Multiplex temperature signals through one conditioning circuit

Question

I'm planning on designing a PCB that takes numerous analog temperature sensor input signals and convert them using an Analog to Digital Converter (ADC) to be read by a computer. The signals need to go through a conditioning circuit prior to being read by the ADC.

I need to read so many temperature signals that it would take up a ton of space and power to have conditioning circuits for each sensor. Would it be practical to first route the temp signals through a multiplexer that then goes to a single conditioning circuit, then gets converted to a digital signal? I don't need high time resolution for the temperature measurements so we could interpret the sensors based on some frequency. I've attached a rough sketch of what I'm suggesting. From searching online so far I haven't seen anyone try to do it that way. I'm wondering if a multiplexer might distort the incoming signals too much and throw off measurements.

Thanks

Answer 1

Multiplexing a single conditioning circuit for use among multiple sensors is a common method to minimize the amount of hardware required.

Temperatures tend to be very slowly changing so cycle times of many seconds is usually acceptable. This allows many sensors to share the hardware.

However, some caution is required to ensure that the multiplexing does not corrupt the incoming signal from the sensor.

Many systems use thermocouples as the sensor but one characteristic of them is that they only produce very small voltages, a few microvolts per degree. So you need a multiplexer that can switch such small voltages wihout significant errors.

Mechanical relays are a common way of doing the switching, often reed relays. They can switch in a few milliseconds and have low voltage offsets. Semiconductor analog switches can also are often used, providing their higher on-resistance is accounted for.

Resistive sensors such as RTDs or thermistors can also be multiplexed although the excitation for those may also have to be switched.

Answer 2

It’s usually better to have a signal conditioning circuit for each input. Failing that, at least a low pass filter and some protection for each input.

It may not be as necessary if the sensors are essentially colocated with the circuitry. For example if you need to measure temperature at a number of points within a module.

But it really depends on the application and how much you care about performance and failures vs. cost. Long runs of sensor wires (perhaps at a refinery or agricultural installation) may demand isolation and lightning protection, for example.