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ADR381ART-R2 Fiches technique(PDF) 11 Page - Analog Devices |
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ADR381ART-R2 Fiches technique(HTML) 11 Page - Analog Devices |
11 / 12 page REV. A ADR380/ADR381 –11– reference is then desirable from the point that an additional operational amplifier is not required for either reinversion (current-switching mode) or amplification (voltage-switching mode) of the DAC output voltage. In general, any positive voltage reference can be converted into a negative voltage refer- ence through the use of an operational amplifier and a pair of matched resistors in an inverting configuration. The disadvan- tage to this approach is that the largest single source of error in the circuit is the relative matching of the resistors used. The circuit in Figure 3 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 equilibrium, adjusts its output to establish the proper relationship between the reference’s VOUT and GND. Thus, any negative output voltage desired can be chosen by simply substituting for the appropriate reference IC. A precaution should be noted with this approach: 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. GND VOUT VIN U1 ADR380 C2 0.1 F 3 +5V –VREF VIN R4 1k C1 1 F C3 1 F 2 R3 100k A1 1 C4 1 F U2 –5V OP195 R5 100 –V +V Figure 3. A Negative Precision Voltage Reference Uses No Precision Resistors Precision Current Source Many times in low power applications, the need arises for a pre- cision current source that can operate on low supply voltages. As shown in Figure 4, the ADR380/ADR381 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 (R1 + P1), 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 90 µA to approximately 5 mA. GND VOUT VIN U1 ADR380 C2 0.1 F 3 VIN C1 1 F C3 1 F 2 R1 1 RL P1 IOUT ISY ADJUST Figure 4. A Precision Current Source Precision High Current Voltage Source In some cases, the user may want higher output current delivered to a load and still achieve better than 0.5% accuracy out of the ADR380/ADR381. The accuracy for a reference is normally specified on the data sheet with no load. However, the output voltage changes with load current. The circuit in Figure 5 provides high current without compro- mising the accuracy of the ADR380/ADR381. By op amp action, VO follows VREF with very low drop in R1. To maintain circuit equilibrium, the op amp also drives the N-Ch MOSFET Q1 into saturation to maintain the current needed at different loads. R2 is optional to prevent oscillation at Q1. In such an approach, hun- dreds of milliamps of load current can be achieved and the current is limited by the thermal limitation of Q1. VIN = VO + 300 mV. GND VOUT VIN U1 ADR380/ ADR381 3 VO 2 R2 100 1 C1 0.001 F Q1 2N7002 +8 –15V R1 100k RL VIN A1 –V +V AD820 Figure 5. ADR380/ADR381 for Precision High Current Voltage Source |
Numéro de pièce similaire - ADR381ART-R2 |
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Description similaire - ADR381ART-R2 |
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