Datasheet LT1031, LH0070 (Analog Devices) - 7

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