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I got my first function generator today. The generator is a SDG1032X.

It gives you the option to switch between 50 Ω output and HiZ (high impedance) output by long pressing the output buttons.

It does not however specify what ohm rating the HiZ actually is.

All it says in the manual is:

High Impedance: displayed as HiZ;

Load: the default is 50 Ω and the range is 50 Ω to 100 kΩ.

Does this mean anything over 100 kΩ is considered HiZ?

Would it damage the generator if I hooked it up to a circuit in the wrong range?

FYI it has a 50 Ω ground; and my BNC cable is 50 Ω.

Any additional explanation is welcome too!

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    \$\begingroup\$ I find it ridiculous that someone voted to close this as a use of electronic devices question. A function generator is one of the essential tools in the field of electrical engineering and more importantly a tool exclusive to this field. Next I guess a question about the LM317 will be closed because it's an electronic device. \$\endgroup\$
    – pipe
    Commented Jun 22 at 21:16

2 Answers 2

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The setting simply selects what load impedance you have so that the display reads the voltage you get at the load.

For example if the unloaded output is 2Vpp, it will be 1Vpp when you connect a 50 ohm load, because the generator has a 50 ohm output impedance.

So when you tell your generator what kind of load you connect, it will show the correct voltage at load. If you set it incorrectly, it does not affect the generator output or damage it in any way, but the voltage displayed does not match what reaches the load.

So it's the load that may damage if you select an incorrect setting, and the voltage is either half or double than what is displayed on screen.

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The "Hi-Z feature" is simply a way for you to tell the signal generator what load it is connected to. The reason this feature exists is so that the signal generator can more accurately display the voltage amplitude at the load. More detailed explanation below.


Would it damage the generator if I hooked it up to a circuit in the wrong range?

Answer: no, there is no risk of damage - all that happens is the displayed value for output amplitude changes.

The signal generator has a fixed output impedance of 50Ω; this does not change, it is always present with all settings, as the manual explains on page 67 (refer image below).


enter image description here


Link for above image:
https://int.siglent.com/upload_file/user/SDG1000/SDG1000_UserManual_UM02010-E08B.pdf


There are several practical reasons for this, one of which is to protect the sig-gen from being overloaded when the output is short-circuited.

When you connect your sig-gen to a load, this is what the overall circuit looks like (assuming the cable is ideal, ie: has negligible impedance compared to [Rout] and [R_load]):

schematic

simulate this circuit – Schematic created using CircuitLab

[Node1] is the output of the ideal sig-generator with an output impedance of 0Ω, this is inside the sig-gen and the user (you) cannot get to it.

[Node2] is the output terminal of the sig-gen, which is what you connect your cable to.


Here is the key idea:

The sig-gen controls the voltage at [Node1], but displays the voltage at [Node2] according to what you set for parameter: enter image description here

If this is set to "Hi-Z" then the displayed value is simply the same as the controlled value.


The voltage at the load [Node2] is not the same as the voltage at [Node1]; there will be a voltage drop due to [Rout]. The sig-gen does not "know" the voltage at [Node2] it only "knows" voltage at [Node1] - it "knows" this because you tell it, by setting up the parameters associated with voltage amplitude.

You, as the user, want to know the voltage at the load, which (for the purposes of this discussion) we shall assume to be the same as the voltage at [Node2].

If the load is 1kΩ as shown in the above schematic, the voltage at the load will be almost (but not quite) the same as the voltage at [Node1]. You can calculate it accurately from the usual resistor-divider equation:
[Node2 volts] = [Node1 volts] * ( 1000Ω / (1000Ω + 50Ω) )
= 0.952 [Node1].

So the voltage at the load will be 95% of the voltage at [Node1] - which is not perfect, but accurate enough for most uses. If you set the sig-gen to output a DC voltage of 1.00V with no AC component, here is what the Sig-Gen will display as the output voltage for different values of the parameter "Load Setting":

# Load Setting Sig-Gen Display Actual V measured at load
1 Hi-Z 1.00Vdc 0.95Vdc
2 50Ω 0.50Vdc 0.95Vdc
3 1kΩ 0.95Vdc 0.95Vdc

Note: Row #3 of the above table may not be correct. According to the manual, it seems you can set this parameter freely to any value, however, I have not used this product myself so cannot be sure this is how it will work; I suggest you try it out yourself - you can't do any damage to the sig-gen by changing this parameter setting.

If you set the sig-gen to output a sinewave of 1kHz & 1.00Vp-p, then a scope connected across the load will show the sinewave to have an amplitude of 0.95Vp-p.

The 50Ω output impedance is industry-standard practice, done to ensure that the unit can drive standard 50Ω co-ax cables and other 50Ω standard loads to avoid the problems caused by incorrectly terminated transmission lines (that is a separate topic).

Below: the relevant section of the manual, for convenience.



enter image description here

enter image description here

enter image description here

Link for above images:
https://int.siglent.com/upload_file/user/SDG1000/SDG1000_UserManual_UM02010-E08B.pdf


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