Constant current excitation for HMU and HDU

Constant current excitation for HMU and HDU pressure sensors
1
INTRODUCTION
Silicon based piezoresistive pressure sensors use a
Wheatstone bridge implanted into a thin diaphragm
to transform pressure into an electrical output. When
pressure is applied to the diaphragm, the electrical
resistivity changes due to the mechanical stress
(piezoresistive effect). If the bridge circuit is supplied
with a voltage, a sensor output signal proportional to
pressure is generated.
The bridge resistors as well as the pressure
sensitivity of the silicon diaphragm are temperature
dependent and change over the operating
temperature range of the sensor. For all
piezoresistive silicon pressure sensors the bridge
resistance increases with increasing temperature
while the pressure sensitivity decreases with
increasing temperature. In order to maintain a
temperature stable output voltage all piezoresistive
pressure sensors have to be temperature
compensated. Different methods such as passive
compensation with resistor networks or active
compensation with op-amps as well as ASIC or
microcontroller based digital compensation can be
used. Under certain conditions a temperature
compensation can also be achieved by applying a
constant current across the sensor bridge.
3
CIRCUIT DESIGN
Two examples of a constant current excitation circuit
for First Sensors HMU and HDU pressure sensors
are shown in Fig. 4 and 5.
3.1 Constant current source with op-amp
The circuit shown in Fig. 4 designs a constant
current source from a single operational amplifier.
The current source is controlled by a ±1 % band-gap
reference diode ZR.
The bridge current IB is defined by
IB =
(VR − VO )
R2
where
VR
VO
R2
= Diode reference voltage (1.235 V ±1 %)
= Amplifier offset voltage (~0 V)
= Current set resistor (820 Ω)
Selecting amplifier A1 with an offset voltage <1 mV
and a ±0.1 % tolerance resistor R2 with a standard
value of 820 Ω delivers a current of IB = 1.51 mA with
a typical accuracy of ±1.2 %.
3.2 Constant current source with shunt regulator
2
CONSTANT CURRENT EXCITATION
Constant current excitation is a simple and low cost
method for temperature compensation of span which
uses the internal characteristics of the silicon sensor
element. First Sensors HMU and HDU pressure
sensor elements are designed in such a way that the
bridge resistance as well as the sensitivity change
over temperature with nearly the same absolute
magnitude but with opposite signs (compare Fig. 1
and 2). Therefore, if the sensor is supplied with a
constant current the voltage across the bridge
increases with increasing temperature and
compensates for the decreasing sensitivity. The
result is a self-compensated HMU/HDU sensor with
a typical span error of less than ±1.5 % FSS over an
operating temperature range of -20...+50 °C (see Fig. 3).
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The circuit shown in Fig. 5 uses a LMV431A
adjustable precision shunt regulator to achieve a
constant current through the sensor bridge.
The bridge current IB is defined by
IB =
VR
R2
where
VR
R2
= Transistor reference voltage (1.235 V ±1 %)
= Current set resistor (820 Ω)
Selecting a ±0.1 % tolerance resistor R2 with a
standard value of 820 Ω delivers a current of
IB = 1.51 mA.
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Bridge resistance [Ohm]
Constant current excitation for HMU and HDU pressure sensors
Temperature [° C]
Sensitivity [mV/(psi*Vs)]
Fig. 1: Bridge resistance over temperature for HMU and HDU pressure sensors
Temperature [° C]
Span error [%FSS]
Fig. 2: Sensitivity over temperature for HMU and HDU pressure sensors
Temperature [° C]
Fig. 3: Typical span error over temperature for constant current excitation of HMU and HDU pressure sensors
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Constant current excitation for HMU and HDU pressure sensors
V+
R1
VR
IB = 1.51 mA
+
VO
A1
-
C1
HMU/
HDU
Sensor
-Vout
ZR
R2
+Vout
GND
V+ = >10 V
A1 = LT1490A or LTC1051
ZR = LT1034-1.2 or LT1004-1.2
C1 = 0.1 µF
R1 = Select according to LT1034-1.2 or LT1004-1.2 data sheet
R2 = 820 Ohm
Fig. 4: Constant current source with op-amp for HMU and HDU pressure sensors
V+
R1
HMU/
HDU
Sensor
-Vout
+Vout
IB = 1.51 mA
T1
Z1
VR
R2
GND
V+ = >10 V
T1 = BC847
Z1 = LMV431A
R1 = Select according to LMV431A data sheet, e.g. 62 kOhm
R2 = 820 Ohm
Fig. 5: Constant current source with shunt regulator for HMU and HDU pressure sensors
First Sensors does not assume any liability arising out of the application or use of any product or circuit described herein, neither does
it convey any license under its patent rights nor the rights of others.
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