LMH6629 oscillates at about 800 MHz

Hi,

we have now completely redesigned our circuit to fit onto one board and added the RC filter between the inputs (180 Ohm, 10 pF). Sadly, it had nearly no effect. We now see a very sharp oscillation at about 300 MHz and 1.4 GHz. I have attached our most recent PCB.

Most interesting to see was, that the 600 fF capacitor had no effect. We got nearly the same results without it. Also, adding the pair of wires to trim the circuit, always greatly increased the amplitudes of these oscillations.

If you have any additional ideas, please let me know. I have attached our PCB, the Diode is directly soldered into it.

Best Regards,

Johannes

Yes, I think, we will try using another operational amplifier. Thank you for your help. :)

Hi,

Here are my comments in no particular order:

1. You could obtain the LMH6629 EVAL board here (http://nationalsamples.ti.com/evm/LMH6629SDEVAL.html?OrderOnline=Order+Online)  and verify that it is stable first. Then, you could possibly modify it to use your APD?
2. Experiment with the LMH6629 COMP pin state (LLP-8 package only)
3. Experiment with an RC across the inputs as Figure 4 shows. The datasheet has more explanation on this:
4. Terminate the LMH6629 in 100ohm (50ohm series and 50ohm shunt to ground) directly at the output. The 50ohm shunt would be your oscilloscope / spectrum Analyzer input impedance
5. Use the oscilloscope probe tip to touch each and every device pin (while monitoring the output on another scope channel) and look for which pin is most sensitive to the probing. This could provide some clues as to the source of you instability
6. Isolate your APD from the input using a series resistance (for experimentation only) to see if the photo diode capacitance is a culprit
7. Get the low value (0.1uF) ceramic supply decoupling caps as close as possible (< 0.2") to the LMH6629 supply pins with the return path currents away from the sensitive input pins. From your layout view, pin 2 (V-) traces is fairly long and serpentine before you see any visible decoupling caps!

Hope this helps.

Thanks,

Hooman

Hello, can you explain the 90 Mhz limit, i dont understund, if the GBWP is 4 Ghz?, and when say the minimun stable gain (g = +10) if i use g = +100?

Hi,

Here is what the datasheet states regarding "Constraint 2" (90MHz) that you asked about:

" “Constraint 2” places an upper limit on the feedback phase lead network frequency to make sure it is fully effective in the frequency range when loop gain approaches 0dB."

Explained differently, if the product of R_c x C_c is too small, it might only provide the necessary noise gain increase at too high a frequency to be useful! "Constraint 2" ensures that the values selected for these components is such that their effect is helpful in the frequency of interest. The full effect of a zero in the transfer function can be achieved in about a decade above it.

I'm sorry but I could not follow your 2nd point (about g= +100). Please restate it differently or provide more details so that I can respond.

Thanks,
Hooman

Hello,

I need an photodiodo amplifier, the photodiode have Isc max 100 uA and cd = 40pf, and i need a Voutput max in 10 V with 200MHz band width. I believe that several steps are needed. The LMH6629   (SOT23-5), apparently has his best performance, with g = 10 and Maximum output of 200 mV, for small signal and large-signal 2 Vpp.

Could you give me a recommendation?

thank you

Can you give me your opinion about this circuit?

Hi,

I have not done a  complete analysis of your circuit (at least with simulation), but these things come to my mind just looking at it:

1. Using a current feedback amplifier like the LMH6703 imposes restrictions on the upper feedback resistor / gain range of the 1st stage (as you've chosen a low value of 300ohm for that). Most Transimpedance amplifiers take most of their gain in the 1st stage and for that they choose a low noise device. I think you'll not get the best possible SNR by distributing your gain in cascaded stages, as you have done. The theory is to maximize the first stage gain with a low noise device.

2. If you choose to use the LMH6703, I'd recommend the +/-5V supplies. +/-6V is the upper limit on supply voltage and leaves no margin for tolerances

If you send me the TINA schematic of your circuit we could compare the SNR / bandwidth against the ideal case of low noise / high gain input stage and then you can decide if the performance you'll get with this circuit is acceptable or not. Note that the LMH6703 TINA model includes the noise performance (including the 1/f noise region) and is accurate enough for first-pass analysis of the output noise and SNR.

