Experienced designers of precision analog and mixed-signal systems know that the performance of the voltage reference, whether stand-alone or integrated into an analog-to-digital converter (ADC), is critical to overall system performance. Both the absolute accuracy and the stability of the reference, which is largely but not entirely a function of temperature changes, set performance limits. Although the accuracy can be calibrated and compensated with some effort, it takes a lot of work to assess thermal drift and integrate it into a correction scheme.
Therefore, the emphasis is on an inherently stable reference such as the buried 7.05V voltage reference, compensated by the ADR1399 furnace from Analog Devices. (Fig. 1). This precision four-pin shunt reference with integrated microheater exhibits excellent temperature stability with a drift of only 0.2 ppm / Â° C over a wide range of voltage, temperature and quiescent current conditions. Long term stability is 7 ppm / âkHr.
The maximum dynamic impedance is 0.08 Î© and the typical reference noise is 1.84 ÂµV rms from 10 Hz to 1 kHz. Applications of ADR1399 include ultra-stable digital voltmeters, precision calibration equipment, and any system or signal chain that requires highly repeatable ADC readings.
The ADR1399 heater operates from a 9.5-40V supply and typically draws 8.5-21mA, with the greatest steady-state current demand at an ambient temperature of -55 Â° C. , of cours. Heating current can exceed 100mA on start-up until it thermally stabilizes within seconds (Fig. 2).
Two operational amplifier servo loops inside the ADR1399 help maximize stability. A loop maintains a fixed ratio of Zener and VTO BE currents, with the total current defined by an external pull-up resistor or current source. The second operational amplifier loop maintains the device chip at a non-adjustable set temperature of approximately 95 Â° C, thus preventing most external fluctuations in ambient temperature from affecting the operating temperature. These loops also contribute to the excellent load response of the ADR1399.
Key details on PCB packaging and placement
The packaging and installation of the PCB is critical for a device such as this reference. The entire unit is housed in a single, hermetically sealed, four-pin TO-46 package measuring approximately 5mm in diameter and 2.5mm in height, which is surrounded by plastic thermal insulation. This increases thermal resistance and smooths out fluctuations due to changes in ambient temperature and reduces the heating power required.
Another design technique that increases thermal resistance is to reduce the solid copper planes of the associated PCB near the device, as they remove heat (which, ironically and unlike most cases, is a feature that you don’t want in this situation). It is also effective to elevate the device on its cables about 1cm above the surface of the board, again to reduce the thermal path and allow stagnant air to thermally insulate the device. somewhat.
Other subtle considerations of layout and use can maximize the performance of a precision voltage reference like this. TO-46 box wires, made of Kovar, are typically soldered to a copper track in a conventional board design. However, Kovar copper junctions also induce thermocouple voltages of 35 ÂµV / Â° C, which is approximately 25 times higher than the typical temperature coefficient of ADR1399.
Therefore, to minimize thermocouple induced voltage errors, the series junctions with the critical pins should always see the same temperature as the corresponding junction in the return path, with the need to avoid temperature gradients at both. points where the Zener pins come in contact with the PCB.
The ADR1399 is priced at $ 11.81 (100 pieces). Although this is a functionally “simple” four terminal device that does one thing and strives to do it well, users may still prefer to rate its performance for their situation. To facilitate this, the associated EVAL-ADR1399 evaluation board, priced at $ 49 (Fig. 3), allows efficient prototyping. It includes an edge-mounted SMA connector as well as a pair of connectors for basic banana plugs and is powered by a standard USB connection.