Discussion in 'General Electronics Chat' started by noza, Aug 18, 2008.
The purpose of the input impedance being set to 100ohms does anyone know?
If it's a meter, that figure is more likely to be 100Mohms. The input resistance is high so as to disturb the circuit being measured as little as possible.
Old-style "needle" D'Arsonval movement multimeters were typically rated for 10k Ohms/Volt, or 10,000 Ohms per Volt. This amount of load could have a significant effect due to loading on sensitive analog circuits, and resulted in the measured voltage being different from the actual voltage present without the load of the meter.
10k Ohms/Volt represents a 0.1mA current load. If the circuit was capable of supplying a 1mA current, the 0.1mA load would be off by 10%.
Modern DMMs (digital multimeters) have input impedances in the range of hundreds of megOhms, or nearly infinite. This presents an extremely small load on the circuit being measured in comparison to the old style multimeter, thus the voltage in the circuit can be much more accurately measured without the necessity of figuring in the resistance of the meter.
The old-style multimeters still have their uses. DMM's are usually rather useless when rapidly changing signal levels are involved; the numbers on the display just roll. With the old-style D'Arsonval meters, you can see the needle move and get a good idea what the average signal level is without resorting to adding circuitry.
You can even use an old-style meter in conjunction with a sweep frequency generator to tune a filter. You can't do that with a DMM.
Consider the input impedance as being in parallel to the component who's voltage you are measuring the voltage across. Draw up the equivalent circuit of two resistances in parallel where the value of the input impedance is very large (approaching infinity) - look at how the current is drawn through the two resistances - nearly all current is through the resistance who's voltage is being measured. This illustrates that measuring the voltage/resistance is not (or as little as possible) disturbing the system which you are measuring by the act of measuring it.