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OP493FS-REEL Datasheet(PDF) 11 Page - Analog Devices |
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OP493FS-REEL Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page REV. B OP193/OP293/OP493 –11– 6 5 7 4 1 2 3 V– V+ OP193 100k 100k Figure 5. High Resolution Offset Nulling Circuit A Micropower False-Ground Generator Some single-supply circuits work best when inputs are biased above ground, typically at 1/2 of the supply voltage. In these cases a false ground can be created by using a voltage divider buffered by an amplifier. One such circuit is shown in Figure 6. This circuit will generate a false-ground reference at 1/2 of the supply voltage, while drawing only about 27 µA from a 5 V supply. The circuit includes compensation to allow for a 1 µF bypass capacitor at the false-ground output. The benefit of a large capacitor is that not only does the false ground present a very low dc resistance to the load, but its ac impedance is low as well. The OP193 can both sink and source more than 5 mA, which improves recovery time from transients in the load current. 6 7 2 3 10k OP193 100 4 5V OR 12V 0.022 F 1 F 240k 240k 1 F 2.5V OR 6V Figure 6. A Micropower False-Ground Generator A Battery-Powered Voltage Reference The circuit of Figure 7 is a battery-powered voltage reference that draws only 17 µA of supply current. At this level, two AA alkaline cells can power this reference for more than 18 months. At an output voltage of 1.23 V @ 25 °C, drift of the reference is only 5.5 µV/°C over the industrial temperature range. Load regulation is 85 µV/mA with line regulation at 120 µV/V. Design of the reference is based on the Brokaw bandgap core technique. Scaling of resistors R1 and R2 produces unequal currents in Q1 and Q2. The resulting ∆VBE across R3 creates a temperature-proportional voltage (PTAT) which, in turn, pro- duces a larger temperature-proportional voltage across R4 and R5, V1. The temperature coefficient of V1 cancels (first order) the complementary to absolute temperature (CTAT) coefficient of VBE1. When adjusted to 1.23 V @ 25 °C, output voltage tempco is at a minimum. Bandgap references can have start-up problems. With no current in R1 and R2, the OP193 is beyond its positive input range limit and has an undefined output state. Shorting Pin 5 (an offset adjust pin) to ground forces the output high under these circumstances and ensures reliable startup without significantly degrading the OP193’s offset drift. 6 7 2 3 C1 1000pF OP193 VBE2 4 R2 1.5M Q1 VOUT (1.23V @ 25°C) 5 R1 240k V+ (2.5V TO 36V) Q2 1 2 3 7 6 5 VBE1 MAT-01AH VBE R3 68k R5 20k OUTPUT ADJUST R4 130k V1 Figure 7. A Battery-Powered Voltage Reference A Single-Supply Current Monitor Current monitoring essentially consists of amplifying the voltage drop across a resistor placed in series with the current to be measured. The difficulty is that only small voltage drops can be tolerated, and with low precision op amps this greatly limits the overall resolution. The single-supply current monitor of Figure 8 has a resolution of 10 µA and is capable of monitoring 30 mA of current. This range can be adjusted by changing the current sense resistor R1. When measuring total system current, it may be necessary to include the supply current of the current moni- tor, which bypasses the current sense resistor, in the final result. This current can be measured and calibrated (together with the residual offset) by adjustment of the offset trim potentiometer, R2. This produces a deliberate temperature dependent offset. However, the supply current of the OP193 is also proportional to temperature, and the two effects tend to track. Current in R4 and R5, which also bypasses R1, can be adjusted via a gain trim. 6 7 2 3 OP193 4 R2 100k VOUT = 100mV/mA(ITEST) 5 V+ 1 R2 9.9k R3 100k R5 100 R1 1 TO CIRCUIT UNDER TEST ITEST Figure 8. Single-Supply Current Monitor |
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