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AD693 データシート(PDF) 5 Page - Analog Devices |
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AD693 データシート(HTML) 5 Page - Analog Devices |
5 / 12 page AD693 REV. A –5– converter’s inverting input (Pin 12). Arranging the zero offset in this way makes the zero signal output current independent of input span. When the input to the signal amp is zero, the noninverting input of the V/I is at 6.2 V. Since the standard offsets are laser trimmed at the factory, adjustment is seldom necessary except to accommodate the zero offset of the actual source. (See “Adjusting Zero.”) SIGNAL AMPLIFIER The Signal Amplifier is an instrumentation amplifier used to buffer and scale the input to match the desired span. Inputs applied to the Signal Amplifier (at Pins 17 and 18) are amplified and referred to the 6.2 V reference output in much the same way as the level translation occurs in the V/I converter. Signals from the two preamplifiers are subtracted, the difference is amplified, and the result is fed back to the upper preamp to minimize the difference. Since the two preamps are identical, this minimum will occur when the voltage at the upper preamp just matches the differential input applied to the Signal Amplifier at the left. Since the signal which is applied to the V/I is attenuated across the two 800 Ω resistors before driving the upper preamp, it will necessarily be an amplified version of the signal applied between Pins 17 and 18. By changing this attenuation, you can control the span referred to the Signal Amplifier. To illustrate: a 75 mV signal applied to the V/I results in a 20 mA loop current. Nominally, 15 mV is applied to offset the zero to 4 mA leaving a 60 mV range to correspond to the span. And, since the nominal attenuation of the resistors connected to Pins 16, 15 and 14 is 2.00, a 30 mV input signal will be doubled to result in 20 mA of loop current. Shorting Pins 15 and 16 results in unity gain and permits a 60 mV input span. Other choices of span may be implemented with user supplied resistors to modify the attenuation. (See section “Adjusting Input Span.”) The Signal Amplifier is specially designed to accommodate a large common-mode range. Common-mode signals anywhere up to and beyond the 6.2 V reference are easily handled as long as VIN is sufficiently positive. The Signal Amplifier is biased with respect to VIN and requires about 3.5 volts of headroom. The extended range will be useful when measuring sensors driven, for example, by the auxiliary amplifier which may go above the 6.2 V potential. In addition, the PNP input stage will continue to operate normally with common-mode voltages of several hundred mV, negative, with respect to common. This feature accommodates self-generating sensors, such as thermocouples, which may produce small negative normal-mode signals as well as common-mode noise on “grounded” signal sources. AUXILIARY AMPLIFIER The Auxiliary Amplifier is included in the AD693 as a signal conditioning aid. It can be used as an op amp in noninverting applications and has special provisions to provide a controlled current output. Designed with a differential input stage and an unbiased Class A output stage, the amplifier can be resistively loaded to common with the self-contained 100 Ω resistor or with a user supplied resistor. As a functional element, the Auxiliary Amplifier can be used in dynamic bridges and arrangements such as the RTD signal conditioner shown in Figure 17. It can be used to buffer, amplify and combine other signals with the main Signal Amplifier. The Auxiliary Amplifier can also provide other voltages for excitation FUNCTIONAL DESCRIPTION The operation of the AD693 can be understood by dividing the circuit into three functional parts (see Figure 9). First, an instrumentation amplifier front-end buffers and scales the low- level input signal. This amplifier drives the second section, a V/I converter, which provides the 4-to-20mA loop current. The third section, a voltage reference and resistance divider, provides application voltages for setting the various “live zero” currents. In addition to these three main sections, there is an on-chip auxiliary amplifier which can be used for transducer excitation. VOLTAGE-TO-CURRENT (V/I) CONVERTER The output NPN transistor for the V/I section sinks loop current when driven on by a high gain amplifier at its base. The input for this amplifier is derived from the difference in the outputs of the matched preamplifiers having gains, G2. This difference is caused to be small by the large gain, +A, and the negative feedback through the NPN transistor and the loop current sampling resistor between IIN and Boost. The signal across this resistor is compared to the input of the left preamp and servos the loop current until both signals are equal. Accurate voltage-to-current transformation is thereby assured. The preamplifiers employ a special design which allows the active feedback amplifier to operate from the most positive point in the circuit, IIN. The V/I stage is designed to have a nominal transconductance of 0.2666 A/V. Thus, a 75 mV signal applied to the inputs of the V/I (Pin 16, noninverting; Pin 12, inverting) results in a full-scale output current of 20 mA. The current limiter operates as follows: the output of the feed- back preamp is an accurate indication of the loop current. This output is compared to an internal setpoint which backs off the drive to the NPN transistor when the loop current approaches 25 mA. As a result, the loop and the AD693 are protected from the consequences of voltage overdrive at the V/I input. VOLTAGE REFERENCE AND DIVIDER A stabilized bandgap voltage reference and laser-trimmed resistor divider provide for both transducer excitation as well as precalibrated offsets for the V/I converter. When not used for external excitation, the reference should be loaded by approxi- mately 1 mA (6.2 k Ω to common). The 4 mA and 12 mA taps on the resistor divider correspond to –15 mV and –45 mV, respectively, and result in a live zero of 4 mA or 12 mA of loop current when connected to the V/I Figure 9. Functional Flock Diagram |
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同様の説明 - AD693 |
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