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AN1324
A Simple Sensor Interface Amplifier
Prepared by: Warren Schultz
Discrete Applications Engineering

INTRODUCTION
Compensated semiconductor pressure sensors such as is directly compatible with Microcomputer A/D inputs. A
the MPX2000 family are relatively easy to interface with digital description of an Evaluation Board and design considerations
systems. With these sensors and the circuitry that is described are presented as follows.
here, pressure is translated into a 0.5 to 4.5 V output range that
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Figure 1. DEVB173 Sensor Building Block Evaluation Board

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EVALUATION BOARD DESCRIPTION
A summary of the information required to use the Sensor ELECTRICAL CHARACTERISTICS
Mini Block evaluation board, part number DEVB173, is The following electrical characteristics are included as a
presented as follows. A discussion of the design appears guide to operation.
under the heading Design Considerations.
Characteristic Symbol Min Typ Max Units
Power Supply Voltage B+ 10 — 30 Volts
Full Scale Pressure PFS kPa
MPX2010 — — 10
FUNCTION MPX2050 — — 50
The evaluation board shown in Figure 1 is designed to MPX2100 — — 100
MPX2200 — — 200
translate pressure, vacuum, or differential pressure into a
MPX2700 — — 700
single–ended, ground referenced voltage that is suitable for
direct input to microcomputer A/D ports. It has two input ports. Overpressure PMAX — — 700 kPa
P1, the pressure port, is on the top side of the sensor and P2, Full Scale Output VFS — 4.5 — Volts
a vacuum port, is on the bottom side. These ports can be Zero Pressure Offset VOFF — 0.5 — Volts
supplied pressure on P1 or vacuum on P2, or a differential
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Sensitivity SAOUT — 4V/PFS — V/kPa

pressure between P1 and P2. Any of these sources will
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produce equivalent outputs. Quiescent Current ICC — 25 — mA

The output is a ground referenced analog signal. It
nominally supplies 0.5 V at zero pressure and 4.5 V at full CONTENT
scale. A zero adjustment has been made at the factory with Board contents are described in the following parts list and
trim resistor R7. Full scale output is approximately 4 V above schematic. A pin–by–pin circuit description follows in the next
the zero setting. section.

Table 1. Parts List

Designator Qty. Description Value Vendor Part
C1 1 Ceramic Capacitor 0.2 µF
C2 1 Ceramic Capacitor 0.2 µF
C3 1 Ceramic Capacitor 0.001 µF
R1* 1 1/4 Watt Film Resistor 93.1 k 1%
R2 1 1/4 Watt Film Resistor 750 1%
R3 1 1/4 Watt Film Resistor 39.2 k 1%
R4* 1 1/4 Watt Film Resistor 100 1%
R5 1 1/4 Watt Film Resistor 1.33 k 1%
R6 1 1/4 Watt Film Resistor 11 k 1%
R7 1 1/4 Watt Film Resistor Trim
U1 1 Op Amp Motorola MC33272P
U2 1 8 V Regulator Motorola MC78L08ACP
XDCR1 1 Pressure Sensor Motorola MPX2100DP
* For MPX2010 Sensors R1 = 150 k & R4 = 61.9 ohms

PIN–BY–PIN DESCRIPTION
B+: GND:
Input power is supplied at the B+ terminal. Minimum input The terminal labeled GND is intended for use as the power
voltage is 6.8 V and maximum is 30 V. supply return. It is generally advisable to leave enough bare
wire going into this terminal to conveniently provide a
connection for instrumentation ground clips.
OUT:
An analog output is supplied at the OUT terminal. The signal P1, P2:
it provides is nominally 0.5 V at zero pressure and 4.5 V at full Pressure and Vacuum ports P1 and P2 protrude from the
scale. This output is designed to be directly connected to a sensor on the right side of the board. Pressure port P1 is on
microcomputer A/D channel, such as one of the E ports on an the top and vacuum port P2 is on the bottom. Neither port is
MC68HC11. labeled. Maximum safe pressure is 700 kPa.

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B+

U1
3 MC78L08ACP
I 1
O
G 5 8
2 + 7
6 – OUT
C1
0.2 µF C2 3 2 XDCR1 U2B
0.2 µF MPX2000 MC33272
R1
SERIES
4 1 SENSOR 93.1 k 1%
GND

C3
R7 R3 R2 0.001 µF
TRIM 39.2 k U2A 750
3 MC33272 1%
+ 1
2 –
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4
R5 R6
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1.33 k 11 k
1% 1%
NOTES: R4
R7 selected for zero pressure VOUT = 0.5 V 100
For MPX2010 Sensors: 1%
R1 = 150 k
R4 = 61.9 Ohms

Figure 2. Sensor Mini Block

+8 V
5 8
+ 7
6 – OUT
3 2 XDCR1 U2B
MPX2000 MC33272
R1
SERIES
4 1 SENSOR 93.1 k
R2 1%
U2A 750
3 MC33272 1%
+ 1
2 –
VOFFSET 4
R6
12.4 k
1%
R4
100
1%

