.3V low dropout voltage regulator. Converts +5V from USB to +3.3V needed by ADC and EEPROM chip. You can see empty pads around it – because of that you can install voltage regulator in SO8 package, instead of smaller TO-23-5.
- AK93C45A 1Kbit Serial CMOS EEPROM memory made by AKM. It’s used by ADC (#8) to store USB device related info, like: Vendor ID, Product ID in Device Descriptor, and String Descriptor.
- AK5371 two channel, 16bit analog-to-digital converter with 20dB Pre-amplifier, +24dB to -31dB Programmable Gain Amplifier, on-chip PLL and USB interface. It takes analog signal from microphone, converts it to digital form and then sends it through USB to your computer.
- 11.2896MHz crystal oscillator. Which is a typical crystal value given in AK5371 datasheet.
- PNP (MMBT3906) and NPN (MMBT3904) transistors, which switch off +5V from USB to microphone board and LED, when device goes to sleep mode.
- Isolated resistor network (four separate resistors in one package). It looks like this res net used as a pull-up on some digital pins. But I’m wondering why is it so big? Maybe because this board was assembled by hands, using solder paste stencil and reflow oven? This board’s assembly method is just my guess, but it looks like it really was assembled by hand.
- Caps for VREF, AGND and the rightmost is connected between two variable gain amplifiers to eliminate DC offset. See AK5371 datasheet for more info.
- Cap between VCOM (common voltage reference) and analog ground. See AK5371 datasheet for more info.
On the bottom I drew vertical red line to separate digital and analog grounds. Usually in schematics name GND is given to digital ground and AGND to analog ground. Sometimes it’s more than two grounds can be present in schematic. If you don’t know what is a difference between analog and digital ground – just Google for it!
Analog ground (on the right) connected with digital ground (on the left) through an inductor #5 (on top) in one spot and through another inductor with USB GND in another spot (yellow circle).
Microphone board is powered from +5V. There’s three solder pads on the right: P1 is GND, P3 is +5V and P2 is input from microphone board.
It looks like that small black three-legged device is a specialized JFET transistor for condenser microphones.
Microphone itself connected between ground (P1 blue wire) and IN (solder bubble between JFET and red resistor).
Power +5V (P3, yellow wire) goes through two RC filters in series (R30, C37; R23, C28). Then to voltage divider (R21, R22). Output of the voltage divider is connected to C27. So it should be very little ripple and noise after C27, which is very important in audio applications.
Then we have R19, R20 and JFET to decrease condenser microphone output impedance. (I have very little experience in audio circuits design, so correct me if I said something wrong).
That’s pretty much all what I wanted to tell you about this mic. I recommend you to look through AK5371 datasheet, which you can find down below.
And some more photos:
- AK5371 2ch A/D Converter with USB I/F
- AK93C45A 1Kbit Serial CMOS EEPROM
- MMBT3904 (2N3904) General Purpose NPN Bipolar Transistor
- MMBT3906 (2N3906) General Purpose PNP Bipolar Transistor
Tell me guys, do you like this type of articles?