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ADR01BR Datasheet(PDF) 17 Page  Analog Devices 

ADR01BR Datasheet(HTML) 17 Page  Analog Devices 
17 / 20 page ADR01/ADR02/ADR03/ADR06 Rev. F  Page 17 of 20 To optimize the resolution of this circuit, dualsupply op amps should be used because the ground potential of ADR02 can swing from –5 V at zero scale to VL at full scale of the potenti ometer setting. PROGRAMMABLE 4 TO 20 mA CURRENT TRANSMITTER Because of their precision, adequate current handling, and small footprint, the devices are suitable as the reference sources for many high performance converter circuits. One of these applications is the multichannel 16bit 4 to 20 mA current transmitter in the industrial control market (see Figure 42). This circuit employs a Howland current pump at the output, which yields better efficiency, a lower component count, and a higher voltage compliance than the conventional design with op amps and MOSFETs. In this circuit, if the resistors are matched such that R1 = R1′, R2 = R2′, R3 = R3′, the load current is N REF L D V R R ) R (R I 2 3 1 3 2 × × ′ + = (2) where D is similarly the decimal equivalent of the DAC input code and N is the number of bits of the DAC. According to Equation 2, R3′ can be used to set the sensitivity. R3′ can be made as small as necessary to achieve the current needed within U4 output current driving capability. On the other hand, other resistors can be kept high to conserve power. U1 15V VIN VOUT GND TEMP TRIM U1 = ADR01/ADR02/ADR03/ADR06, REF01 U2 = AD5543/AD5544/AD5554 U3, U4 = AD8512 U2 5V 10V +15V –15V VDD VREF GND RF IO IO AD5544 DIGITAL INPUT CODE 20%–100% FULL SCALE U3 VX 0V TO –10V R1 150k Ω R2 15k Ω U4 C1 10pF VP R3 50 Ω AD8512 R3' 50 Ω VL R1' 150k Ω LOAD 500 Ω 4–20mA VN VO R2' 15k Ω Figure 42. Programmable 4 to 20 mA Transmitter In this circuit, the AD8512 is capable of delivering 20 mA of current, and the voltage compliance approaches 15 V. The Howland current pump yields a potentially infinite output impedance, which is highly desirable, but resistance matching is critical in this application. The output impedance can be deter mined using Equation 3. As can be seen by this equation, if the resistors are perfectly matched, ZO is infinite. On the other hand, if they are not matched, ZO is either positive or negative. If the latter is true, oscillation may occur. For this reason, a capacitor, C1, in the range of 1 pF to 10 pF should be connected between VP and the output terminal of U4, to filter any oscillation. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ′ ′ ′ = = 1 R1R2 R2 R1 R1 I V Z t t O (3) In this circuit, an ADR01 provides the stable 10.000 V reference for the AD5544 quad 16bit DAC. The resolution of the adjust able current is 0.3 µA/step, and the total worstcase INL error is merely 4 LSB. Such error is equivalent to 1.2 µA or a 0.006% system error, which is well below most systems’ requirements. The result is shown in Figure 43 with measurement taken at 25°C and 70°C; total system error of 4 LSB at both 25°C and 70°C. 5 –1 0 65536 8192 16384 24576 32768 40960 49152 57344 4 3 2 1 0 CODE (Decimal) RL = 500Ω IL = 0mA TO 20mA 25 oC 70 oC Figure 43. Result of Programmable 4 to 20 mA Current Transmitter Precision Boosted Output Regulator A precision voltage output with boosted current capability can be realized with the circuit shown in Figure 44. In this circuit, U2 forces VO to be equal to VREF by regulating the turnon of N1, thereby making the load current furnished by VIN. In this configuration, a 50 mA load is achievable at VIN of 15 V. Moderate heat is generated on the MOSFET, and higher current can be achieved with a replacement of a larger device. In addition, for a heavy capacitive load with a fast edging input signal, a buffer should be added at the output to enhance the transient response. U2 15V N1 200 Ω U1 ADR01/ ADR02/ ADR03/ ADR06 VIN VOUT TEMP TRIM GND V– V+ OP1177 2N7002 VIN VO RL 1 µF CL Figure 44. Precision Boosted Output Regulator 
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