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REF192 Datasheet(PDF) 19 Page - Analog Devices |
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REF192 Datasheet(HTML) 19 Page - Analog Devices |
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19 / 24 page  REV. E REF19x Series –19– The circuit illustrated in Figure 5 avoids the need for tightly matched resistors with the use of an active integrator circuit. In this circuit, the output of the voltage reference provides the input drive for the integrator. The integrator, to maintain circuit equi- librium, adjusts its output to establish the proper relationship between the reference’s VOUT and GND. Thus, any desired negative output voltage can be chosen by simply substituting for the appropriate reference IC. The sleep feature is maintained in the circuit with the simple addition of a PNP transistor and a 10 k Ω resistor. One caveat with this approach should be men- tioned: although rail-to-rail output amplifiers work best in the application, these operational amplifiers require a finite amount (mV) of headroom when required to provide any load current. The choice for the circuit’s negative supply should take this issue into account. A1 +5V –5V 100 1 F 1k 1 F –VREF 100k REF19x VIN GND VOUT SLEEP 10k SLEEP TTL/CMOS A1 = 1/2 OP295, 1/2 OP291 VIN 10k 4 2 3 6 2N3906 Figure 5. A Negative Precision Voltage Reference Uses No Precision Resistors Stacking Reference ICs for Arbitrary Outputs Some applications may require two reference voltage sources that are a combined sum of standard outputs. The circuit of Figure 6 shows how this “stacked output” reference can be implemented. U2 REF19x (SEE TABLE) R1 3.9k (SEE TEXT) C1 0.1 F +VS VS > VOUT2 +0.15V VIN COMMON VOUT COMMON OUTPUT TABLE U1/U2 REF192/REF192 REF192/REF194 REF192/REF195 VOUT1 (V) 2.5 2.5 2.5 VOUT2 (V) 5.0 7.0 7.5 +VOUT2 VO (U2) C2 1 F C3 0.1 F +VOUT1 VO (U1) C4 1 F U1 REF19x (SEE TABLE) Figure 6. Stacking Voltage References with the REF19x Two reference ICs are used, fed from a common unregulated input, VS. The outputs of the individual ICs are simply con- nected in series as shown, which provides two output voltages, VOUT1 and VOUT2. VOUT1 is the terminal voltage of U1, while VOUT2 is the sum of this voltage and the terminal voltage of U2. U1 and U2 are simply chosen for the two voltages that supply the required outputs (see Table I). If, for example, both U1 and U2 are REF192s, the two outputs are 2.5 V and 5.0 V. While this concept is simple, some cautions are in order. Since the lower reference circuit must sink a small bias current from U2 (50 µA to 100 µA), plus the base current from the series PNP output transistor in U2, either the external load of U1 or R1 must provide a path for this current. If the U1 minimum load is not well defined, resistor R1 should be used, set to a value that will conservatively pass 600 µA of current with the applicable VOUT1 across it. Note that the two U1 and U2 reference circuits are locally treated as macrocells, each having its own bypasses at input and output for best stability. Both U1 and U2 in this circuit can source dc currents up to their full rating. The minimum input voltage, VS, is determined by the sum of the outputs, VOUT2, plus the dropout voltage of U2. A related variation on stacking two three-terminal references is shown in Figure 6, where U1, a REF192, is stacked with a two-terminal reference diode such as the AD589. Like the three- terminal stacked reference above, this circuit provides two outputs, VOUT1 and VOUT2, which are the individual terminal voltages of D1 and U1 respectively. Here this is 1.235 and 2.5, which pro- vides a VOUT2 of 3.735 V. When using two-terminal reference diodes such as D1, the rated minimum and maximum device currents must be observed and the maximum load current from VOUT1 can be no greater than the current set up by R1 and VO(U1). In the case with VO(U1) equal to 2.5 V, R1 provides a 500 µA bias to D1, so the maximum load current available at VOUT1 is 450 µA or less. U1 REF192 R1 4.99k (SEE TEXT) C1 0.1 F +VS VS > VOUT2 +0.15V VIN COMMON VOUT COMMON +VOUT2 3.735V VO (U1) C2 1 F D1 AD589 VO (D1) C3 1 F +VOUT1 1.235V Figure 7. Stacking Voltage References with the REF19x A Precision Current Source Many times, in low power applications, the need arises for a precision current source that can operate on low supply voltages. As shown in Figure 8, any one of the devices in the REF19x family of references can be configured as a precision current source. The circuit configuration illustrated is a floating current source with a grounded load. The reference’s output voltage is bootstrapped across RSET, which sets the output current into the load. With this configuration, circuit precision is maintained for load currents in the range from the reference’s supply current (typically, 30 µA) to approximately 30 mA. The low dropout voltage of these devices maximizes the current source’s output voltage compliance without excess headroom. |
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Similar Description - REF192 |
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