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REF192ES-REEL7 Datasheet(PDF) 21 Page - Analog Devices |
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REF192ES-REEL7 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 28 page  REF19x Series Rev. I | Page 21 of 28 One caveat to this approach is that 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. 100Ω 1µF 1kΩ 1µF –VREF REF19x VIN GND VOUT 100kΩ SLEEP TTL/CMOS A1 = 1/2 OP295, 1/2 OP291 VIN 10kΩ 2N3906 3 6 2 4 SLEEP 10kΩ +5V –5V A1 Figure 24. 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 shown in Figure 25 shows how this stacked output reference can be implemented. R1 3.9kΩ (SEE TEXT) C1 0.1µF +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 C2 1µF C3 0.1µF +VOUT1 C4 1µF U2 REF19x (SEE TABLE) 2 6 3 4 U1 REF19x (SEE TABLE) 2 6 3 4 + + VO (U2) VO (U1) Figure 25. 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 connected in series, as shown in Figure 25, which provide 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 chosen for the two voltages that supply the required outputs (see Output Table in Figure 25). 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 needed. 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 conservatively passes 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 3-terminal references is shown in Figure 26, where U1, a REF192, is stacked with a 2-terminal reference diode, such as the AD589. Like the 3-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 V and 2.5 V, which provides a VOUT2 of 3.735 V. When using 2-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 setup by R1 and VO (U1). When VO (U1) is 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. D1 AD589 R1 4.99kΩ (SEE TEXT) C1 0.1µF +VS VS > VOUT2 + 0.15V VIN COMMON VOUT COMMON +VOUT2 3.735V C2 1µF +VOUT1 1.235V C3 1µF U1 REF192 2 6 3 4 + + VO (U1) VO (D1) Figure 26. Stacking Voltage References with the REF192 PRECISION CURRENT SOURCE In low power applications, the need often arises for a precision current source that can operate on low supply voltages. As shown in Figure 27, 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 output voltage of the reference 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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