3
\$\begingroup\$

I have an analog sensor that requires constant supply voltage and outputs a fraction of the supply voltage. I need to design a module that will be able to output a digital signal representing the sensor output.

First I just paired it with an ADC:

enter image description here

But I quickly realized that this was not a very good idea since sensor output (and concequently the ADC output) depends on the supply voltage. Although supply voltage fluctuations aren't big, they are still big enough to make sensor readings useless.

This led me to the idea of ​​using some kind of voltage reference IC. As far as I understand you can't directly use the output of reference voltage IC to supply power to a sensor since it can't provide any meaningful current (sensor may need tens of mA). So I probably need some kind of adjustable linear regulator paired with a voltage reference IC that will be cabable of outputting enough current for the sensor while maintaining output voltage equal to reference. However I have no idea how to make such a circuit or is it even a correct approach.

enter image description here

It feels like there must be plenty of circuits for this kind of appications but I wasn't able to find anything concrete (probably because I couldn't accurately describe the problem I'm facing).

So the questions I have are:

  1. What type of voltage reference I should use?
  2. Is the aforementioned approach (a voltage regulator with vref ic somehow connected to feedback pin) adequate for the described situation?
  3. If the approach in question 2 is adequate, how would the circuit look like? If not, what is the correct approach?
\$\endgroup\$
4
  • 2
    \$\begingroup\$ ”But I quickly realized that this was not a very good idea since sensor output (and concequently the ADC output) depends on the supply voltage.” A search term for this is ratiometric. If you use a linear regulator with enough precise output from the factory, say 0.5 %, would that be good enough for you? Can you measure Vcc and scale your measured value accordingly? \$\endgroup\$ Commented Jan 9 at 12:00
  • \$\begingroup\$ A linear regulator with precise enough output probably would be ok. For some reason I thought that it would significantly fluctuate with temperature changes. Technically I can measure Vcc and scale the measured value accordingly but I'd prefer not to in this particular case. \$\endgroup\$ Commented Jan 9 at 12:14
  • \$\begingroup\$ There are plenty of ratiometric ADCs available. It's a common technique for sensor interface. \$\endgroup\$ Commented Jan 9 at 12:34
  • \$\begingroup\$ YOu should learn to specify what you need by listing requirements or "Design Specs" like here, what signal and noise levels did you see? what PS variations? thus what PSRR is needed or stability in supply? What Signal BW and noise BW? thus what SNR is needed? get it? \$\endgroup\$ Commented Jan 9 at 17:28

2 Answers 2

Reset to default
5
\$\begingroup\$

Ratiometric sensors (as a complete product) as you have are typically provided for relatively low-accuracy situations, and you can just configure your ADC to use the supply voltage as a reference and be done with it. There may be some noise or other artifacts if your supply voltage is too noisy. Most MCU ADCs have the ability to use the supply voltage as a reference, even if they have an internal reference option. You might want a filtered version of Vdd for the analog stuff.

However, this statement concerns me a bit:

Although supply voltage fluctuations aren't big, they are still big enough to make sensor readings useless.

Specifically, the "useless" part, as opposed to say "not ideal". You should read the sensor specifications (as with any datasheet) with a jaundiced eye and make sure you're not trying to make a silk purse from a sow's ear.

In some cases it might make sense to use a separate regulator for the analog circuitry so that it is less noisy, even if the slower changes cancel out.

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Thank you for the answer! The ADC that I initially selected didn't have the option to supply external reference voltage so I got a bit confused at first. So basically I should use a separate regulator and use its output as power for the sensor and reference for the ADC. As for my statement about readings being useless - "not as precise as I needed them to be" is closer to reality. \$\endgroup\$ Commented Jan 9 at 13:19
  • \$\begingroup\$ @Kekers_Dev If it's necessary to use a separate regulator or filter you can, but it's only for noise reduction. If the noise is not a problem then you can just slap them across the power and call it a day. There are a lot of variables. If you do choose to use a separate regulator you need to consider supply sequencing and if any harm could result if one supply comes up or goes down before the other. \$\endgroup\$ Commented Jan 9 at 13:22
3
\$\begingroup\$

I quickly realized that this was not a very good idea since sensor output (and consequently the ADC output) depends on the supply voltage. Although supply voltage fluctuations aren't big, they are still big enough to make sensor readings useless.

If the output rises and falls ratiometrically with the power rail then you don't need to do anything because the ADC reference voltage and input change proportionally and, this results in the correct (and constant) digital value despite changes in Vcc. It's called a ratiometric measurement and is widely used for devices like pressure sensors and strain gauges etc..

As an example consider a sensor that nominally produces +2.5 volts out when the supply rail is +5 volts. If the ADC reference is also connected to 5 volts then the digital output will be half full-scale. If the supply rose to +5.5 volts then the ADC will still produce a digital output that is half full-scale. This is what ratiometric means.

Guide to Ratiometric Sensor Applications: -

A ratiometric measurement provides a low-cost method to interface a microcontroller to a sensor. The key to an effective ratiometric measurement is to use a common excitation voltage for both the sensor and the microcontroller’s Analog to Digital Converter (ADC). As this excitation voltage naturally drifts, it impacts both sensor and ADC. However, this is mitigated by the ratiometric properties of each device. As an example, consider an excitation voltage drift from 4.9 VDC to 4.2 VDC, the ratio for both the sensor and ADC is equal. Consequently, the binary result of the ADC remains unchanged.

This led me to the idea of ​​using some kind of voltage reference IC.

If you can use ratiometric measurements then it's unlikely to be needed.

\$\endgroup\$
4
  • \$\begingroup\$ Can you please elaborate a bit? I thought ADC reference voltage stays constant and consequently the digital value changes with sensor output. Are you talking about some special type of ADCs? \$\endgroup\$ Commented Jan 9 at 12:47
  • \$\begingroup\$ @Kekers_Dev notes added to my answer. \$\endgroup\$ Commented Jan 9 at 13:25
  • \$\begingroup\$ Thank you, I mixed up the "on-board" vref of the ADC with externally profided vref \$\endgroup\$ Commented Jan 9 at 13:30
  • \$\begingroup\$ I don't know for a fact that they exist, but I wouldn't be surprised to see an ADC with a built-in voltage reference that's independent of the supply. I could see a use case for one powered off 5 V with an internal 4.096 V reference, or a 3.3 V one with an internal 2.048 V or 1.024 V reference. But the majority of ADCs do work as you described, with either an external reference (which can be tied to the supply) or using their own power supply as the reference. \$\endgroup\$ Commented Jan 9 at 15:46

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.