Datasheet ADR1399 (Analog Devices) - 10

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ADR1399 APPLICATIONS INFORMATION AVOIDING THERMOCOUPLE ERRORS
Thermocouples occur whenever two dissimilar metals form a junc- tion. For example, the TO-46 package leads are made of Kovar and are usually soldered to a copper trace in a PCB design. Kovar copper junctions are known to cause thermocouple voltages of 35 μV/°C, which is about 25 times higher than the typical tempera- ture coefficient of the ADR1399. To minimize thermocouple induced voltage errors, ensure that junctions in series with critical pins
Figure 20. Single-Supply Operation
always see the same temperature as the corresponding junction in the return path. For the TO-46 of the ADR1399, this results in the need to avoid temperature gradients at the two points where the Zener pins contact the PCB.
SHUNT DYNAMIC IMPEDANCE AND CAPACITIVE LOAD
The ADR1399 offers much reduced output shunt dynamic impe- dance over the LM399. At first, users might dismiss this advance- ment as not critical. However, considering the need for high stability
Figure 21. Split Supply Operation
in the presence of supply fluctuations and RSHUNT drift, the low dynamic impedance of the ADR1399 is advantageous. Consider, for example the effect of a 0.1% change in supply voltage on a 15 V supply running 3 mA into an ADR1399 through a 2.67 kΩ RSHUNT. The extra supply voltage causes an extra 5.6 μA to flow, which on the 40 mΩ dynamic impedance of the ADR1399 increases the reference voltage by 0.22 μV. The same supply shift onto an LM399 design and into its typical 0.5 Ω dynamic impedance induces a much more considerable 2.8 μV of reference
Figure 22. Buffered Operation
shift. Therefore, the improvement in dynamic impedance allows a much better opportunity for maintaining high stability in the most critical shunt reference output voltage. A similar simple calculation can be done for effects due to changes in the RSHUNT value. One of the trade-offs of achieving the much reduced dynamic impedance, however, is an increased sensitivity to direct capacitive loading. The LM399 was stable with practically any capacitive load. The ADR1399 starts to ring with direct capacitive loads of more than a few hundred pF, and oscillates with 10 nF direct. The ADR1399 is optimized for an external compensation series network
Figure 23. Negative Heater Supply with Positive Reference
of 5 Ω and 1 μF, as is shown in most of the typical application figures (see the Typical Applications section). If updating a legacy design with too much capacitance for the ADR1399, and there is nowhere to add a series 5 Ω, try reducing the capacitance to less than 1 nF. Another single-element passive found to work directly with ADR1399 is a 10 μF tantalum capacitor, even though the series ohms can measure less than 5 Ω on an impedance analyzer.
TYPICAL APPLICATIONS Figure 24. Parallel References for Lower Noise
Figure 20 through Figure 24 show basic connections for single-sup- ply, split supply, buffered references, negative heater supply with positive reference, and parallel references for lower noise opera- tion.
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Document Outline Features Applications Pin Configuration General Description Specifications Electrical Characteristics Absolute Maximum Ratings Thermal Resistance ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Operating Set Temperature Thermal Resistance Applications Information Avoiding Thermocouple Errors Shunt Dynamic Impedance and Capacitive Load Typical Applications Outline Dimensions Ordering Guide