HA-5002 Unity gain buffer but doubles the output Voltage?

Go To Last Post
5 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi guys,

I found a component (HA-5002) that's like perfect for my application.

Heres the datasheet: http://www.intersil.com%2Fdata%2Ffn%2Ffn2921.pdf

 

The datasheet states it is a current buffer, ie unity gain gain amp.

 

Ok thats fine expect I need to take a signal and transmit it over a coaxial cable and get the same output after the input and output loads. In this case both are 75ohms so I have to drive two 75ohm resistors in series. Similar to the example given (Unity Gain buffer.png attached, which is at top of page 6 of the datasheet), except they use 50 ohm resistors. They show a scope trace where Vin = Vout.

 

As I see it Rm, and RL make a potential divider with Vout halfway in between, surely Vout = 1/2 Vin? The datasheet clearly shows otherwise.

I even simulated it (simulated.png attached) and got the result I expected, Vout = 1/2 Vin because of the potential divider network. PS the green trace is the output.

 

I can't see what I am overlooking? Is it some transmission line magic?

 

Attachment(s): 

Just a noob in this crazy world trying to get some electrons to obey me.

Last Edited: Mon. Feb 22, 2016 - 09:48 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

High speed amplifier systems that drive transmission lines are usually 2X. That is because of the "gain" of 1/2 in the reverse termination resistor, Rm and load, Rl. Your Unity Gain buffer.png shows things the way they really are.

 

What the spec sheet may not tell you is that the AMPLIFIER has a gain of 2X. What it DOES tell you is that it is 1X from the input to the output. Yes, that is a 2X amplifier in a circuit that they call "Unity Gain Buffer".

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi Jim, thanks for your reply, it is a great help.

Okay so that makes a bit more sense.

 

So for my circuit I simulated, if I actually built it, wou are saying I see Vout= Vin?

If yes, is there something wrong with my simulation?

I got the Spice model from the manufacturers website directly.

 

Kind Regards

Just a noob in this crazy world trying to get some electrons to obey me.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

If you measure V at the output pin of the op-amp, you should get 2X.

 

If you measure the output at the far end of the back-terminated transmission line, you should get 1X.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi,

 

In the datasheet there are other examples where the gain was really 1 and I couldn't reconcile why sometimes it seems amplified, and sometimes it is not.

 

Although I wish the gain of 2x was the case, after very carefully reading the datasheet over and over, I noticed some faint numbers, at the top of Unity Gain buffer.png. They are the different volts/per division (1V and 500mV), hence the output signal is half the input as per the simulation (Vout = 1/2 Vin).

 

Thanks for the help!

Just a noob in this crazy world trying to get some electrons to obey me.