Electronic Design

Sample 64 Temperature Points For Pennies Per Channel With Multiplexed Delta VBE Thermometry

Delta VBE ( VBE)-based thermometry is based on the “PTAT VBE effect,” which says that for an ideal transistor, the VBE corresponding to changes in collector current is exactly proportional to absolute temperature. Cheap small-signal transistors conform closely enough to the ideal model that VBE circuits can implement accurate and cost-effective thermometers needing no calibration. An earlier Idea for Design descriBEd how a new variation on VBE, “Tri-Current VBE,” extends its utility by making VBE practically immune to parasitic interconnection resistances.1

This design idea builds on that idea (see the figure). It aggressively exploits the ohmic-error immunity of the Tri-Current VBE principle by adding a front end comprising a massive 8 × 8 matrix of temperature-sensor transistors, thus achieving uniquely costeffective temperature measurement. The per-channel cost actually approaches the price of a single garden-variety small signal transistor, literally pennies. The simplicity of X-Y matrix interconnection, which needs only 16 conductors for 64 channels, adds further to the economy of the technique.

Of course, multiplexing of the transistor matrix could never work accurately without effective cancellation of ohmic error. That’s because the IR voltage noise introduced by the interaction of modulation currents with “Collector” and “Emitter” multiplexer on-resistances is nearly as large as the VBE signal itself. But as explained in the reference article, the four-step modulation sequence is specifically designed to allow discrimination, subtraction, and cancellation of the linear (ohmic) signal components.

Synchronous demodulation of the resulting ac signal accumulates the (I1 + I2) ?VBE response as V1 on C1, and the (I1 + I3) response as V2 on C2. Differential summation (V = 2V1 – V2 = 198.4 µV/K), offset (VC = V - 0.05412 = 0 @ 273K), and scaling (VO = 504 VC = 0.1 V/ °C) yields the output VO = 0 V @ 0°C, 10 V = @ 100°C, with an untrimmed accuracy of better than ±1°C. To achieve this accuracy requires 0.1% resistors (note the asterisk *) and a 2.500-V reference of similar precision. But the 64 temperature sensors themselves are, by contrast, any common small signal general-purpose NPN such as the venerable 2N2222 or 2N3904.

While the resulting thermometer is accurate and cheap, it’s not especially fast. The maximum multiplexer scanning rate is limited by modulation frequency and RC settling times to about 10 points per second. Fortunately, adequate temperature measurement scan periods are usually more appropriately measured in minutes than milliseconds. So this is usually fast enough.

Reference: 1.“Triple-Current-Modulation ?VBE Thermometry Cancels Ohmic Error Sources,” W. Stephen Woodward, Electronic Design, June 25, 2009.

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