OPA 2604 buffer

Hi-

I want to use an opa 2604 op amp as a buffer setup like the model on page 12 (figure 9) on the datasheet. I’m not an EE, so please excuse any ignorance.

http://focus.ti.com/lit/ds/sbos006/sbos006.pdf

The worst case load senearo, this buffer will need to drive around 500ohms, (best 640ohms) at unity gain.

A coupe of questions for you guys –

1. Whats the function of R1 in this model?
2. My math says if R2 is 0ohms, then It will be unity. Is there a better way to use R1/R2 to get unity gain?
3. Does Vout/ Itotal give the absolute load this buffer can drive? I get about 25 ohms.

Gain 3.472=3.472(1+0/20000)
Output load 3.472/0.1361 = 25.51ohms?? dosent seem like a real figure to me.

Thanks


:guinness: :guinness:

edited for clarity

[quote author="sonicmook56"]The worst case load senearo, this buffer will need to drive around 500ohms, (best 640ohms) at unity gain.

A coupe of questions for you guys –

1. Whats the function of R1 in this model?[/quote]

R1 and R2 form a classic feedback network for a non-inverting opamp circuit. Gain = 1 + R2/R1.

2. My math says if R2 is 0ohms, then It will be unity. Is there a better way to use R1/R2 to get unity gain?

Click to expand...

Yes: omit R1.

3. Does Vout/ Itotal give the absolute load this buffer can drive? I get about 25 ohms.

Gain 3.472=3.472(1+0/20000)
Output load 3.472/0.1361 = 25.51ohms

Click to expand...

Where does that figure of 3.472 come from?

Peace,
Paul

[quote author="pstamler"]

Where does that figure of 3.472 come from?

[/quote]

Thanks Paul.

3.472 is Vpp for +4dBu. I figured that was a good place to start when trying to figure out the total load. I could probably just use "1" for the math?

How would one choose the correct value of R1.... 10k? 20k? 100k? why?

Does Vout/ Itotal give the absolute load this buffer can drive? I get about 25 ohms.

Click to expand...

No because you'd get horribly high distortion. I'd say 600 ohm is the lowest load you should try. If you need more, a discrete output stage looks like a way better solution.

Samuel

Put in a BUF634 in serie with the OPA output before the feedback resistor then you will be able to drive heavy loads.
max 250mA
2000v/uS

I only need to drive 500ohms in the worst case scenario.

The 2604 shoud be able to do this set up as in figure 9 of the datasheet... :?: no?

[quote author="sonicmook56"]I only need to drive 500ohms in the worst case scenario.

The 2604 shoud be able to do this set up as in figure 9 of the datasheet... :?: no?[/quote]
For unity gain, just build figure 9 with R1 omitted and R2 as a short circuit. Make sure you put adequate decoupling on the amplifier(s): 100nF ceramic across pins 4 and 8 (if OPA2604) or between pins 4 and 7 on both devices if using 2xOPA604.

PS your avatar isn't working

I'm finding the 604 series to lack greatly in a lot of aspects.

Look at some of the opamps designed to drive cables, a lot of them can drive down to 75R easily.

I use tris to drive lines:

[quote author="sonicmook56"]I only need to drive 500ohms in the worst case scenario.

The 2604 shoud be able to do this set up as in figure 9 of the datasheet... :?: no?[/quote]

Yes. A single amplifier should be able to drive 600 ohms, so the double-amplifier shown should do 300 ohms okay.

For stability, do the 100nF cap between V+ and V-, but also put caps from V+ to ground and V- to ground, close to the amp's pins. Put 100 ohms in series with the output to avoid instability when driving long lines -- it'll lose you 1.6dB of level, but that's not usually a big deal.

Peace,
Paul

Thanks for the info. Is there a formula to find out how much of a load and opamp setup like this could drive?