LT1021 UUWUAPPLICATIONS INFORMATION minimized. In the current output circuit, R1 determines Kelvin Connections output current. It should have a TC commensurate with the Although the LT1021 does not have true force/sense LT1021 or track closely with the feedback resistor around capability at its outputs, significant improvements in ground the op amp. loop and line loss problems can be achieved with proper hook-up. In series mode operation, the ground pin of the LT1021 carries only ≈1mA and can be used as a sense LT1021-7 line, greatly reducing ground loop and loss problems on IN OUT R1* the low side of the reference. The high side supplies load GND R2* 200 14k Ω current so line resistance must be kept low. Twelve feet of 1% 1% R3 VOUT 10k 6.950V #22 gauge hook-up wire or 1 foot of 0.025 inch printed circuit trace will create 2mV loss at 10mA output current. TC TRACKING TO 50ppm/°C 1021 AI03 This is equivalent to 1LSB in a 10V, 12-bit system. The following circuits show proper hook-up to minimize LT1021-7 errors due to ground loops and line losses. Losses in the IN OUT output lead can be greatly reduced by adding a PNP boost GND R1* transistor if load currents are 5mA or higher. R2 can be R2** 7.15k 182k added to further reduce current in the output sense lead. R3 50k 1.000mA Standard Series Mode – OP AMP LT1021 KEEP THIS LINE RESISTANCE LOW + INPUT IN OUT 1021 AI04 GND + LOAD *RESISTOR TC DETERMINES IOUT TC **TC ≤ (10 • R1) TC. R2 AND R3 SCALE WITH R1 FOR DIFFERENT OUTPUT CURRENTS GROUND RETURN 1021 AI05 Capacitive Loading and Transient Response The LT1021 is stable with all capacitive loads, but for Series Mode with Boost Transistor 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 INPUT R1 transient response worst-case at light load currents. Be- 220Ω cause of internal current drain on the output, actual worst- 2N3906 case occurs at ILOAD = 0 on LT1021-5, ILOAD = – 0.8mA (sinking) on LT1021-7 and I IN LOAD = 1.4mA (sink- LT1021 ing) on LT1021-10. Significantly better load transient OUT response is obtained by moving slightly away from these GND LOAD R2* 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 10µF GROUND RETURN 1021 AI06 solid tantalum capacitor with several ohms in series *OPTIONAL—REDUCES CURRENT IN OUTPUT SENSE LEAD provides optimum output bypass. R2 = 2.4k (LT1021-5), 3k (LT1021-7), 5.6k (LT1021-10) 1021fc 10
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