Datasheet AD693 (Analog Devices) - 5

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