low input impedance of amplifer

[jimmykk]

I have a question regarding the input impedance of amplifier AD605 . It has an input impedance of 175 Ohm while output Impedance is 2 Ohm.
Why is it having such a low input impedance ....shouldn't it be in the range of kOhmS or MOhms

 

[LvW]

https://www.analog.com/media/en/technical-documentation/data-sheets/AD605.pdf

In this data sheet I find a value of 2MegOhm.

 

[jimmykk]

I think you are talking about the input resistance provided by the gain control interface ....isnt it different from the actual input resistance of the amplifier.

 

[FvM]

Why is it having such a low input impedance ....shouldn't it be in the range of kOhmS or MOhms

Did you eventually read the datasheet? The differential inout goes to the attenuator ladder rather than an amplifier.

 

[jimmykk]

Sorry.....but can you please explain about the attenuator ladder what does -48db means here.......and what is the actual input impedance offered by the amplifier

 

[FvM]

As the block diagram shows, there's no direct access to the differntial amplifier input. Instead a differntial ended attenuator network with 175 impedance is connected in front of it. The attenuation can be programmed between 0 and 48 db via VGN control voltage. I imagine that each ladder node drives a differential pair, teh attenuation is set by programming their bias current respectively. But circuit details aren't revealed in the datasheet.

 

[jimmykk]

OK....I understand that 175 ohm is not the input impedance of the amplifier but what is it then. i am cofused here

 

[KlausST]

Hi,

I understand that 175 ohm is not the input impedance of the amplifier but what is it then.

--> A string of resistors.

Klaus