In part 2, we ll see how to interface an unamplified uncompensated pressure sensor with PSoC3 and the system performance. We ll use the Honeywell NBPMANS015PGUNV for interfacing with PSoC3. The important specifications of Honeywell NBPMANS015PGUNV are listed below.
Supply voltage = 5V
Pressure Range: 0 -15 psi (Gage)
Sensitivity (max): 6.9 mV / psi (25°C)
Sensitivity (min): 3.3 mV / psi (25°C)
Temp Coefficient of sensitivity (max): -3.8 %
Offset: 35.6 mV (50°C)
Temperature coefficient of offset: 1.5%
Specified temp range: -40 to 125 °C
Bridge resistance (max): 5.9 k (50°C)
Accuracy: +/-0.25 % FSS BFSL
Sensor operation:
This type of pressure sensor is a piezo-resistive sensor (Wheatstone s bridge) driven by a voltage supply. The bridge output voltage is directly proportional to the applied pressure and the supply voltage. The primary sources of error to be factored in while designing with this type of sensor is the sensor offset error, span error and temperature coefficient of span and offset (since the sensor is temperature uncompensated, temperature coefficient of span and offset play a major role in the final error).
Design:
The design parameters of concern are the ADC resolution, input range and reference.
ADC input range:
This parameter is dependent on the maximum voltage output, V0, from the pressure sensor. At 5V supply and using the maximum offset and sensitivity possible, we get;
V0 (max) = 6.9 * 15 + 35.6 = 139.1 mV
ADC input range should be greater than +/-0.256 V.
Resolution:
1/1000th of the full scale resolution is sufficient in pressure sensing applications.
Pressure resolution = 15 psi/1000 = 0.015 psi
Voltage resolution = 49.543 mV/1000 = 49.543uV
This requires a 16-bit ADC in +/-1.024V range or 14-bit ADC in +/-0.256V range.
Reference:
A ratiometric reference should be used in this case. Hence PSoC reference should be configured for internal vdda/4 , where vdda = 5 volts.
PSoC Creator Top Design:
The ADC has three channels, one for sensing pressure and the other two used for temperature measurement. The RTD temperature is measured as described in AN70698. ADC configuration for the pressure sensing channel is shown below.
Note that +/-Vref/4 range can also be used for this configuration in 14-bit mode.
Pressure Equation:
From the measured ADC voltage, the pressure is calculated from the equation below;
P = (Vo / S) * Pr
P Pressure (in psi)
V0 Bridge output voltage in mV
S Span in mV
Pr Rated Pressure (in psi)
Calibrations required:
Room Temperature calibration:
Since span has a very high tolerance, we have to calibrate span before using it. Pressure sensor offset should also be calibrated before use.
Offset Calibration:
Offset of the pressure sensor has to be corrected by giving a zero pressure input and measuring the ADC output voltage, Voff.
Voff = Voffp + Voffs
Voffp Pressure sensor offset
Voffs signal chain offset
Span/Gain Calibration:
The span of the pressure sensor is calibrated by applying a full scale pressure input to the pressure sensor and measuring the ADC output voltage, Vfs.
S = Vfs
(Where S is the Span)
By doing span calibration we are calibrating both the span error of the sensor and gain error of the ADC.
Temperature calibration:
Both the pressure sensor offset and span varies with temperature and they have to be calibrated. But the sensor datasheet doesn t provide information on the span or offset calibration. It provides only the limits of the error. If the characteristic curve is found by experiment, we can correct for both span and offset temperature coefficient accurately.
List of all errors:
|
S.No
|
Parameter
|
Error at 10 psi (in psi)
|
|
Sensor
|
|
1
|
Offset
|
0 *
|
|
2
|
Span error
|
0 *
|
|
3
|
Temperature coefficient of offset (50 °C)
|
1.5 (Can be calibrated)
|
|
4
|
Temperature coefficient of span (50 °C)
|
-0.6 (Can be calibrated)
|
|
5
|
Non-linearity
|
0.0375
|
|
Signal Chain
|
|
5
|
Offset
|
0
|
|
6
|
Gain error
|
0 *
|
|
7
|
Offset drift (at 50°C)
|
< 0.004
|
|
8
|
Gain drift (at 50°C)
|
0.01 (can be calibrated)
|
|
9
|
INL
|
0.02
|
*Note: Assumes calibration source has zero error.
ADC INL and the sensor non-linearity are the only factors that can t be calibrated and will affect the final measurement.
PSoC Value:
Apart from integrating the analog front end, ADC and the MCU, providing a separate channel for accurate temperature measurement, PSoC can integrate miscellaneous features suchascapsense, segment LCD drive and communications protocols. Designing with PSoC creator can reduce the design time considerably. The BOM cost and board size can also be significantly reduced.
In the next part we ll see how to interface unamplified compensated pressure sensor with PSoC3.
By Praveen Sekar