LMH6624 unexpected frequency response (peak at 250 MHz)

Gefeba

Prodigy 120 points

Other Parts Discussed in Thread: LMH6624

Dear all,

I'm using an LMH6624 for a high speed transimpedance amplifier. The topology is pretty simple:

- input photodiode

- feedback resistor (which sets the gain) capacitor (to increase the phase margin)

- the positive input of the OpAmp is connected to ground

With a 1Kohm resistor and a ~10 pF capacitor to compensate I get a flat frequency response up to 60/70 MHz. (see attached picture)

That is basically ok but all of a sudden there is a peak at 250 MHz that can't be explained from the information in the datasheet.

Does anyone have any idea about what could cause this?

Thanks a lot in advance.

Best regards

over 10 years ago

0 Samir Cherian over 10 years ago

TI__Genius 12631 points

Hello,

I plotted the Aol for the LMH6624 and it looks like the 2nd pole of the device kicks in around 220MHz . This is affecting the closed loop response. What is the photo-diode capacitance of your system?5661.tinadiag.emf

Thanks,
Samir

0 Hooman Hashemi over 10 years ago in reply to Samir Cherian

TI__Mastermind 22825 points

Hi Samir and user4200690,

Minimum Noise Gain: Please also consider the possibility that the capacitor across the feedback resistor may have to be lowered because the LMH6624 noise gain must be higher that 10V/V, similar to the explanation in the E2E post below:

http://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10/p/285141/994987.aspx#994987

With too large a value for CF, you could be encountering instability and that could explain the 250MHz you had noted.

Non-Inverting high frequency impedance to ground: From a slightly different angle, I've had cases where the Transimpedance amplifier stability is improved if the non-inverting input AC impedance to ground is increased, as shown in this schematic, where R4 is the high frequency impedance this input sees:

http://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10/p/294380/1026520.aspx#1026520

You may want to try this approach if all else does not improve your 250MHz peaking.

Regards,

Hooman

0 Gefeba over 10 years ago in reply to Samir Cherian

Prodigy 120 points

First, thanks a lot for answering.

Before I go deep into them, I will just make some short comments.

1. The Cpd is roughly 40 pF. The PD is biased at 5V

2. Is it possible to simulate myself or get that Aol information from the LMH6624? I checked again the datasheet and the "open loop frequency response over T" graphs are not really clear.

3. I'll check the lower Cf and non-inverting input impedance approaches in detail

Will be back! Thanks again

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LMH6624 unexpected frequency response (peak at 250 MHz)