Non-inverting amp, Current feedback amplifier, insufficient gain.

[zoulzubazz]

hey guys,
trying to test the usability of the ADA4870 high speed high current opamp, datasheet here:

https://www.analog.com/media/en/technical-documentation/data-sheets/ADA4870.pdf

I have connected the ADA4870 in a configuration (as shown in picture attached with post) very similar to the evaluation board presented in figure 5 here

https://www.analog.com/media/en/tec...luation-documentation/EVAL-ADA4870_UG-685.pdf

the amp in connected as an non-inverting amplifier set for a gain of 5 (have tried gain of 10 too) but the setup is unable to provide a gain greater than one. The supply is +-17V and the input is 2Vp-p. A clean sinusoid is reproduced but not at the level of gain required. Could someone please shed some light on what could be wrong. Thanks very much.

 

[FvM]

Circuit should work. Signal frequency?

I presume, you attenuate the generator signal to 1/3 intentionally?

 

[zoulzubazz]

hey FvM,

the desired frequency of operation around 10MHz.

when i sweep through frequency at a gain of 10 the gain starts to drop at about 500kHz but according to the data sheet this should occur at the vicinity of 10MHz.

also regarding attenuating the signal to 1/3rd, I found that adding a 50ohm resistor between the dc blocking cap and the non-inverting input significantly reduced the output noise of the circuit.

thanks very much.

 

[FvM]

I have no reason to doubt the Av=5 and Av=10 gain curves in the datasheet. Something must be wrong in your setup, but hard to guess from a distance.

The 50 ohm resistor at Vinp shouldn't be necessary, a bias resistor of 1 or 10 k must work, a higher series capacitor value may be necessary if you worry about low frequency noise.