3–205
Motorola Sensor Device Data
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Figure 5. Instrument Amplifier Transfer Function with
Spread of the Device to Device Offset Variation
V1, V2
5 Vdc
1/2
VCC
VEE
VCC
V1
V2X
V1X
V2
VPS
(mV)
0
5
10
15
20
Figure 5 shows the transfer function of different instrument
amplifiers used in the same application. The same sort of
random errors are generated by crossing the inputs of the
instrument amplifier. The spread of the differential output
voltage (V1–V2) and (V2x–V1x) is due to the unsigned voltage
offset and its absolute value. Figures 6 and 7 show the
unit–to–unit variations of both the offset and the bias current
of the dual op amp MC33078.
Vio (mV)
T
(
°
C)
+2
+1
0
–1
–2
–50
–25
0
25
50
75
100
125
Figure 6. Input Offset Voltage versus Temperature
UNIT 1
UNIT 2
UNIT 3
To realize such a system, the designer must provide a
calibration procedure which is very time consuming. Some
extra potentiometers must be implemented for setting both the
offset and the Full Scale Output with a complex temperature
compensation network circuit.
The new proposed solution will reduce or eliminate any
calibration procedure.
Figure 7. Input Bias Current versus Temperature
lib (nA)
T
(
°
C)
600
450
300
150
0
–50
–25
0
25
50
75
100
125
UNIT 1
UNIT 2
MCU CONTRIBUTION
As shown in Figure 5, crossing the instrument amplifier
inputs generated their mutual differences which can be
computed by the MCU.
1
2
4
3
Rg
Rf
–
+
+
–
V1
V2
P
0 V
Figure 8. Crossing of the Instrument Amplifier
Input Using a Port of the MCU
éé
éé
+VS
Figure 8 shows the analog switches on the front of the
instrument amplifier and the total symmetry of the chain. The
residual resistance RDS(on) of the switches does not ntroduce
errors due to the high input impedance of the instrument
amplifier.
With the aid of two analog switches, the MCU successively
converts the output signals V1, V2.
Four conversions are necessary to compute the final result.
First, two conversions of V1 and V2 are executed and stored
in the registers R1, R2. Then, the analog switches are
commuted in the opposite position and the two last
conversions of V2x and V1x are executed and stored in the
registers R2x and R1x. Then, the MCU computes the following
equation:
RESULT = (R1 – R2) + (R2x – R1x) (2)
F
Freescale Semiconductor, Inc.
n
.