APPLICATIONS EXAMPLES
Instrumentation
Strain Gauge Bridge Amplifier
The circuit below shows a simple strain gauge circuit with a gain of 100 (40dB) and operated from a single supply. The chart illustrates the transfer function of the circuit for a single order-of-magnitude signal differential range from the bridge beginning with 5mV up to 50mV. The circuit is operated from a single 5V supply, but could equally as well be configured to use a dual balanced supply. It is immediately evident that the wide common-mode output range of the NE5234 is very advantageous in handling this wide range of signals with good linearity due to this feature.
A variation on this particular idea is the remote strain gauge circuit operating from a three wire line, one of which is the shield. This full-differential input circuit has balanced
input resistance to afford good common-mode noise rejection characteristics. Resistors are metal film or deposited carbon. Supply leads must be carefully bypassed close to the NE/SA5234 with ceramic or chip monolithic capacitors to give optimum noise performance. As shown, an auxiliary sub-regulator may be added to improve the overall DC stability of the bridge signal voltage. A regulator capable of providing the necessary few milliamperes at somewhat reduced voltage for the transducer is shown in one of the following examples. This makes use of one of the op amps in the same device package to provide the voltage regulation. Note that the use of multiple op amps within a single package minimizes the possibility of thermal drift and mismatched response from various DC parameters.
Multiple sets of transducers may be constructed from The NE/SA5234 or the NE5234D surface mount device to form a compact and stable instrumentation package. This is useful for transducer applications in
the measurement of pressure, strain, position and temperature, which have similar circuit configurations. First order temperature compensation of the transducers such as semiconductor strain gauges, or resistive units may be achieved by using one of the gauges as a reference device only. It is thermally coupled to the same member as the active gauge, as shown in the example. (Figure 18)
A 4 to 20mA Current Loop
Some instrumentation installations require the 4-20mA current loop. This addition to the above bridge transducer circuit examples is demonstrated in Figure 16.
This circuit makes use of the remote transducer bridge previously described and adds current loop signaling capability. The voltage-to-current converter consists of an additional op amp from the same NE/SA5234 package combined with a single transistor to
drive the current loop. The sensitivity is actually in mA/V, or transconductance, which is equal to 1/RSH. This sensitivity in this particular example is set to 4mA/V. Thus, with a bridge amplifier having a differential gain of 100, an input of 10mV will produce a 4mA output current and 50mV will produce a 20mA output. Of course the line resistance plus receiver resistance must be within the voltage compliance range of the supply voltage to guarantee linear operation over the total range. A negative supply may be used if it is preferred to have the current loop referenced to ground.
DC Regulators and Servos
Closely related to DC and low frequency AC linear transducers are DC regulators and servo circuits. The proliferation of many battery, and solar powered remote instrumentation packages results in a need for adaptable circuits which may readily be made up from existing stock ICs. The examples given here are quite simple, but can be very useful to the designer when economy and size are at a premium.
Solar Regulator for 3-Volt CMOS
Working with small instrumentation packages which are to operate from solar photovoltaic cells may bring a need for simple sub-regulators for MOS circuits requiring only a few milliamperes of drain current. Figure 19 shows a simple low voltage regulator making use of the particularly excellent DC characteristics of the NE/SA5234. The regulator becomes an integral part of any functional analog signal processing package such as an environmental data instrumentation unit. The low current drain of the the typical 3V or 5V MOS digital IC allows one sub regulator to serve up to 10 or more such devices. If the instrument package is to be subjected to wide temperature variations, the SA5234 is recommended. A second op amp in the package may serve as a low battery alarm with tone modulator as in radio links, or simple logic level comparator. Overcurrent protection is easily added within the regulator loop to detect short circuit failures and automatically limit the current.
DC Servo-amps
Servo control systems for low voltage motor drives require high gain-accuracy and good DC stability for many applications. Applications such as the position control of air flow vanes, servo valves, and optical lenses or apertures, are typical examples. Figure 20 demonstrates one simple DC motor servo application with position control feedback. The motor is a 3V permanent magnet rotor type used in micro-position applications and is adaptable to battery supply environments.
Position information is received from a multi-turn potentiometer to give adequate resolution. The input voltage may be generated from another potentiometer which is remote from the motor drive unit proper, or from a D/A converter output for micro processor controlled systems. The input voltage range is 1.0 to 3.0V and the supply voltage is 4.5V.
