I was thinking, since these sensors(ADXL325 or 327) are not really that fast, a triple adc is not necessary, since acquisition and conversion times of modern adc's are incredibly fast, the difference in output while sequentially sampling for
the same event is negligible. As such, a programmable amplifier with multiple inputs might simplify circuit design. I was thinking of using an MCP23008 for the following programmable features :
# To switch the 2 bandwidth capacitors (per channel)in parallel on or off.
That would give 4 different(2 bit) capacitor values for each channel.
Each channel has 2 mosfet's switching 2 capacitors in parallel.
That would give me 4 bandwidth ranges. 3 channels * (2 *mosfet + 2 *capacitor). Two pins used for capacitor selection it is. Creating a bandwidth range of 1Hz to 500 Hz in 4 steps.
Forgot to mention, when both switchable capacitors are off or disconnected, a 10nF capacitor is always present at the output of the ADXL327.
# An MCP6G03 comes to mind. a single amplifier with an enable input and a gain select input where gain is 1,10 or 50. Three of these where the outputs are connected together as wired OR (by putting in series 3 160 Ohm resistors for each output).
Then 1 pin is needed to select the amount of amplification between 1, 10 or 50.
# And 3 pins are needed to select one of three opamps.
# 1 pin to activate the selftest of the ADXL sensor.
That is 7 pins to control a bandwidth range, selftest, gain , and input select all through an I2C interface.
This all at a fraction of the price but with an increase afcourse of PCB real estate.
But in SMD with size 0603 components really not that big. Even size 0805 are still small.
MCP6G03 :
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en028127
datasheet :
http://ww1.microchip.com/downloads/en/DeviceDoc/22004b.pdf
MCP23008 :
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en021393
datasheet :
http://ww1.microchip.com/downloads/en/DeviceDoc/21919e.pdf
EDIT:
I noticed that the MCP6G03 is hard to come by.
As such, 3 MCP6G01's together with 3 free adc inputs on the microcontroller might be a better solution. Or 3 74HC1G66 analog gates combined with three MCP6G01's. Each in series with the output of an MCP6G01. Then a similar function is created as with the MCP6G03.
A bit enthusiastic i am : An MCP6G04 with an ADG658 would be easily routed.
When i have drawn a schematic, i can post it if you desire but i do not know where to post pdf, although i could make a low compression jpg....