3–392
Motorola Sensor Device Data
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AMPLIFIER SELECTION AND
AMPLIFIER INDUCED ERRORS
A differential amplifier is needed to convert the differential
output of the MPX2010 sensor to a high level ground–
referenced (single–ended). The classic three–op amp instru-
mentation amplifier can be used. However, it requires additional
components (3 op–amps and possibly a split power supply). An
instrumentation topology shown in Figure 1 requires only a
single supply and only 2 op–amps and 1% resistors.
Figure 1. MPX2010 Amplifier Circuit
VREF
+VCC
R+S2
R+S1
+
–
R1
R2
2
3
1
R3
+
–
R4
6
7
5
U1B
VOUT_FS
+VCC
4
3
X1
1
2
S+
PRESSURE SENSOR
S
U1A
The circuit uses a voltage divider R+S1 and R+S2 to provide
the reference (level shift), U1A and U1B are non–inverting
amplifiers arranged in a differential configuration with gain
resistors R1, R2, R3 and R4. Note that U1B is the main gain
stage and it has the most gain. It is recommended to place a
0.015
μ
F capacitor in it’s feedback loop (in parallel with R4) to
reduce noise. The amplifier output can be characterized with
the equation below:
R4
R3
(4)
Gain
1
(5)
Voffset
VREFR2·R1
R1·R3
VSCM
R2·R4
R1·R3
1
(6)
Vout
(S
S ) Gain
Voffset
(7)
where (S
S )
Sensor differential output
Sensor offset
Equation 4 is the differential gain of the amplifier and
equation 5 is the resulting offset voltage of the amplifier.
The above equations assume that the amplifier is close to
ideal (high AOL , low input offset voltage and low input offset
bias currents). Since an ideal op–amp is hard to come by, the
customer should select an op–amp based on cost and perfor-
mance. Below are some points to keep in mind when selecting
an op–amp and designing the amplifier circuit.
Note that the ratio R2*R4/R1*R3 controls the system offset
as well as the common mode error of the amplifier. Mis-
matches in these resistors will result in an offset and common
mode error which appear as offset. It is therefore recom-
mended to use 1% metal film resistors to reduce these errors.
Also, Vref source impedance should be minimized in compari-
son with R1 in order to reduce common mode error.
Amplifier input offset and input bias currents can induce
errors. For example, an input offset (Vio) of the amplifier can
become significant when the closed–loop gain of the amplifier
is increased. Furthermore, there is also a temperature coeffi-
cient of the input voltage offset which contribute an additional
error across temperature. If the input bias current of the ampli-
fier is not taken into account in the design, it can also become
a source of error. A technique to reduce this error is to match
the impedance the source impedance of what the op–amp
input pins sees.
It is important to note that high performance op–amps are
more expensive. An MC33272 op–amp has a low input offset
and low input bias current which is suitable for the two–op amp
amplifier design. We can see that there is a tradeoff between
accuracy and cost when designing a solution with the
MPX2010.
F
Freescale Semiconductor, Inc.
n
.