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Philips Semiconductors Application note
AN1651Using the NE/SA5234 amplifier
1991 Oct
11
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/R
SH
. 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.