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LH0070 Datasheet(PDF) 7 Page - Linear Technology |
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LH0070 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LT1031/LH0070 1031fb APPLICATIO S I FOR ATIO Standard Series Mode LT1031 • TA05 INPUT LOAD + KEEP THIS LINE RESISTANCE LOW GROUND RETURN LT1031 GND IN OUT Capacitive Loading and Transient Response The LT1031 is stable with all capacitive loads, but for optimum settling with load transients, output capacitance should be under 1000pF. The output stage of the reference is class AB with a fairly low idling current. This makes transient response worst-case at light load currents. Because of internal current drain on the output, actual worst-case occurs at ILOAD = 1.4mA (sinking). Significantly better load transient response is obtained by moving slightly away from these points. See Load Transient Response curves for details. In general, best transient response is obtained when the output is sourcing current. In critical applications, a 10 µF solid tantalum capacitor with several ohms in series provides optimum output bypass. Kelvin Connections Although the LT1031 does not have true force/sense capability at its outputs, significant improvements in ground loop and line loss problems can be achieved with proper hook-up. In series mode operation, the ground pin of the LT1031 carries only ≈1mA and can be used as a sense line, greatly reducing ground loop and loss problems on the low side of the reference. The high side supplies load current so line resistance must be kept low. Twelve feet of #22 gauge hook up wire or 1 foot of 0.025 inch printed circuit trace will create 2mV loss at 10mA output current. This is equivalent to 1LSB in a 10V, 12-bit system. The following circuits show proper hook-up to minimize errors due to ground loops and line losses. Losses in the output lead can be greatly reduced by adding a PNP boost transistor if load currents are 5mA or higher. R2 can be added to further reduce current in the output sense lead. Effects of Air Movement on Low Frequency Noise The LT1031 has very low noise because of the buried zener used in its design. In the 0.1Hz to 10Hz band, peak-to-peak noise is about 0.5ppm of the DC output. To achieve this low noise, however, care must be taken to shield the reference from ambient air turbulence. Air movement can create noise because of thermoelectric differences between IC package leads (especially kovar lead TO-5) and printed circuit board materials and/or sockets. Power dissipation in the reference, even though it rarely exceeds 20mW, is enough to cause small temperature gradients in the package leads. Variations in thermal resistance, caused by uneven airflow, create differential lead temperatures, thereby causing thermoelectric voltage noise at the output of the reference. The XY plotter trace shown on the following page dramatically illustrates this effect. The first half of the plot was done with the LT1031 shielded from ambient air with a small foam cup. The cup was then removed for the second half of the trace. Ambient in both cases was a lab environment with no excessive air turbu- lence from air conditioners, opening/closing doors, etc. Removing the foam cup increases the output noise by almost an order of magnitude in the 0.01Hz to 1Hz band! The kovar leads of the TO-5 (H) package are the primary culprit. Alloy 42 and copper lead frames used on dual-in- line packages are not nearly as sensitive to thermally generated noise because they are intrinsically matched. Series Mode with Boost Transistor LT1031 GND OUT IN 2N3906 INPUT LOAD *OPTIONAL—REDUCES CURRENT IN OUTPUT SENSE LEAD GROUND RETURN R1 220 Ω R2* 5.6k LT1031 • TA06 |
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