OpAmp datasheet gains: GAIN(dB) vs. G?

[raff5184]

Hi all,
I am studying the datasheet of this AD811 opAmp
http://www.analog.com/media/en/technical-documentation/data-sheets/AD811.pdf

What is the difference in Figure 6 between the GAIN (dB) on the y axis and the G = +2?
Similarly in Figure 29 there are 3 different gains: GAIN (dB), G = +1 (top left) and Gain = + 10 (caption)

Finally, in the table 3 (page 12) I need to choose R_FB for the closed loop gain. Do the positive values for the gain correspond to the non-inverting configuration and the negative values to the inverting configuration? I mean, what's the difference between +1, +10, and -1, -10 etc?

Thank you

[ogden]

Hi all,
I am studying the datasheet of this AD811 opAmp
http://www.analog.com/media/en/technical-documentation/data-sheets/AD811.pdf

What is the difference in Figure 6 between the GAIN (dB) on the y axis and the G = +2?
Similarly in Figure 29 there are 3 different gains: GAIN (dB), G = +1 (top left) and Gain = + 10 (caption)

Finally, in the table 3 (page 12) I need to choose R_FB for the closed loop gain. Do the positive values for the gain correspond to the non-inverting configuration and the negative values to the inverting configuration? I mean, what's the difference between +1, +10, and -1, -10 etc?

Thank you

Figure 6 shows GBW (Gain Bandwidth) for amplification factor 2. Gain Bandwidth (beasured in dB) is defined by opamp's frequency characteristics, amplification gain/ratio is defined by feedback resistors, their ratio. Hope it helps. If not - do not hesitate to ask further, thou after reading doc below

Further reading:
http://www.ti.com/lit/an/sloa011/sloa011.pdf

Do the positive values for the gain correspond to the non-inverting configuration and the negative values to the inverting configuration?

Yes, exactly.

[raff5184]

Figure 6 shows GBW (Gain Bandwidth) for amplification factor 2. Gain Bandwidth (beasured in dB) is defined by opamp's frequency characteristics, amplification gain/ratio is defined by feedback resistors, their ratio. Hope it helps. If not - do not hesitate to ask further, thou after reading doc below

First of all thank you.
Since this opamp is a current feedback I thought that it is incorrect to use the concept of GBW. By GBW I mean gain-bandwidth product

[ogden]

Since this opamp is a current feedback I thought that it is incorrect to use the concept of GBW. By GBW I mean gain-bandwidth product

Briefly looking into datasheet I missed that this is current feedback opamp, but anyway GBW for such apply. After all General Description says "The -3 dB bandwidth of 120 MHz at a gain of +2" which indeed is GBW at -3dB compression point, specified.

[raff5184]

Another question about this device. I build the circuit on Figure 31, with R_FB=442 and R_G=44 and the 1uF two capacitors.
My input signal is a 700kHz 1Vp_p sinusoid. In output a see a 50-60MHz sinusoid, and it is not amplified. What do you think I might be doing wrong?
Indeed I see the 60MHz output even though there is no input. So if the input is there or not, doesnt change the output

I'm attaching a sketch of the circuit

[ogden]

What do you think I might be doing wrong?

Seems, you are not reading datasheet well enough. That amp requires bipolar supply _and_ proper decoupling. You cannot create ground for such amplifier just using resistor divider and 1uF(!) caps  No wonder that thing gets feedback and generates. Why you need 140MHz video amp for 0.7 MHz signal? Maybe try LM324N or similar for starters? If you really need this amp for some reason running - find some AD high speed amp reference design PCB and replicate it's layout, follow all the hi-speed amplifier design rules.

[raff5184]

I don't have experience with this type of opamp. So I was reading general technical documentation from AD but nothing was mentioned about decoupling, bipolar supply... And I was following the circuit on the datasheet and don't know how to generate a bipolar power supply

Thank you though

[rstofer]

Fig 25 and 31 in the datasheet show connections to +Vs and -Vs as well as bypass capacitors to ground.  From this you can deduce that a bipolar power supply is needed.  This can be as low as +- 5V up to +- 15V.  The table on Page 4 only shows data for those two voltages but notice that in every case, the power supply is bipolar.

You need a +- 15V supply.  This is pretty common for dual supply op amps.

[ogden]

I don't have experience with this type of opamp.

Those require hi-freq experience. Because fast amplifiers tend to feedback in case of mistakes.

So I was reading general technical documentation from AD but nothing was mentioned about decoupling, bipolar supply...

When you read specified for +/- 5V to +/- 15V operation - it means bipolar supply. Decoupling is *basic* rule for fast circuits. Run some internet search about decoupling. It's basically 0.1uF ceramic capacitor on each power pin as close you can get it to the pin connected to huge ground plane which runs all over the board and below IC too. Better you use 2-layer board with whole bottom layer as ground plane... There's lot to learn if you want to use such fast amps properly. I advise to put this one aside and take some "generic" amp which is not so fast - you won't have to fight with feedback so badly.

Did you put this amp on breadboard? 

And I was following the circuit on the datasheet and don't know how to generate a bipolar power supply

You just *have* supply with output of +5V *and* -5V (or other *two* outputs of both positive/negative voltages up-to +/- 15V accordingly). Hey, do you know what internet search is?

http://www.bristolwatch.com/ele/power_supplies.htm

[raff5184]

It's basically 0.1uF ceramic capacitor on each power pin as close you can get it to the pin connected to huge ground plane which runs all over the board and below IC too.

And I did put the two 0.1uF but you said it can't be done as I did.

Did you put this amp on breadboard? 

Yes, I have the breadboard version of the opAmp

Hey, do you know what internet search is?

http://www.bristolwatch.com/ele/power_supplies.htm

Sorry I was imprecise, I know what bipolar supply is, and that's what I tried to do with the resistor divider, but apparently it is incorrect

[Zero999]

I don't see why this shouldn't work from a single supply. AC couple both the input and output signals.

A breadboard is the most likely cause of the problems you're experiencing. The parasitics (capacitance between the tracks and the long inductive component leads) will begin to have a significant effect at these frequencies. Use proper ground plane construction.

[ogden]

I don't see why this shouldn't work from a single supply. AC couple both the input and output signals.

Sure it will work - if virtual ground is made correctly. However if hi-speed opamp can pull lousy ground all over the place - then as we can see, it does not work.

Yes, I have the breadboard version of the opAmp

Forget to make 140MHz amp work on the breadboard. It is close to impossible. You can possibly succeed using "dead bug" style.

Better just pick some 1MHz GBW opamp and end your struggle.