The SSP1492 (Product Brief) can interface directly with capacitive sensor elements. Our solutions can measure down to femtoFarads (0.001pFs) and soon even to attoFarads (0.000001pFs). The SSP1492 easily handles up to 12 sensors per system and provides fast response (<500usec) as well. Outputs include SPI, I2C, or PWM. Depending on your application, our solutions can provide an 8051 MCU core along with RAM, ROM, and a EEPROM.

The SSP1401 (Product Brief) allows up to 6 capacitive inputs and outputs to industry standard SPI. Sensor manufacturers use this product with accelerometers, magnetometers, humidity, tilt, and pressure sensors. The high resolution, fast response time, and absence of MCU allow for a very cost-effective solution.

This application brief will demonstrate the use of the SSP1492 with a capacitive-based MEMS acceleration sensor. The SSP1492 can interface directly to capacitive sensor elements and provide all signal processing needs to derive a calibrated signal using a minimal number of external components.

The VTI SCG30V three-axis acceleration sensing element contains 4 differential capacitive inertial sensor cells that change capacitance in relation to its orientation to gravity. The unit is also sensitive to accelerations due to motion. The SCG30V uses 4 moving masses with an off center fulcrum. This creates a torsional movement of the internal mass. The moving mass is electrically connected as a common plate between two fixed plates, hence there are two capacitive outputs per cell. The 4 moving masses or mass electrodes are electrically tied to one another and serve as the common for all 8 capacitor outputs. The capacitive signal outputs are labeled 1F,1B,2F,2B,3F,3B,4F,4B. The number represents the moving mass cell number and the 'B' and 'F' relate to the Forward and Back capacitive sensing plate in that cell. The relationships of the 8 capacitive signal to Cartesian gravity(or acceleration) axes are given in the following table.



The SSP1492 measures the 8 capacitive signals by electrically selecting each capacitor individually into a relaxation oscillator as shown in the diagram below. The selection is done by alternatively grounding a give capacitor and electrically floating (high impedance) the remaining capacitors. This is accomplished with the SSP1492's 8 general-purpose input/output channels (GPIO), see Figure 1 Below. A buffer, not shown, is used between the accelerometer common output and the relaxation oscillator input to reduce the capacitance loading of the RC timing elements on the relaxation oscillator to increase its dynamic range.




All SSP1492 signal processing functions and hardware peripherals are accessible as commands that are sent to the SSP1492 through either the I2C or SPI interface. Alternatively, a small program to orchestrate the measurement process and perform all the required math function to derive the Cartesian oriented acceleration values can be uploaded into the macro program space by the host processor. Figure 2 is a schematic of an actual Accelerometer solution using the VTI SCG30V. Code used in the macro program for this application example is available upon request.

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Create an entire smart sensor solution within hours with only a sensor and as few as 2 external passive components. Internal ROM already contains ready-to-use signal processing functions. No programming is required to operate system as a slave from I2C or SPI interface. Click here to learn more or for an Evaluation Kit!