ICP Piezo Force Sensor Interface to microcontroller

Question

I need some advice on interfacing an ICP piezo force sensor from PCB Piezotronics to a microcontroller. The sensor has a low impedance output of 0-5 volts superimposed on a DC offset of 10 volts. The data must be collected in DC coupled mode for time response reasons. The signal will be low frequency, below 1 KHz. Basically what I would like to do is level shift the DC offset and send the signal into an 0-5V capable ADC with some sort of serial interface, say SPI. I cannot use an AC coupling capacitor due to the DC coupling requirement. The system will have single sided power available up to 24V, with 5V and 3.3V for other components. There are a myriad of ways to do this it seems but I have found no direct examples on the web. I was thinking of using an AD8276 unity gain difference amp with the ICP force sensor signal as the in+, 10 volts as the in-, 0 volts as the REF and sending the resulting OUT signal possibly to a AD7192 Sigma-delta ADC. We use the AD7192 currently for load cells and it works great. Does this approach sound reasonable or is there a better way. This is a new project so we are open to an suugestions 
 Thanks for your help.

ADIApproved · Answer

Greg, 
 I am considering mid-reference to be the middle of the ADC's reference. In other words, if you supply your ADC with 5V reference, then it would be 2.5V. You will want to drive the reference pin this way because your signal will be bipolar, and this way you can acquire it with the ADC. However, and like I said before, you may benefit from getting twice as much signal range if you implement the differential drive. I had a little more time to sketch it, so I am attaching it to this post. While the previous idea would allow you to acquire signals from from your sensor in the range slightly less than 7.5V to 12.5V (assuming 10V offset) the differential drive would allow you to go from a little more than 5 to a little less than 15V. 
 Radj is a different story. You want to adjust the voltage going into the input of the first instrumentation amplifier to set your "zero" coming from the sensor. The voltage resulting from this divider should be around 10V, as you've correctly pointed out. 
 Vref can be 5V. There's a number of options. I'm not the apps engineer for references, but I know that if you wanted to run directly off 24V, you can use something like AD586. However, 24V input for a 5V reference is a lot of power and you may want to stay away from that. That means that you should drop that voltage with a regulator or more simply with a zener. Then you can feed it into a reference like ADR435. I believe you can also buffer the reference voltage with an amplifier with enough current drive to power the ADC. An example on how to do this can be found on page 16 of the following data sheet (AD7690). 
 Regards, 
 Gustavo

ADIApproved · Answer

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

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ICP Piezo Force Sensor Interface to microcontroller

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