I am working on my school project - DAQ unit for strain measurements with AC excited Wheatstone's bridge. Device will contain analog input board (from application note AN-683), digital board (ADC converter, uProccesor and board-to-PC interface, etc.) and measurement software on PC.
Currently I am working on analog input board from application note and i have few questions for experienced engineers. I have built electronics before, but this is my first time doing something with measuring, higher resolutions, noise etc.
- excitation is going to be sine wave of 2.5 V and 400Hz, and will not exceed 1kHz
- supply voltages of AD8221, AD630 and OP1177 are +-15V
1) If I am using AN-683, what should I do with unused pins 3,4,5,6 18,20 and 7? I think that 3,4,5,6 could be just soldered to package pad and left floating, but I am not sure what to do with OPamps inputs 18,20 and open-collector pin 7
2) What type of resistor should I use to set Gain within AD8221 instrumental amplifier? Will smd1206 resistor with 0.1 % accuracy be ok?
3) I am willing to replace cascade of RC filters at the AD630's output with Active 4th order Bessel filter integrated circuit. Should I be concerned of capacitive and resistive loading of AD630's output in this application if there is another OPamp (OP1177) at the output of filter? I do not understand effects of capacitive and resistive loading effects in this application very well.
4) In this particular application note, the output of AD630 is DC voltage (rectified product of excitation voltage and bridge output multiplication). What is mathematical relation between input of AD630 (amplified bridge output) and output of AD630? I am curious how i should connect DC filtered output with strain change at Wheatstone bridge.
Thank you very much for your time with helping me. Datasheets and application note are attached.