ADC input filter design for DC input signal

Question

My plan is to use an ADS1115 ADC to read a low impedance 0-5V output signal from a level transducer. The ADC data sheet gives guidance for ADC input filtering. This data sheet, and even other input filter design notes all assume a relatively high frequency input signal (High frequency relative to my application).

My application is a level transducer that will mostly be 0v for the majority of its life. I will only be monitoring the input signal for a high level event. If a high water level were to occur, it would happen very slowly, it would take over 10 minutes just for the level to increase an inch. Once the level reaches a high threshold; however, level reading accuracy is critical.

My design is power critical, runs on battery. I chose my sampling rate to be 64 samples per second. The entire system will wake from sleep, take a few samples, average them, and go back to sleep (sleep duration will be a few seconds). The transducer is mounted directly onto PCB via one or two inches of wire. Industrial environment, very noisy environment as wel.

Given this application, is a RC circuit really necessary? I would imagine just removing the resistor from the RC circuit and letting just capacitor filter all AC noise would be enough for my application? I would of course still use the differential capacitor as per the data sheet. Or would it be better to make a RC filter with a very low cut off? That was my original plan, but the use of precision resistors and required class 1 caps would eat into board space and allotted budget; I didn't want to go this route unless it would be necessary.

Thank you for taking the time to read!

Answer 1

An anti aliasing filter should be always present. It would be better to sample at high sample rate, use a FIR filter to eliminate the environment noise 50/60Hz, thus the anti-alias filter can be a small RC with low TAU value. Sampling at 64sps would imply large RC filter and it is very close to 60Hz, so most probably you will pick lots of noise/garbage.

See this example:

Youtube , source PDF

You could use 1kHz sampling rate with a 300Hz cutt-off anti-alias RC filter. $$f_{cuttoff}=\frac{1}{2\pi RC}$$

You said, your source has low impedance, so you don't need a buffer. Let we neglect the source impedance (=0) and we calculate as the entire resistance is the filter resistance R=1kOhm.

$$C=\frac{1}{2\pi Rf_{cuttoff}}=\frac{1}{2\pi\cdot 1k\Omega\cdot 300Hz}\approx 5.3\cdot 10^{-7}F\approx 0.56\mu F$$

^{simulate this circuit – Schematic created using CircuitLab}

Above it is an anti-alias filter 300Hz cutt-off. Then you sample and filter with low pass FIR filter, for example 128 taps. You would get a precise measurement without environment noise.

Answer 2

Your sampling frequency is 64 Hz. You need an anti-aliasing filter that cuts off much before that. That is just basic Analog Design. Even if there is no other source of noise, there could possibly be lighning off in the distance or ESD close at hand. I suggest 2200 Ohms and 10uF. This will give you a cutoff frequency of around 7 Hz. A 10uF ceramic capacitor is not too large physically. The 2200 Ohm resistor could possibly add some tiny DC error to your ADC reading, but you can calibrate it out (which you will probably have to do anyway). If it does lead to error and you don't want to calibrate you can consider a smaller R and bigger C or bite the bullet and buffer the signal, then filter it (so you will only have offset error which can be uV if you choose an appropriate buffer).

At the minimum, though, I would design the PCB with a place to put a 10uF cap next to the ADC and an 0402 or 0603 resistor in series. Because I have been down this road before. It is much better to have a place to put the components and not need them than to need them and have no place to put them. The cost of a small SMT resistor is totally insignificant, even in high volume production. The space is pretty small, too. It seems that you are at peace with adding a capacitor, which makes your reticence concerning the tiny cheap resistor very perplexing to me. Remember that the filtering is a function of the product of R and C. Adding an R greater than zero means your capacitor can be smaller.

Answer 3

My design is power critical, runs on battery. I chose my sampling rate to be 64 samples per second. The entire system will wake from sleep, take a few samples, average them, and go back to sleep (sleep duration will be a few seconds).

At 64 S/s you can actually go back to sleep between samples.

Also, the power consumption of filtering and ADC combined will be low compared to the power used for your I²C bus'es pull-ups, just saying.

Given this application, is a RC circuit really necessary?

Yes.

Think of an RC filter as simple voltage divider, just that the "lower" resistor is a capacitor. Since a capacitor has a lower effective resistance for higher frequency, that makes it a frequency-dependent voltage divider.

Without the "upper" resistor, no voltage drop will be seen.

Why on earth would you want to save a resistor? That's literally the easiest, safest, most accurate and cheapest type of part you can get.

So, yes, without the R you don't get a filter.

Or would it be better to make a RC filter with a very low cut off?

"Very low" is a very relative term.

The theory is clear: Whatever input signal above 32 Hz reaches your ADC will get aliased into your signal of interest. So. Know where your signal of interest "ends" (i.e. its highest frequency component). You want to put your cut-off as closely as possible above that; the further above the cut-off frequency unwanted signal components are, the better they are suppressed.

Notice that a single stage of RC filtering isn't always sufficient. You need to start with a drawing of your spectrum to know what kind of filter oyu need.

hat was my original plan, but the use of precision resistors and required class 1 caps would eat into board space and allotted budge

Nothing says you need precision resistors. Standard 1% would totally do, since you never ever get capacitors even in the same order of magnitude of precision as you can get resistors! And you were about to hand-wavy omit the resistor, so I don't think you've actually calculated which accuracy you need.

The current going into your ADC is going to be << 1 µA (see the datasheet's input impedance table). So, the power lost in your resistor will be absolutely negligible – you'll want a large resistor value, anyway, to achieve a low cutoff frequency with a reasonably small capacitor.

So, go with the smallest 1% precision resistor you can solder. You can buy these down to 01005, at every distributor. They are narrower than your ADC's pins are wide. A resistor is not your space problem!