Q
I've got two questions:
1. Is there any performance difference between old RTD connection (in ADuC824
datasheet with ref resistor connected to EXC) and new RTD connection (in
ADuC834 datasheet with ref resistor connected to AGND)?
2. Can I connect two RTDs serially (4 way connection each) and measure their
resistance on AIN1-2 and AIN3-4? What would be suggested decoupling (if any)?
A
There is no performance difference between the two implementations. The circuit
shown on the ADuC834 datasheet is advantageous as it uses one less resistor.
The key requirements are:
Maximize the full-scale voltage drop across the RTD to maximize resolution
Keep the total voltage drop across the sense resistors and RTD within the
voltage compliance range of the current source (VDD-0.6V)
And ensure you meet the input common mode requirements of the ADC input
(AGND+100mV to VDD-100mV).
Yes it is possible to use multiple RTDs with the same current source (in
series). Again, simply ensure you meet the voltage compliance range of the
current source and the input range of the ADC.
Decoupling requirements will depend on how harsh an environement the RTD are
placed in. If the RTDs are on the same board as the ADuC, not exposed to noise
and the ADuC power supply is well decoupled, you probably will not require any
decoupling capacitors on the analog inputs.
However, if the RTDs are located remotely via long cables in an electrically
harsh environment (lots of magnetic field and EMI) you will need to look at
adding capacitors to filter the spurious signals. 10nF would be a good starting
point but I suspect a little trial and error coupled with a reasonable estimate
of the likely interference / noise signals will provide the best solution.