Thanks,

Hooman

0624.Photodiode I.TSC

I send the TINA file, can you recomend some sustitute for the LMH6703c?, i need 200MHz band width, with in this case Vout = 5 Vpp, i read the datasheet about transimpedance amplifier and if you want big band width you need set a low gain, is tru?, your help is good for my in this mater

thanks 

eneas

Hi,

I've not verified this with simulation, but if you used a low noise device on the front, you should get higher SNR. Here are a couple of possibilities which work with 10V supplies:

OPA847 (0.85nV/RtHz, 2.5pA/RtHz, 3.9GHz GBW)

LMH6624 (0.92nV/RtHz, 2.3pA/RtHz, 1.5GHz GBW)

In simulation, you'd want to increase the 1st stage Rf as high as possible, without running out of bandwidth, for best SNR. TINA already includes the LMH6624 model. For the OPA847, you could get the OPA847 Reference Design (which includes the OPA847 model) here:

http://www.ti.com/litv/tsc/sboc098c

Thanks,

Hooman

Hi Hooman:

I have a question about the COMP pin for the LMH6629. When the LMH6629 is used for transimpedance amplifier, the gain is mostly defined by the current and feedback resistor, the output voltage is basically V=IR. Is that true? Will it change the output voltage by a factor of 10 or 4 if I connect the COMP pin to the Vcc or ground to make it 10V/V gain or 4V/V gain.

thanks,

Jason

Hi Jason,

With respect to the COMP pin:

It is best to think of this pin not in terms of closed loop gain but what it does to the open loop transfer function. With COMP pin High, your have higher bandwidth because the response reaches higher in frequency and vice versa for COMP pin Low voltage. So, when it comes to the stability of the transimpedance amplifier (which is always a challenge), you would want to be mindful of the open loop transfer function shift with COMP pin, shown below, and what you need to do to stabilize the loop.

Figure 15 in the datasheet shows the relationship between the open loop response and external compensation across RF graphically.

Regards,

Hooman

Thanks Hooman, it's clear to me now.

regards

Jason

Thanks Hooman:

I'm using the LMH6629 as AC coupled TIA, but the output noise level is much higher than the simulation. In the simulation is around 10nV/sqrt(Hz), but in the measurement, it goes up to 60nV/sqrt(Hz), and there is another LNA after the TIA, the SNR is only 20dB for -50dBm signal with 25MHz~60MHz bandwidth. Is something related to the layout?

Jason

Hi Jason,

It is hard to say whether your symptoms are related to the layout or not without more information. Do you have anything to share (schematic, layout, TINA simulation file, conditions)?

Please let me know.

Regards,

Hooman

Hi Hooman:

Attached is the AC coupled simulation. I used one DC coupled TIA LMH6629 before, the output noise is close to the simulation results though still 20% higher. I don't have my PIN diode spice model, so I just used the one in the TINA, the capacitance is around 1.5pF. Our application is between 20MHz to 80MHz.

thanks,

Jason

Hi Jason,

With your TINA circuit, I measure 13nV/RtHz output noise at VF1 or 26nV/RtHz at device output. The 1kohm PIN diode biasing resistor (R2) contributes 21nV/RtHz [4.07nV/RtHz x (5k / 1k) ]. Is that an external resistor in your circuit or built-in with your PIN diode? Because, with such a big contribution from it, I'd make sure that it is in fact 1kohm. Can you insert a series inductor (500uH or larger) with it to eliminate its noise contribution and still maintain DC biasing?

With the input coupling capacitor C1 opened, TINA shows about 15.8nV/RtHz at the device output. What do you read if you did that? If you disconnect the input, you may have to include a 1.5pF shunt cap to ground at the inverting input for direct equivalence.

If your frequency response is "peaking" a lot, your noise gain may be higher than you / simulation expect and that could raise your output noise. However, right now your noise is so dominated by that biasing resistor (R2) which I don't think peaking is your issue. Once you knock that down (if possible), then other noise contribution may become dominant, which they are not now. A clue as to whether or not you got questionable frequency response / noise gain peaking might be in tying the COMP pin Low and seeing if your noise changes? Or you could try and actually measure frequency response by applying a current input (possibly through a common base amplifier collector). Another possibility: You could use the LMH6629SDEVAL board from TI to reduce your concerns about layout issues.

Hope this helps.

Regards,

Hooman

Thanks Hooman

The circuit is not peaking. The output of the TIA is pretty flat. I can use the EVAL board to test

Jason

I tried switch the COMP pin low or high. There is very small changes between them. And also when I check the wider spectrum, I find there is a peak at 3rd harmonic, and it's strong.

Any updates?