Choosing the correct DMM to meet your needs
Take the Digital Multi Meter (DMM) for example – the most versatile of all DC instruments. On RS’s website alone there are 608 different DMMs, ranging in price from a few tens of pounds up to around £8k (as of Feb 2020).
So, which one should you choose?
The first question should always be “what measurement functionality do I require”? Most benchtop DMMs offer a core set of measurements such as Current (DC and AC), Voltage (DC and AC) and Resistance (2 wire and 4 wire). With the latest and greatest instruments, such as Keithley’s DMM7510, additional measurement functions include Temperature, Capacitance, Frequency, Period.
In addition, some DMMs like the Keithley DMM7510 have high-speed digitizers (up to 1 M samples/s, up to 18 bits Digital to Analog Convertor). This enables you to look at changing voltages and currents in far finer detail than using traditional methods, usually an oscilloscope.
Applications include measuring low-level ripple voltages that cannot be measured using the standard functionality of a DMM or using an oscilloscope (see above). It can, however, be achieved using the instrument’s high-speed digitizers.
Resolution of a DMM
One of the main differentiators between DMMs are the number of “digits” that the instrument displays. This can range from 2½ to 8½.
The ½ digit represents the first digit on the display and is typically limited in the number of values that it can show, for example in a 2V range, the first digit can only be a 0, 1, or 2. It cannot be a 4, for example. All the remaining digits can be any value between 0 and 9.
Note – the resolution is the least significant digit on the scale (Range) that you have selected.
Here we have the Keithley DMM7510 7½ digit DMM showing a reading in the 100mV range, with a resolution of 10nV.
So how many digits do you need for your application?
Most DMMs with a low number of digits (2½ to 4½) are used mainly for maintenance type applications, and where the resolution required is not great. These are typically hand-held, and battery-powered for portability.
Here we have the 3½ digit handheld Fluke 115 DMM, and the 6½ digit benchtop DMM6500 from Keithley.
For example, if the question is around whether a mains voltage is present or not, you don’t need a high-resolution DMM. However, if you need to measure the mains voltage to the nearest mV, you will need a minimum of a 6½ digit DMM, and preferably a higher number of digits.
Getting the best out of your instrument
Once you have chosen your DMM it is important to know how to use the built-in features that allow you to improve the quality of your measurements.
There are additional features within the Keithley range of DMMs that can improve the quality of your measurements. Let’s look at a few issues that you might see with your measurement:
Minimising effects of electrical noise – one of the biggest factors in noisy measurements is 50Hz noise generated from the mains supply to your office/Lab/factory. No matter how much shielding you use around your device under test it is virtually impossible to eliminate. However, with Keithley’s range of DMMs, we can filter out 50Hz noise using a feature called NPLC (Number of Power Line Cycles).
If, for example, the integration period is set to 1 PLC, the integration period for the measurement is set to 1/50Hz, i.e. 20ms. The integral, or average, of a full sine wave is zero, therefore almost eliminating unwanted 50Hz noise. This feature can dramatically improve the quality and repeatability of your measurements.
Thermoelectric Voltages – Otherwise known as Thermoelectric EMFs, they are the most common source of errors in low resistance measurements. These voltages are generated when conductors made of dissimilar materials are joined together and different parts of the circuit are at different temperatures. The net result is that a thermocouple is formed, and the voltage drop varies with temperature
Offset Compensation is the feature built into many of Keithley’s DMMs that eliminate the effects of Thermoelectric EMF in resistance measurements.
There are many other features in our DMMs and other instruments that can be used to improve the quality of your measurements. In addition, the setup of your measurements also plays a crucial role in determining the quality of your measurements.
For more information, the Low-Level Measurements Handbook from Keithley offers a wealth of information on the appropriate equipment and techniques for many different testing scenarios. Please contact me if you'd like a copy.