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.
We will look into how to decode it later.
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