Datasheet AD693 (Analog Devices) - 8

AD693
An alternative arrangement, allowing wide range span adjust- ment between two set ranges, is shown in Figure 13. R  S  S1 and R = R − 1.0024 E 2 D RS2 are calculated to be 90% of the values determined from the  S − 60 mV  previous formulae. The smallest value is then placed in series and R with the wiper of the 1.5 kΩ potentiometer shown in the figure. E1 = 412 RE2 For example, to adjust the span between 25 mV and 40 mV, RS1 Figure 14 shows a scheme for adjusting the modified span and and RS2 are calculated to be 2000 Ω and 800 Ω, respectively. 4 mA offset via RE3 and RE4. The trim procedure is to first The smaller value, 800 Ω, is then reduced by 10% to cover the connect both signal inputs to the 6.2 V Reference, set RE4 to possible ranges of resistance in the AD693 and that value is put zero and then adjust RE3 so that 4 mA flows in the current loop. in place. 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 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. Figure 14. Adjusting for Spans between 60 mV and In devising your own adjustment scheme, remember that you 100 mV (RE1 and RE2) with Fine-Scale Adjust (RE3 and RE4) should adjust the gain such that the desired span voltage at the After adjusting RE3 place the desired full scale (S) across the Signal Amplifier input translates to 60 mV at the output. Note signal inputs and adjust RE4 so that 20 mA flows in the current also that the full differential voltage applied to the V/I converter loop. An attenuated portion of the input signal is now added is 75 mV; in the 4-20 mA mode, –15 mV is applied to the into the V/I zero to maintain the 75 mV maximum differential. inverting input (zero pin) by the Divider Network and +60 mV If there is some small offset at the input to the Signal Amplifier, is applied to the noninverting input by the Signal Amplifier. In it may be necessary to repeat the two adjustments. 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%
LOCAL-POWERED OPERATION FOR 0–20 mA OUTPUT
larger than that calculated for a 4-20 mA output. The AD693 is designed for local-powered, three-wire systems as Finally, the external resistance from P2 to 6.2 V should not be well as two-wire loops. All its usual ranges are available in three- made less than 1 kΩ unless the voltage reference is loaded to at wire operation, and in addition, the 0–20 mA range can be used. least 1.0 mA. (A simple load resistor can be used to meet this The 0-20 mA convention offers slightly more resolution and requirement if a low value potentiometer is desired.) In no case may simplify the loop receiver, two reasons why it is sometimes should the resistance from P2 to 6.2 V be less than 200 Ω. preferred.
Input Spans Between 60 mV and 100 mV
The arrangement, illustrated in Figure 15, results in a 0–20 mA Input spans of up to 100 mV can be obtained by adding an transmitter where the precalibrated span is 37.5 mV. Con- offset proportional to the output signal into the zero pin of the necting P1 to P2 will double the span to 75 mV. Sensor input V/I converter. This can be accomplished with two resistors and and excitation is unchanged from the two-wire mode except for adjusted via the optional trim scheme shown in Figure 14. The the 25% increase in span. Many sensors are ratiometric so that resistor divider formed by RE1 and RE2 from the output of the an increase in excitation can be used instead of a span Signal Amplifier modifies the differential input voltage range adjustment. applied to the V/I converter. In the local-powered mode, increases in excitation are made In order to determine the fixed resistor values, RE1 and RE2, first easier. Voltage compliance at the IIN terminal is also improved; measure the source resistance (RD) of the internal divider network. the loop voltage may be permitted to fall to 6 volts at the This can be accomplished (power supply disconnected) by AD693, easing the trade-off between loop voltage and loop measuring the resistance between the 4 mA of offset (Pin 13) resistance. Note that the load resistor, RL, should meter the and common (Pin 6) with the 6.2 V reference (Pin 14) connected current into Pin 10, IIN, so as not to confuse the loop current to common. The measured value, RD, is then used to calculate with the local power supply current. RE1 and RE2 via the following formula: –8– REV. A