Introduction to differential pressure sensor
A differential pressure sensor is a type of sensor widely used in engineering, industry and science to measure the difference between two pressures.
（Pressure sensor）
Its working principle is based on the relationship between pressure and pressure.
A differential pressure sensor usually consists of two input channels connected to the measured medium to measure the pressure at two different locations in the measured medium. The two channels can be separated, or they can be connected by a membrane or pipe.
When the pressure in the measured medium is different, the pressure in the two channels will also be different. The differential pressure sensor measures the difference in pressure in the two channels and converts it into a corresponding electrical output.
The commonly used differential pressure sensors are piezoelectric sensors based on piezoelectric effect and resistance sensors based on resistance effect.
Piezoelectric differential pressure sensors take advantage of the properties of piezoelectric materials, which generate electric charges when pressure is applied to the measured medium. After amplification and conversion, this charge signal can be used as the output signal of the differential pressure sensor.
Resistive differential pressure sensors use changes in resistance to measure differential pressure. One of the common resistive differential pressure sensors is based on a metal film structure. When the pressure is applied to the measured medium, the metal film will be deformed, and then change the resistance value.
By measuring the change of resistance value, the output signal of the differential pressure sensor can be obtained. Differential pressure sensors usually have high precision, fast response and a wide measurement range. They can be widely used in gas, liquid and solid pressure difference measurement, such as meteorological pressure measurement, liquid level measurement, wind speed measurement, etc.
In the low-temperature industry, the differential pressure sensor can be used as a liquid level detection in the storage tank, as shown in the figure on the right, leading a duct to the liquid phase of the sensor at the bottom of the tank, and then leading a tube to the gas phase at the top, according to the pressure formula:
p=ρgh，Δp = Pa-Pb= Δhρg,
In this way, the H height can be calculated from the density, or converted to the corresponding millimeter column of water.
Taking X manufacturer’s products as an example, the circuit used inside is a resistance bridge circuit, and its output voltage range is 60-160mv, the corresponding range is 0-20Kpa, and each PASCAL corresponds to 5μV.
The wiring diagram is as follows:
（Wiring diagram）
Here, we use 24-bit ADC SSP1220 to collect the voltage data, and then convert it into the corresponding liquid level.
Introduction to the SSP1220
SSP1220 is a precision 24-bit analog-to-digital converter developed and designed by Siproin Microelectronics. It has the following features:
- Two differential inputs or four single-ended inputs,
- A low noise programmable gain amplifier (PGA),
- Two programmable excitation current sources,
- A voltage reference, an oscillator,
- A low-side switch and a precision temperature sensor.
The SSP1220 is capable of performing conversions at sampling data rates up to 2k SPS and is stable within a single cycle. For industrial applications in noisy environments, the digital filter provides both 50Hz and 60Hz rejection at a sampling frequency of 20SPS.
The internal PGA offers up to 128V/V gain. This makes the SSP1220 ideal for small sensor signal measurement applications such as resistive temperature detectors (RTDS), thermocouples, thermistors and resistive bridge sensors. The SSP1220 supports the measurement of pseudo-differential or fully differential signals when using PGA, and can also be configured to disable internal PGA, operating in duty cycle mode with PGA disabled with power consumption as low as 120µA.
The SSP1220 Thin Small Form Factor (TSSOP)-16 package is rated for operating temperature range from -40°C to +125°C.
（SSP1220 pin diagram）
The SSP1220 communicates with the controller through the SPI. The controller works in active mode and the chip works in slave mode. The SSP1220 works only in mode 1, that is, CPOL is set to 0 and CPHA is set to 1. (Other modes are not supported)
In SPI mode 1, SCLK remains low in the idle state and data is transmitted or changed only at the SCLK rising edge. The master device and slave device latch or read data at the SCLK fall edge. The SPI timing requirements are as follows:
（Serial Interface Timing Requirements）
Reference circuit design
1.power supply system, input voltage 3.6-5.5V, here use LDO directly step-down to 3V, the reference voltage is external 2.5V.
（Power supply system diagram）
2.SSP1220 sampling circuit
(SSP1220 Sampling diagram)
（Wiring diagram）
- V+ connected to the upper bridge arm B+;
- V- to lower bridge arm B-;
- S+ is connected to the differential input high end;
- S- connects to the lower end of the differential input;
3.MCU and 485 circuit, we convert the data into 485 output
（485 circuit diagram）
4.Actual test data
Tested in practice：
| Differential pressure value （Kpa） | Voltage value （μV） | Measured value （μV） | Differential pressure value （Kpa） | Voltage value （μV） | Measured value （μV） |
| 0 | 60000 | 61130 | 10 | 110000 | 111142 |
| 1 | 65000 | 66140 | 11 | 115000 | 116151 |
| 2 | 70000 | 71127 | 12 | 120000 | 121158 |
| 3 | 75000 | 76142 | 13 | 125000 | 126149 |
| 4 | 80000 | 81143 | 14 | 130000 | 131161 |
| 5 | 85000 | 86144 | 15 | 135000 | 136149 |
| 6 | 90000 | 91137 | 16 | 140000 | 141175 |
| 7 | 95000 | 96131 | 17 | 145000 | 146162 |
| 8 | 100000 | 101139 | 18 | 150000 | 151151 |
| 9 | 105000 | 106128 | 19 | 155000 | 156152 |
In order to achieve the best accuracy, we use the minimum rate of 20SPS sampling, you can see a fluctuation of about 10μV at the position of 100μV, actually can already meet the accuracy requirements.
