input impedance

[PG1995]

Hi

Please have a see Example 1-5 on the given link:
https://img854.imageshack.us/img854/8376/inputimpedance.jpg

The example mentions input impedance. What is it in simple words? The book says: Because this input impedance is across the measured terminals , a small current flows flows through the multimeter....

What is book saying? Please help me with it. Thank you very much for this.

 

[carbonzit]

For the sake of this example and your understanding of the concept, think impedance = resistance.

Specifically, resistance as measured across the input to a circuit. High impedance means high resistance means little loading to a source connected to the input. Low resistance means the opposite.

 

[KeepItSimpleStupid]

Any measurement of a circuit disturbs the circuit in some way. So, if the input Z is purely resistive and it's 10 Megohms, it will have negligible effect on measuring the voltage across a motor whose internal resistance is 2 ohms. 2 ohms || 10 Meg ohms is pretty close to 2 ohms. The || means in parallel with.

If your circuit had a resistance of 10 G ohms and you put a 10 M-ohm resistor across it, you can't measure it with your meter. 10 G ohms || 10 M ohms is closer to 10 Meg ohms than 10 G ohms and the voltage measured will be nowhere near right.

A DC measurement that you make will suck some current out of the circuit just to measure it and it can be quite small. It just so happens that a 10 Meg ohm input R is a standard number, say for a handheld DVM. With Lab instruments it may change based on range, getting higher with the more sensitive scales.

 

[PG1995]

Thank you, carbonzit, KeepItSimpleStupid.

I think I understand it now. Say, voltmeter measures how much voltage is appearing across its terminals. In the Example 1-5 open circuit voltage is 9 volts and a Rth of 1.5 K. Measuring with a multimeter which has impedance of 200 K ohm will give a reading of 9 * ( 200 K / 201.5 K) or 8.933 volts (I've used voltage divider rule). Error is 0.067.

In the scanned example in my first post the author used the phrase "measured terminals", what does the author mean by it?

The author also says that moving coil has typical sensitivity of 20k ohm per volt. What does the part "20k ohm per volt" mean? That would mean for two volts it would be 40k ohm.

The author also suggests to use a field-effect transistor. What does this mean? If it's complicated, then you can ignore it because perhaps I won't be able to understand it.

Please help me with the above stuff. Thanks a lot. Things would have been really difficult without your help.

 

[Nigel Goodwin]

The author also says that moving coil has typical sensitivity of 20k ohm per volt. What does the part "20k ohm per volt" mean? That would mean for two volts it would be 40k ohm.

It refers to passive analogue meters, they draw current based on their meter sensitivity (most were 50uA FSD meters), so they have a load resistance of 20K ohms per volt - this is per volt on the meter setting (so a 10V range will be 200K, and a 100V range 2M). This is different to digital meters, where they mostly have a fixed loading of 10M - notice that this means that old analogue meters are actually better on the high voltage ranges (1000V range 20M).

 

[KeepItSimpleStupid]

Measured terminals: Where you put the probes of the meter

Before I briefly define FET, I want to mention that DVM's of today had a history and they all started out as Analog:

1) The ohms/volt type which started out as a simple D'Arsonval meter movement.
2) Then we had the VTVM or the Vacuum Tube Voltmeter
3) Then the FET TVM or the Field Effect Transistor Transistorized Voltmeter
4) Now we can out the FET's inside of an OP AMP or Operational Amplifier.

The FET is voltage control device and the Bi-Polar transistor is a Current Control Device. Hence FETs are used to get very low input bias currents.

 

[KeepItSimpleStupid]

Aside: I want to mention, en passing, is that you are learning about DC circuits. You will find that in devices connected to a transmission line will have 50 ohm input and output impedeances. This minimizes reflections. RG58 Coax cable has a characteristic Z of 50 ohms. You'll learn this in Electromagnetic Field Theory.

Furthermore, you will learn that oscilloscopes will have a characteristic Z of 1 M ohm shunted by several pf of capacitance. The standard 10 X probe has a variable capacitor which is adjusted so that square waves are square. This divider is an AC divider and compensates the probe capacitance. With the 10X robe, the input Z still ends up being 10 M ohms without the capacitive componenent. Expensive scopes may have switchable 50 and 1 M ohm inputs.

As always you learn simple concepts and add the complexities later for you have to understand the basics.