Please help me understand some Signal Conditioning basics

[eustas1]

Say I have some sensors producing small analog signals, then some wires (not short but not crazy long, let's say 5-10 ft.) transmitting those to the MCU. I need to condition the signals for the ADC, but also the sensors and wires will be exposed to significant EMI, so I need to protect them too. For sure I need good CMRR. I also need to amplify and maybe offset the signal (for the ADC). This signal is theoretical, but I don't expect it to change very quickly so let's say also a Low Pass Filter.

1.) What order do I do this in? Does it go CMRR --> gain --> low pass? Or CMRR --> low pass --> gain? Does it matter?

2.) Does an instrumentation amplifier replace the CMRR + gain portion of the above? Would that mean I could do an instrumentation amplifier followed by a low pass filter and be done with it?

3.) Should this signal conditioning occur on the sensor side of the cable, or on the MCU side of the cable?

[K C]

Hi,

A lot of the industrial sensor produce a 4 - 20 mA signal. In most case this range is 0 - 100% of the process value.

Current below 4 and above 20 can indicate several things like: Beam block, dirty window, sensor error, line faults, etc.

The impedance of a current input is low typical 50 or 250 Ohm. This can be followed by a low pass filter. 

This filter is also necessary if a HART signal is present to filter out the HART signal from the current measurement.

Normal shielding for long cables should enough as protection.

If you search for the topic as industrial sensor and current loop you will find a lot of information.

Kees

[santo_13]

Best option is to convert the sensor to digital, move the analog to digital conversion close to the sensor and send digital data over long lines to the MCU.