Figure 3. Simplified Schematic

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DESIGN CONSIDERATIONS
When interfacing semiconductor pressure sensors to Gain can be determined by assuming a differential output
microcomputers, the design challenge is how to take a at the sensor and going through the same calculation. To do
relatively small DC coupled differential signal and produce a this assume 100 mV of differential output, which puts pin 3 of
ground referenced output that is suitable for driving A/D U2A at 3.95 V, and pin 5 of U2B at 4.05 V. Therefore, 3.95 V
inputs. A very simple interface circuit that will do this job is is applied to R6, generating 319 µA. This current flowing
shown in Figure 2. It uses one dual op amp and several through R4 produces 31.9 mV, placing pin 1 of U2A at 3950 mV
resistors to amplify and level shift the sensor’s output. To see + 31.9 mV = 3982 mV. The voltage across R2 is then 4050 mV
how this amplifier works, let’s simplify it in Figure 3, and – 3982 mV = 68 mV, which produces a current of 91 µA that
assume VOFFSET is zero. If the common mode voltage at pins flows into R1. The output voltage is then 4.05 V + (91 µA •
93.1 k) = 12.5 V. Dividing 12.5 V by the 100 mV input yields
2 and 4 of the sensor is 4.0 V, then pin 2 of U2A and pin 6 of
a gain of 125, which provides a 4 V span for 32 mV of full scale
U2B are also at 4.0 V. This puts 4.0 V across R6. Assuming
sensor output.
that the current in R4 is equal to the current in R6, 323 µA x
Returning to Figure 2, a 0.5 V VOFFSET is generated by the
100 ohms produces a 32 mV drop across R4 which adds to the
divider consisting of R3, R5, and R7. To keep the input
4.0 V at pin 2. The output voltage at pin 1 of U2A is, therefore, impedance looking into pin 2 of U2A at 12.4 k, R6 is chosen
4.032 V. This puts 4.032 – 4.0 V across R2, producing 43 µA. as 11 k. The divider impedance is then chosen to nominally be
The same current flowing through R1 again produces a 1.4 k, providing a total of 12.4 k. For purposes of analysis, the
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voltage drop of 4.0 V, which sets the output at zero. complete circuit in Figure 2 is then equivalent to Figure 3 with
Substituting a value for VOFFSET other than zero into this a VOFFSET input of 0.5 V.
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calculation reveals that the zero pressure output voltage The resulting 0.5 V to 4.5 V output from pin 7 of U2B is
equals VOFFSET. For this DC output voltage to be independent directly compatible with microprocessor A/D inputs. Over a
of the sensor’s common mode voltage it is necessary to satisfy zero to 50°C temperature range combined accuracy for the
the condition that R1/R2 = R6/R4. sensor and interface is ±5%.

APPLICATION
Using the Sensor Mini Block’s analog output to provide optimum use of microcomputer A/D inputs. A direct
pressure information to a microcomputer is very connection from the evaluation board output to an A/D input
straightforward. The output voltage range which goes from 0.5 is all that is required. Using the MC68HC11 as an example, the
V at zero pressure to 4.5 V at full scale is designed to make output is connected to any of the E ports, such as port E0.

CHANGING SENSORS
In order to change pressure ranges, MPX2050, MPX2100, change the full scale output from 4.5 V at 100 kPa to 4.5 V at
MPX2200, and MPX2700 pressure sensors can be 200 kPa. To make this translation with an MPX2010 requires
substituted directly for each other. When one of these sensors changing R1 from 93.1 k to 150 k and R4 from 100 ohms to
is substituted for another, the 4.5 V full scale output will remain 61.9 ohms. With R1 at 93.1 k and R4 at 100 ohms, full scale
the same and correspond to the new sensor’s full scale span for an MPX2010 is only 2.5 V, producing a nominal full
pressure specification. For example, substituting an scale output voltage of 3.0 V.
MPX2200 200 kPa sensor for an MPX2100 100 kPa unit will

FURTHER SIMPLIFICATION
In non–demanding applications the 7 resistor topology that values in Figure 2, this would mean R3 = 200 k, R5 = 13.3 k,
is shown in Figure 2 can be reduced to 5, by eliminating R6 and R6 = 0, and R7 is open. In an untrimmed system, there is no
R7. Without R7 the zero pressure offset is untrimmed. real disadvantage to doing this, provided that the ratios can be
However, in microprocessor based systems it is relatively sufficiently matched with standard resistor values.
easy to read the zero pressure offset voltage, store it, and The other option is to eliminate R6 and trim R3 with R7. This
calibrate in software. This can be done automatically when the situation is somewhat different. The trimming operation will
unit powers up, or as a calibration procedure. R6 can be throw the ratio off, and reduce common mode rejection.
eliminated (reduced to zero ohms) by directly connecting the Typically several percent of any change in the sensor’s
R3, R5 divider to pin 2. The output impedance of this divider common mode voltage will show up as an output error when
then needs to be choosen such that its ratio with R4 = R1/R2, this configuration is used.
in other words [R3•R5/(R3+R5)]/R4 = R1/R2. Given the

CONCLUSION
Perhaps the most noteworthy aspect to the sensor amplifier one dual op amp and a few resistors. The result is a simple and
described here is its simplicity. The interface between an inexpensive circuit that is capable of measuring pressure,
MPX2000 series sensor and a microcomputer A/D consists of vacuum or differential pressure.

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