floating input for DVM

Digital voltmeters are now in wide-spread use and growing ever more popular. Many of the cheaper types of DVM however suffer from a slight drawback in that they have an earthed input (i.e. one of the input terminals is connected to earth or to a fixed voltage level). In many cases this is not particularly important, however there are situations (if the DVM is used in conjunction with an add-on unit such as an AC millivolt-meter, for example) where it can be something of a nuisance. With the aid of the following circuit, formed around a differential amplifier, any DVM can be provided with a floating input.