A new isothermal multijunction differential thermal element provides fast settling AC to DC converter

An isothermal multijunction differential thermal converter has been fabricated by constructing two separate insulated heaters of a vacuum thermal element and placing them in thermal contact. One heater carries the input signal, while the second heater circuit provides negative feedback to maintain the dual heater structure at a constant temperature. The mass of the heaters and thermocouple temperature sensors is small and the temperature variations are limited to millidegrees Celsius. Therefore, the response time of this thermal converter is a fraction of a second which permits AC-DC conversion 5-10 times faster than conventional thermal devices. Measurement accuracy is comparable to conventional hot-wire single-junction thermal converters and wideband frequency capability beyond 100 MHz makes this thermal converter applicable to precision AC-DC transfer standards and digital AC voltmeters