Issue 155 June 2003
Good Vibrations

SI(G)Ns

With the road-going background (and most of the pins) out of the way, let’s take a look at the thereminesque aspects of the ’33794. The e-field side of the story boils down to little more than the following: a dozen pins, eight electrode connections (E1 to E9), four electrode select lines (A to D), a shield driver (SHIELD) and associated control pin (SHLD DIS), two references (Ref A, Ref B), and the LEVEL output. Here’s how it works.

The four select lines (A to D) control a multiplexer that determines which electrode (or other internal signal as in Table 1) is to be measured. A nominal 120-kHz, 5-V peak-to-peak sine wave is routed to the chosen electrode and to the built-in level check circuit. As an aside, Motorola highlights the purity of the generated sine wave (i.e., –20 dB for second to fourth harmonics, and –60 dB for fifth and beyond), which is no doubt designed that way in order to prevent the chip from interfering with radio reception or the other electronic gadgets found under the hood these days.

Capacitance (i.e., “hand capacitance” in Theremin-speak) between the chosen electrode and ground is detected, amplified, and offset to generate a nominal 0- to 5-V output on the LEVEL pin for connection to your MCU’s A/D converter. Absolute capacitance may vary depending on temperature, aging, or the overall ambient electromagnetic environment, for instance. To that end, low (e.g., 10 pF) and high (e.g., 100 pF) reference capacitors are connected to two lines—Ref A and Ref B—to enable relative measurements and calibration.

It’s likely that packaging and placement concerns will call for the connecting of electrodes to the chip via a length of coax. The problem is that the cable brings along it’s own capacitance between the center conductor and the shield. To solve this problem, connect the SHIELD pin to the coax shield. As a result, the shield is driven to the same level as the electrode pin, in essence making the coax disappear. The shield driver can be turned on and off (SHLD DIS pin), which provides a way to self-test for broken connections or other faults. Toggle the shield driver on and off, and if you don’t see a corresponding change on the LEVEL pin, you’ll know something is amiss.