RICHARD MARKELL
Linear Technology Corp., 1630 McCarthy Blvd.,
Milpitas, CA 95035-7417; (408) 432-1900.
ELECTRONIC DESIGN / JUNE 9, 1997から引用
L
iquid sensors require a media-compatible, solid-state pressure sensor.
The pressure range of the sensor
depends on the height of the column or
tank of fluid that must be sensed. Described here is a way to sense water
height in a tank or column using
EG&G IC Sensors’ Model 90 stainlesssteel diaphragm, 0-15 psig sensor.
Because large chemical or water
tanks are typically located outside in
“tank farms”, it’s insufficient to provide
only an analog interface to a digitization system for level sensing. This is because the very long wires required to
interconnect the system cause IR
drops, noise, and other corruption of
the analog signal. The solution is a system that converts the analog-to-digital
signals at the sensor. In this application, a “liquid height-to-frequency converter” was implemented.
The analog front end of the system
includes the LT1121 linear regulator
for powering the system (Fig. 1). The
LT1121 is a micropower, low-dropout
linear regulator with shutdown. For
micropower applications of this or
other circuits, the ability to shut down
the entire system via single powersupply pin allows the system to operate only when taking data (perhaps
every hour), conserving power.
The LT1121 (U3) converts 12 V to 9
V to power the system. The 12 V may
be obtained from a wall cube or batteries. The LT1034, a 1.2-V reference, is
used with U1D, 1/4 of an LT1079 quad
low-power op amp, to provide a 1.5-mA
current source to the pressure sensor.
The reference voltage also is divided
down by R4, R5, R6, and the 10k potentiometer. It’s used to offset the output
amplifier (U2A) so that the signals
don’t swing around the supply rails.
Op amps U1A and U1B (each 1/4 of
an LT1079) amplify the bridge’s pressure-sensor output and provide a differential signal to U2A (an LT1490).
U2A must be a rail-to-rail op amp; the
system’s analog output is taken from
U2A’s output. The pressure change is
independent of diameter of the water
column, so that a tank of liquid would
produce the same resulting output
voltage as a column of the same height.
The remainder of the circuitry allows for transmission of analog data
over long distances (the circuit was
designed by Jim Williams) (Fig. 2).
The circuit takes a dc input from 0 V to
5 V and converts it to a frequency. For
the pressure circuit in Figure 1, this
translates to approximately 0 Hz to 5
kHz. The voltage-to-frequency converter in Figure 2 has very low power
consumption (26 µA), 0.02% linearity,
60 ppm/°C drift, and 40 ppm/V powersupply rejection.
When operating, C1 switches a
charge pump, consisting of Q5, Q6, and
the 100-pF capacitor, to maintain its
negative input at 0 V. The LT1004s and
associated components form a temperature-compensated reference for the
charge pump. The 100-pF capacitor
charges to a fixed voltage. Consequently, the repetition rate is the circuit’s only degree of freedom to maintain feedback. Comparator C1 pumps
uniform packets of charge to its negative input at a repetition rate precisely
proportional to the input-voltage-derived current. This action ensures that
circuit output frequency is determined
strictly and solely by the input voltage.
Figure 3 shows the output frequency versus column height for two
different Model 90 transducers. Note
the straight lines, which are representative of excellent linearitry.
