Digital multimeter display count range

Question

The display count range is one of the digital multimeter characteristics. We know that a DMM is based on an analog/digital converter, and we know that each ADC has a resolution of n bits.

A 16-bits resolution is 2^16 = 65535 steps. log(65536) = 4.8164799306 digits, so something between 50000 and 60000 counts, but we know too that an ADC can't see above its voltage reference (the maximum value that the ADC can convert).

I have two questions:

1. The number of steps isn't the same as the number of counts?
2. In our example, the number of counts we calculate is 60000. If the reference voltage is 2.5000 V, will the number of counts be 25000?

Update;

At that time I was looking for a handheld digital multimeter of 4.5 digits or higher, the problem was that a lot of them has a low count in the round of 20000, and I knew that it can only be a 16bit ADC or higher, and I wanted to know why it shows only up to 20000 which mean if I measure a 2 volts its ok its gonna show 2.0000v but if I measure 3 Volts I will lose a digit :( it gonna show 03.000v. So I thought at that time it has something to do with the voltage reference and I wanna be sure, that way I can buy a DMM and change it voltage reference I hope I explained my thought better.

Answer 1

1:

From the ADC perspective, counts and steps are the same.

From the user's perspective, a count is the smallest change on the display. This may not be the same as the ADC count.

2:

If we assume a 16-bit ADC and a 2.5V reference, at the ADC there are 65536 steps and each step is:

2.5/2^16 = 38.1469726 uV

The DMM will multiply the ADC output (count) by a scaling factor to produce the display for the user.

Answer 2

We know that a DMM is based on an analog/digital converter, and we know that each ADC has a resolution of n bits.

Dual-slope ADCs are popular in DMMs and they are not limited to resolutions that are binary powers, which is an assumption present in your original question.

You can make a dual-slope convertor with a count range limited by the precision and stability of the components used, rather than requiring it to be a power of 2.

So what you are saying is, there is a big chance that those DMM with a lower count may be based on dual slopes ...

I'm not making claims about particular DMMs. If you want to know what type of ADC a specific DMM uses look in its datasheet or service manual, if available. I'm pointing out that there are ADC architectures that are not tied to binary powers.

... and there are chances to tweak the number of counts by modifying some components?

Where are you going with this? The count range or number of significant digits goes hand in hand with the precision of the ADC (and other) circuitry. They need to be designed in step with each other. You can't just double the voltage reference to convert a 20,000 count meter into a 40,000 count meter and have the figures mean anything (if it even still works). If you want a higher spec DMM you need to buy one. If cost is an issue you can find second-hand ones on auction sites though note the cost of calibration if accuracy rather than just resolution is desired.

I was looking for a handheld digital multimeter of 4.5 digits or higher, the problem was that a lot of them has a low count in the [range?] of 20000

A count of 20,000 is what 4½ digit means so it's not surprising that you found what you asked for. (More strictly the count runs from 0 to 19,999.) A 4¾ digit range is rather more vague and definitions vary between manufacturers. Perhaps that's why we moved to counts to give a clear measure of resolution.

Answer 3

First, SAR ADCs generally can't convert voltages above the reference, but that's not universally true. For example, dual-slope ADCs often can work with inputs +/- double the reference, so a 100mV reference will allow measurements of +/-199.9mV.

The number of steps is the same as the number of counts, they're two words used (perhaps loosely) to mean the same thing.

If it is a 16-bit ADC and the reference is 2.5000V, then there will be \$2^{16}\$ steps or counts, and each step will be ~38.15uV.

The ADC just divides the reference voltage into 'n' steps (ideally it does so monotonically, exactly evenly - no linearity errors- and without noise), it doesn't care if the step size makes sense to a human.

Answer 4

To answer your second question, DMM use resistor dividers to divide down the voltage to a lower value to be read by the ADC. For example, if you were trying to read a 5V signal and the ADC can only read between 0 and 1V, then the Multimeter will have a voltage divider which can divide the voltage by 10 so the ADC only sees 0.5V. The digital circuits in the DMM will know what the divider ratio is and reverse it when it is displayed to the user. This is also why many DMMs require you to choose a maximum voltage so that is can choose the correct divider to use.

Edit to more specifically answer your question:

In your example, we have 60000 counts and a 2.5V reference. This means when the display reaches the 60000 counts (ie 6.0000V, 60.00V, 600.0V). The designer of the DMM will make the voltage divider ratio so that the maximum value of the range can be read by the reference. For the 6.0000V range, the divider ratio will be 2.5V/6V = 0.416. When the voltage goes above 6V, the voltage divider will be switched so that the DMM can read up to 60V - The ratio will be 2.5V/60V = 0.0416.