Sometimes what looks simple is not that simple. You have a quite complex measurement to do, but you want a simple result. What you want to measure is not constant, it's varying in time. Depending on your level of requirement, you could compute one or many properties of the current consumption. These properties will help you to better monitor the system. I propose you 3 different solutions, in ascending complexity.
Solution 1: Average
You want to get a one-value result -> get the average in time. As already proposed by @akellyirl, use a low-pass filter. Compute float y = alpha*input + (1-alpha)*y
for each sample, where alpha
is the smoothing factor. See Wikipedia for the details.
Solution 2: Max + Average
You are interesting in getting the average, and the max value. Monitoring the max value could be interesting for component dimensioning for example.
if (y > max)
max = y;
Solution 3: Standard deviation + Max + Average
Why?
See below charts. There are 3 signals of different shapes. A triangle, a sine, and a spike signal. They are all periodic with same period, same amplitude, same average, and same min and max. But, they have different shapes, and indeed they have a completely different story...
One of the difference is the standard deviation. That's why I suggest you to extend your measurements, and include the standard deviation. The problem is that the standard way to compute it is CPU consuming. Hopefully, there is one solution.
How?
Use the histogram method. Build an histogram of all the measurements, and extract efficiently the statistics (min, max, avg, standard deviation) of the dataset. The histogram groups values together that have the same value, or same range of value. The advantage is to avoid storing all the samples (increasing count in time), and to have a fast computation on a limited number of data.
Before starting acquiring measurements, create an array to store the histogram. It is a 1 dimension integer array, of size 32 for example:
int histo[32];
Depending on the range of the ammeter, adapt below function. For example, if the range is 256mA it means that bin 0 of the histogram will be incremented by value between 0 and 8 mA, bin 1 by value between 8 and 16 mA etc...So, you'll need an integer to represent the histogram bin number:
short int index;
Each time you get a sample, find the corresponding bin index:
index = (short int) floor(yi);
And increment this bin:
histo[index] += 1;
To compute the mean, run this loop:
float mean = 0;
int N = 0;
for (i=0; i < 32 ; i++) {
mean = i * histo[i]; // sum along the histogram
N += i; // count of samples
}
mean /= N; // divide the sum by the count of samples.
mean *= 8; // multiply by the bin width, in mA: Range of 256 mA / 32 bins = 8 mA per bin.
To compute the standard deviation, run this loop:
float std_dev = 0;
for (i=0; i < 32 ; i++) {
std_dev = (i - mean) * (i - mean) * histo[i]; // sum along the histogram
}
std_dev /= N; // divide the sum by the count of samples.
std_dev = sqrt(std_dev); // get the root mean square to finally convert the variance to standard deviation.
The strategy of the histogram method is to make the slow operations on a few number of bins, instead of all the acquired signal samples. The longer the sample size, the better. If you want more details, read this interesting page The Histogram, Pmf and Pdf.
Best Answer
Think about the resolution and range. My trusty (non-yellow-brown) classic Fluke 8026B has a 200nS range. The resolution is 0.1nS.
That means that I can read resistance to a resolution of 10% up to \$1\over 1nS\$ = 1G\$\Omega\$, and see leakage in the 5~10G\$\Omega\$ range.
You could have a 10G\$\Omega\$ range on the meter, either directly measured or calculated, but the number of significant digits at the higher values would probably be only one or two unless the meter was made significantly more expensive.
So it just makes sense to offer a conductance range for making coarse high-resistance measurements. The resistance ranges on the meter go up to 19.9M ohms, so it's mostly useful for the 20M ohm to 10G ohm range.
The resolution of conductance goes down as the accuracy deteriorates (at very high resistance values) but the meter never goes off scale, even for an open circuit, as shown in the photo. Resistance does the opposite, so it will go off-scale for a resistance of more than 19.99M ohms.
Similarly, conductance wouldn't be ideal for very low resistances- the resolution would go up as the resistance approaches zero. It's better to start losing significant digits in the display as a hint to the user, and to allow the meter to not go over-range even if the leads are shorted together.