I'm working on a wave energy project that requires digitzing the following sensors (all at 10 Hz, 50 samples/sec in aggregate):
-- Two type T thermocouples converting -20 to +100 Celsius, ADS1248's internal diode will be used as the "cold junction". Software will perform the cold junction compensation.
-- One three wire RTD, converting -20 to +100 Celsius
-- Two pressure sensors that produce 0 to 10 volts
The proposed connection of the hardware and configuration of the ADS1248 follows:
The two thermocouples will be converted differentially. An external differential RC filter will be used on each thermocouple to reject noise from nearby, kilowatt range DC to DC converters. One side of each thermocouple will be biased to mid-supply. The internal reference will be used for conversions.
The RTD will be connected as described on page 57 of the ADS1248 datasheet except that REFP1 and REFN1 will be used to measure the voltage drop across RBIAS caused by the RTD's excitation current.
The 0 to 10V input from the pressure transducer will be divided by four (0 to 2.5V) and then converted single ended. The internal reference will be used for conversions. The resistive divider will be followed by a unity gain buffer to drive the ADS1248 inputs. Is this buffer necessary? I'm concerned about any high impedance source interacting with the ADC input switching currents to produce measurement errors.
What is the best way to isolate inputs from the ADC input switching currents?
What suggestions do you have for protecting the inputs of the ADS1248?
What is the recommended method to deal with ununsed analog inputs?
Thanks again for your help,
Wayne