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.
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).