<P> Digital ammeter designs use a shunt resistor to produce a calibrated voltage proportional to the current flowing . This voltage is then measured by a digital voltmeter, through use of an analog to digital converter (ADC); the digital display is calibrated to display the current through the shunt . Such instruments are often calibrated to indicate the RMS value for a sine wave only, but many designs will indicate true RMS within limitations of the wave crest factor . </P> <P> There is also a range of devices referred to as integrating ammeters . In these ammeters the current is summed over time, giving as a result the product of current and time; which is proportional to the electrical charge transferred with that current . These can be used for metering energy (the charge needs to be multiplied by the voltage to give energy) or for estimating the charge of a battery or capacitor . </P> <P> A picoammeter, or pico ammeter, measures very low electric current, usually from the picoampere range at the lower end to the milliampere range at the upper end . Picoammeters are used for sensitive measurements where the current being measured is below the theoretical limits of sensitivity of other devices, such as Multimeters . </P> <P> Most picoammeters use a "virtual short" technique and have several different measurement ranges that must be switched between to cover multiple decades of measurement . Other modern picoammeters use log compression and a "current sink" method that eliminates range switching and associated voltage spikes . Special design and usage considerations must be observed in order to reduce leakage current which may swamp measurements such as special insulators and driven shields, triaxial cable is often used for probe connections . </P>

When is an ammeter said to be sensitive
find me the text answering this question