Data Acquisition through Microphone Input

We like to make use of the PC audio device for data acquisition.  Here we'll do some testing to understand the capability.  Most of the recent laptops have only one 3.5mm 4-contact jack; the connection is normally tip: left audio, 1st ring: right audio, 2nd ring: return, sleeve: microphone input.   The common 3-contact headphone plug shorts out the microphone input.    The microphone input supplies a small current to power microphones with a FET amplifier.   For the laptop that I have, the open circuit microphone input voltage is 3.168V and short circuit current 1.46mA; the source impedance is about 2K Ohm.  A typical microphone draws about 0.15mA.

Python sounddevice module can be used for acquiring the signal from the microphone input.    The output seems to be limited to [-1, 1].  The microphone properties settings of output level and boost affect the gain.  For 100 output level and 10dB boost, the gain seems to be x50; 20mVpp input has value of 1 peak-to-peak and above 40mVpp input returns 2 peak-to-peak.  Interestingly, although the output is limited to [-1, 1], it does not show clipping effect.  We do a frequency sweep from 20Hz to 20KHz and look at the power spectrum density.  The sample rate is 96KHz.

There is a sharp cutoff at 15KHz and low frequency limit around 100Hz.   The resolution seems to be 16bit.  There is another cutoff about 22KHz (perhaps for anti-aliasing).

Here is 2KHz square wave input,

The upper cutoff frequency is unfortunate because the sample rate seems to be capable of up to 384KHz.   We'll have use frequency conversion for signal outside of the frequency range.  

Fluke 87

We've looked at a few of very cheap multimeters (1, 2, 3, 4).  How do they compare with the venerable Fluke DMM?  Here we'll look at an older model of Fluke 87 DMM, which costs hundreds of dollars.

The date code shown on the PCB is 1986; so perhaps Fluke 87 was first released in 1987.  The service manual is available; it contains the full schematics and parts list.  Besides the main ASIC (which does dual slope A/D conversion with a16-bit counter for 4.5-digit resolution) , there are a few ICs, a reference voltage (LM285), a RMS to DC converter (AD737), a dual opamp (TLC27M21), and a hex inverter.  The rotary knob switch is more robust than those sliding on the PCB tracks.  The input overload protection circuit is much more comprehensive, including 1500V spark gap surge protector, 910V MOVs, thermistor, a fusible resistor (1KΩ), fuses and the PCB has slots for creepage.  The DMM has a CAT III rating (1000V working voltage and 8000V transient).   Split jacks are used to detect if probes are plugged correctly for the current measurement.  Also a number of film capacitors are used and as well as a precision resistor pack.  There are two adjustable resistors and capacitors for calibration.

One factory test feature is the ultrasonic data communication.  The DMM can be put in the ultrasonic data mode by holding the HOLD button while powering up.  A periodic burst of buzzing can be heard.  We sample the speaker output at 100KHz for 2 seconds. We see it has a burst of 350ms every 600ms.  

Zoom in; we see the structure of the data.  

Zoom in further, we see 16.7KHz pulses.

We will look into how to decode it later.