Selecting Surface Mount Packaging Formats for Digital Environmental Sensors
Selecting surface mount sensor packages requires balancing land-pattern mechanical stress and reflow limits against bus integration effort and unit yield cost.
Ratiometric Analog
Output Correlation
Signal characteristics of a sensor output vary in direct proportion to the supply voltage provided to the device. A ratiometric analog signal eliminates the need for a precision voltage reference at the analog to digital converter. The output voltage is expressed as a fraction of the supply, typically ranging from 10 to 90 percent.
Noise Rejection
Power supply fluctuations affect both the sensor and the converter simultaneously, which cancels out the error in the digital reading. If the supply drops by five percent, the ratiometric analog output also drops by five percent, maintaining the same digital count. This behavior simplifies the design of automotive and industrial systems.
Calibration Factor
Scaling of the measurement must account for the transfer function defined in the datasheet. Most ratiometric analog sensors are calibrated at 5.0 volts but remain accurate across a wider range such as 4.5 to 5.5 volts. Drift in the supply does not manifest as a measurement error because the ratio remains constant.
This assumes that the reference voltage for the converter is the same as the sensor supply. High impedance inputs are required to prevent loading effects from distorting the ratio.
Interference Limit
External noise coupled into the signal line cannot be rejected by the ratiometric relationship. Shielding and filtering are still necessary to protect the low level signal from electromagnetic interference.
Sensor Packages Mounting and Electrical Interfaces
Land pattern geometry, bus capacitance control, and driver timing dictate assembly yield and operational stability across sensor package formats.