Amprobe ACD-10-Pro - Manual (Page 7)

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Note: DC 400.0mV range is designed with 1000M Ω high input impedance for least
current drain in measuring small signals, and can cope better with most commercially
available voltage output transducers/adapters. The non-zero display reading is normal
when the meter inputs are open circuit, which will not affect actual measurement
accuracy. The meter will show close to zero reading when the inputs are shorted. Open
input is actually a floating condition, which is not a zero-volt-input condition.
Note: Hz input sensitivity varies automatically with function range selected while
activating the Hz function. Lowest range has the highest sensitivity, and the highest
range has the lowest sensitivity. Activating the Hz function while measuring the specific
function signal (auto ranging) can automatically select the best sensitivity range to avoid
electrical noise which may cause unstable Hz reading. However, if the Hz reading shows
zero due to insufficient sensitivity, activate the Hz function BEFORE measuring the
specific function signal. The meter is then in the lowest range and can select the highest
sensitivity.