ICP Piezo Force Sensor Interface to microcontroller

Greg,

My understanding from glancing over the ICP sensor documentation is that even if tension is never applied to the sensor, there is a discharge time constant. In other words, if you think of a constant force pulse that will raise the voltage from 10V to 15V, there will be an exponential voltage drop. If the pulse is long enough to drop to, say 14V, then when the force is removed, the output would drop to 9V and eventually go back to 10V.

In other words, even though you're dc coupled to the sensor, there will be an ac coupled like behavior. However, if your application won't see a long pulse (and therefore the recovery voltage will be small), then you can modify the first schematic to drive the reference to a smaller voltage (for instance, less than 1V) with a resistor divider and an amplifier like the OP1177. This is shown on page 20 of the AD8226 data sheet.  The nice thing about the fully differential output is that you get the negative swing without giving up your full positive swing.

If you have less than 18V available, then you should be able to use ADR435 for your system reference. I am attaching a variation to the last circuit to generate a more stable offset null for the sensor output (assuming Vref=5). R1 will allow the circuit to adjust below 10V and Radj will move the voltage higher. R1 probably can be something like 1k and Radj should be at least more than twice R1.

Regards,

Gustavo

PS> I have attached a second schematic showing an example on how to implement a programmable offset null. Note that if you prefer the original implementation using a potentiometer (for manual adjustment), it would be a good idea to add a series resistor to limit the adjustment range. Good luck!

Message was edited by: Gustavo.Castro. Added programmable offset null

Greg,

My understanding from glancing over the ICP sensor documentation is that even if tension is never applied to the sensor, there is a discharge time constant. In other words, if you think of a constant force pulse that will raise the voltage from 10V to 15V, there will be an exponential voltage drop. If the pulse is long enough to drop to, say 14V, then when the force is removed, the output would drop to 9V and eventually go back to 10V.

In other words, even though you're dc coupled to the sensor, there will be an ac coupled like behavior. However, if your application won't see a long pulse (and therefore the recovery voltage will be small), then you can modify the first schematic to drive the reference to a smaller voltage (for instance, less than 1V) with a resistor divider and an amplifier like the OP1177. This is shown on page 20 of the AD8226 data sheet.  The nice thing about the fully differential output is that you get the negative swing without giving up your full positive swing.

If you have less than 18V available, then you should be able to use ADR435 for your system reference. I am attaching a variation to the last circuit to generate a more stable offset null for the sensor output (assuming Vref=5). R1 will allow the circuit to adjust below 10V and Radj will move the voltage higher. R1 probably can be something like 1k and Radj should be at least more than twice R1.

Regards,

Gustavo

PS> I have attached a second schematic showing an example on how to implement a programmable offset null. Note that if you prefer the original implementation using a potentiometer (for manual adjustment), it would be a good idea to add a series resistor to limit the adjustment range. Good luck!

Message was edited by: Gustavo.Castro. Added programmable offset null