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ADA4930-2 Gain Peaking

Question asked by JWGerber on Jan 22, 2016
Latest reply on Mar 16, 2016 by JWGerber

Hi there,

We have an intermittend problem while using the ADA4930-2 in a set of drivers. Intermittendly we pick up a 65 MHz oscillation on one of the 2 channels, which seems to disappear when cycling the devices power.

 

The amplifier set consists of a 7 V/V amp driving another 7 V/V amp, driving the 1.5 V/V final amp (driving the ADC).

The amps have LPF's inbetween them.

 

My first guess would be too much stray capacitance on the input is resulting in gain peaking resulting in the oscillation.

It does not explain why it disappears on voltage cycling, though.

* I have evaluated the phase margin and that sits at 60-80°.

The datasheet refers to a min Rg of 301R for unity gain, but I notice that the given examples also only use Rf = 301R with various values for Rg for higher gain. Is there a recommended upper limit for Rf for noise? - Rf was selected as 536R in out design, so I am thinking of using a lower value to reduce the ratio of stray capacitance to set resistors, in effect moving the peaking frequency.

If that fails, a cap in parallel with Rf would limit the bandwidth.

 

My second suspect is that the final amplifier drives differentially into a 4p7 cap (i.e. no series resistance) and then into the ADC plus terminating resistor. usually opamp do not like driving into a cap, but I see no mentioning of that in the datasheet and Fig. 60 shows the opamp driving into the LP.

 

Do these sound like the correct approach and is there something else that may also be a problem?

 

Thanks,

Johann

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