Data Sheet

ADR1399

APPLICATIONS INFORMATION

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Rev. A | 13 of 16

AVOIDING THERMOCOUPLE ERRORS

Thermocouples occur whenever two dissimilar metals form a junc-

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

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 reduced output shunt dynamic impedance

over the LM399, which is often dismissed as an advancement that

is not critical. However, considering the need for high stability in the

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

μA to flow, which, on the 40 mΩ dynamic impedance of the TO-46

version 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 considerable 2.8 μV of

reference shift. Therefore, the improvement in dynamic impedance

allows a better opportunity for maintaining high stability in the most

critical shunt reference output voltage. Similarly, one can calculate

One of the trade-offs of achieving the reduced dynamic impedance,

however, is an increased sensitivity to direct capacitive loading.

The LM399 is stable with a wide range of capacitive load. The

ADR1399 starts to ring with direct capacitive loads of more than a

few hundred pF, and oscillates with 10 nF directly. The ADR1399

is optimized for an external compensation series network of ~5 Ω

and 1 μF, as shown in Figure 36 to Figure 41 (see the Typical

Applications section). If updating a legacy design with excessive

capacitance for the ADR1399, and there is nowhere to add a series

5 Ω, reduce the capacitance to less than 1 nF if possible. Another

single element passive that works directly with the ADR1399 is a

10 μF tantalum capacitor, even though the series resistance can

measure less than 5 Ω on an impedance analyzer.

With the 8-terminal LCC version of the ADR1399, including new

force and sense pins, the dynamic impedance is lower. With the

+REFF and +REFS shorted together, and the −REFF and −REFS

shorted together, and sensing across the ±REFS pins, the dynamic

impedance is 11 mΩ in magnitude, with 180 degrees of phase at

low frequency, or −11 mΩ at DC.

TYPICAL APPLICATIONS

Figure 36 through Figure 41 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.

Figure 36. Single-Supply Operation

Figure 37. Split Supply Operation

Figure 38. Buffered Operation

Figure 39. Negative Heater Supply with Positive Reference

Figure 40. Parallel References for Lower Noise