Datenblatt-Suchmaschine für elektronische Bauteile |
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AD693AQ Datenblatt(PDF) 8 Page - Analog Devices |
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AD693AQ Datenblatt(HTML) 8 Page - Analog Devices |
8 / 12 page AD693 REV. A –8– R E 2 = RD S S − 60 mV − 1.0024 and RE1 = 412 RE2 Figure 14 shows a scheme for adjusting the modified span and 4 mA offset via RE3 and RE4. The trim procedure is to first connect both signal inputs to the 6.2 V Reference, set RE4 to zero and then adjust RE3 so that 4 mA flows in the current loop. This in effect, creates a divider with the same ratio as the internal divider that sets the 4 mA zero level (–15 mV with respect to 6.2 V). As long as the input signal remains zero the voltage at Pin 12, the zero adjust, will remain at –15 mV with respect to 6.2 V. Figure 14. Adjusting for Spans between 60 mV and 100 mV (RE1 and RE2) with Fine-Scale Adjust (RE3 and RE4) After adjusting RE3 place the desired full scale (S) across the signal inputs and adjust RE4 so that 20 mA flows in the current loop. An attenuated portion of the input signal is now added into the V/I zero to maintain the 75 mV maximum differential. If there is some small offset at the input to the Signal Amplifier, it may be necessary to repeat the two adjustments. LOCAL-POWERED OPERATION FOR 0–20 mA OUTPUT The AD693 is designed for local-powered, three-wire systems as well as two-wire loops. All its usual ranges are available in three- wire operation, and in addition, the 0–20 mA range can be used. The 0-20 mA convention offers slightly more resolution and may simplify the loop receiver, two reasons why it is sometimes preferred. The arrangement, illustrated in Figure 15, results in a 0–20 mA transmitter where the precalibrated span is 37.5 mV. Con- necting P1 to P2 will double the span to 75 mV. Sensor input and excitation is unchanged from the two-wire mode except for the 25% increase in span. Many sensors are ratiometric so that an increase in excitation can be used instead of a span adjustment. In the local-powered mode, increases in excitation are made easier. Voltage compliance at the IIN terminal is also improved; the loop voltage may be permitted to fall to 6 volts at the AD693, easing the trade-off between loop voltage and loop resistance. Note that the load resistor, RL, should meter the current into Pin 10, IIN, so as not to confuse the loop current with the local power supply current. An alternative arrangement, allowing wide range span adjust- ment between two set ranges, is shown in Figure 13. RS1 and RS2 are calculated to be 90% of the values determined from the previous formulae. The smallest value is then placed in series with the wiper of the 1.5 k Ω potentiometer shown in the figure. For example, to adjust the span between 25 mV and 40 mV, RS1 and RS2 are calculated to be 2000 Ω and 800 Ω, respectively. The smaller value, 800 Ω, is then reduced by 10% to cover the possible ranges of resistance in the AD693 and that value is put in place. Figure 13. Wide Range Span Adjustment A number of other arrangements can be used to set the span as long as they are compatible with the pretrimmed noninverting gain of two. The span adjustment can even include thermistors or other sensitive elements to compensate the span of a sensor. In devising your own adjustment scheme, remember that you should adjust the gain such that the desired span voltage at the Signal Amplifier input translates to 60 mV at the output. Note also that the full differential voltage applied to the V/I converter is 75 mV; in the 4-20 mA mode, –15 mV is applied to the inverting input (zero pin) by the Divider Network and +60 mV is applied to the noninverting input by the Signal Amplifier. In the 0–20 mA mode, the total 75 mV must be applied by the Signal Amplifier. As a result, the total span voltage will be 25% larger than that calculated for a 4-20 mA output. Finally, the external resistance from P2 to 6.2 V should not be made less than 1 k Ω unless the voltage reference is loaded to at least 1.0 mA. (A simple load resistor can be used to meet this requirement if a low value potentiometer is desired.) In no case should the resistance from P2 to 6.2 V be less than 200 Ω. Input Spans Between 60 mV and 100 mV Input spans of up to 100 mV can be obtained by adding an offset proportional to the output signal into the zero pin of the V/I converter. This can be accomplished with two resistors and adjusted via the optional trim scheme shown in Figure 14. The resistor divider formed by RE1 and RE2 from the output of the Signal Amplifier modifies the differential input voltage range applied to the V/I converter. In order to determine the fixed resistor values, RE1 and RE2, first measure the source resistance (RD) of the internal divider network. This can be accomplished (power supply disconnected) by measuring the resistance between the 4 mA of offset (Pin 13) and common (Pin 6) with the 6.2 V reference (Pin 14) connected to common. The measured value, RD, is then used to calculate RE1 and RE2 via the following formula: |
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