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Motorola Sensor Device Data
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Prepared by: Eric Jacobsen and Jeff Baum
Sensor Applications Engineering
Motorola Signal Products Division
Phoenix, AZ
INTRODUCTION
Although
temperature compensated monolithic sensor IC’s are
commercially available today, there are many applications
where the flexibility of designing custom signal–conditioning
is of great benefit. Perhaps the need for a versatile low–level
sensor output is best illustrated by considering two particular
cases that frequently occur: (1) the user is in a prototyping
phase of development and needs the ability to make changes
rapidly to the overall transfer function of the combined
sensor/amplifier subsystem, (2) the specific desired transfer
function does not exist in a fully signal–conditioned,
precision–trimmed sensor product (e.g., a signal–conditioned
device is precision trimmed over a different pressure range
than that of the application of interest). In such cases, it is
obvious that there will always be a need for low–level,
nonsignal–conditioned sensors. Given this need, there is also
a need for sensor interface amplifier circuits that can signal
condition the “raw” sensor output to a usable level. These
circuits should also be user friendly, simple, and cost effective.
Today’s unamplified solid–state sensors typically have an
output voltage of tens of millivolts (Motorola’s basic 10 kPa
pressure sensor, MPX10, has a typical full–scale output of
58 mV, when powered with a 5 V supply). Therefore, a gain
stage is needed to obtain a signal large enough for additional
processing. This additional processing may include
digitization by a microcontroller’s analog to digital (A/D)
converter, input to a comparator, etc. Although the
signal–conditioning circuits described here are applicable to
low–level, differential–voltage output sensors in general, the
focus of this paper will be on interfacing pressure sensors to
amplifier circuits.
This paper presents a basic two operational–amplifier
signal–conditioning circuit that provides the desired
characteristics of an instrumentation amplifier interface:
High input impedance
Low output impedance
Differential to single–ended conversion of the pressure
sensor signal
High gain capability
fully
signal–conditioned,
calibrated,
and
For this two op–amp circuit, additional modifications to the
circuit allow (1) gain adjustment without compromising
common mode rejection and (2) both positive and negative dc
level shifts of the zero pressure offset. Varying the gain and
offset is desirable since full–scale span and zero pressure
offset voltages of pressure sensors will vary somewhat from
unit to unit. Thus, a variable gain is desirable to fine tune the
sensor’s full–scale span, and a positive or negative dc level
shift (offset adjustment) of the pressure sensor signal is
needed to translate the pressure sensor’s signal–conditioned
output span to a specific level (e.g., within the high and low
reference voltages of an A/D converter).
For the two op–amp gain stage, this paper will present the
derivation of the transfer function and simplified transfer
function for pressure sensor applications, the derivation and
explanation of the gain stage with a gain adjust feature, and
the derivation and explanation of the gain stage with the dc
level shift modification.
Adding another amplifier stage provides an alternative
method of creating a negative dc voltage level shift. This
stage is cascaded with the output from the two op–amp stage
(Note:gain of the two op–amp stage will be reduced due to
additional gain provided by the second amplifier stage). For
this three op–amp stage, the derivation of the transfer
function, simplified transfer function, and the explanation of
the negative dc level shift feature will be presented.
GENERAL NOTE ON OFFSET ADJUSTMENT
Pressure sensor interface circuits may require either a
positive or a negative dc level shift to adjust the zero pressure
offset voltage. As described above, if the signal–conditioned
pressure sensor voltage is input to an A/D, the sensor’s
output dynamic range must be positioned within the high and
low reference voltages of the A/D; i.e., the zero pressure
offset voltage must be greater than (or equal to) the low
reference voltage and the full–scale pressure voltage must
be less than (or equal to) the high reference voltage (see
Figure 1). Otherwise, voltages above the high reference will
be digitally converted as 255 decimal (for 8–bit A/D), and
voltages below the low reference will be converted as 0. This
creates a nonlinearity in the analog–to–digital conversion.
SEMICONDUCTOR APPLICATION NOTE
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