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Differential wideband DAC output with THS4509

Other Parts Discussed in Thread: THS4509, DAC5681, THS4513

5153.THS4509_DAC.zip

I have designed a differential wideband transimpedance amplifier for a high speed DAC (DAC5681). This DAC has an output compliance range of AVDD+-0.5 V. I have chosen a transimpedance architecture as circuit has to operate from DC to 300 MHz. See schematic in attached ZIP file.

The problem is that this circuit oscillates wildly at about 2 GHz.

Let me explain the design decissions and some measurements I have taken.

The transimpedance stage is simmilar to this described in SBAA150 application note, Example 3.

The PCB was carefully layed out. It is a 8-layer one, and I have removed the ground plane under bias resistors (R22 and R23), feedback elements (R21, R27, C47, C49) and amplifier input and output, including output resistors (R25 and R26). The ground plane removal is valid except for the more distant layer (L7) to shorten current return path. See gerber files detail in attached file.

After many tests involving 4 boards and many experiments, I decided to hand assemble the offending components in a new PCB, and I found simmilar vehaviour, thus DAC can be considered out of the oscillation equation. Even more, in my previous experiments I found that the output common mode setting to mid supply is not responsible of oscillation, so the circuit around it is not assembled. Just R22, R23, C47, R21, U5, R27, C49, R25, R26 and all decoupling components (FB7, C45, C46, FB8, C43, C44) are assembled.

With the component values show, the oscillating frequency is 1.38 GHz and -8 dBm (over 50 R, AC coupled to spectrum analyzer). Gain is 2.3 (7 dB), too close to minimum.

The I reduced R22 and R23 to 11R, increasing gain to 4.5, a safest figure. Oscillation frequency ¡rises! to 1.82 GHz and -13 dBm.

Following some advices in SBAA150 application note, I solder 10 pF in parallel to the amplifier input (in differential mode). Oscillation goes up to 2.25 GHz and -16 dBm.

The I used 100 R for feedback resistors R21 and R27. Oscillation remains at 2.25 GHz and -16 dBm. No changes.

Then , I tested the circuit shown in Figure 21 of SBAA150, using an 130 R in parallel to 12 pF and feedback of 100R and 3p5. Oscillation frequency is 1.9 GHz and -13 dBm.

Oscillation has been measured in 4 series boards (with DAC) and the new one with just the offending circuits.

I have also tested with more decoupling, higher frequency decoupling (like 100nF uW grade capacitors), output traces isolation: using local load termination and attenuation for coaxial cable used for measurement. I have cut traces in DAC input and feedback components, soldering them capacitor over resistor. Nothing works: the circuit is addict to oscillation. I have been able to see oscillation frequency variations depending on the load placed in the non measurement port (either, open, 50R load to GND or AC coupled 50R to GND) but this does not surprises me much as the output stage biasing conditions the nonlinear behavious that sets the oscillation level.

It seems to me that the problem is the second pole of the amplifier (not a very clever guessing). I have done some TINA simulations and my own analysis using douple pole models and, despite minor instability with original designs, all other should be perfectly stable. This means probably the problem is due to component/board paratistics. The GBWP of the device is 3 GHz, so at 2 GHz the device has very small gain, and positive feedback has to be very strong. How can the output phase shift so much?

If you have any clue, please, tell me.

Best regards

Luis Miguel

  • Luis Miguel,

    I simulated your circuit. DC and AC sims look ok, but when I run a transient, it appears to be oscillating (or other). I will have to spend more time with it to see if I can find a topology that will work better. I have some ideas and will get back to you in the next few days.

  • Luis Miguel,

    I worked more on your circuit and think I have something that will work and not so much different from what you have. I attach Tina files to show your original circuit (basically the same for the simulator anyway) and the new circuit.

    If you use "intial conditions" option in Tina transient simulation with the all caps set to 0V except the cap on Vocm (set this to mid supply, 2.5V to speed up simulation), your original circuit ocillates at about 6GHz . The new cicuit does not. Note that using the option where it calculates the operating point both circuits seem to work fine.

    Having capacitance at the feedback summing point is always a problem for high speed op amps that requires careful consideration to properly compensate. As rule I try to avoid if I can. In the modified circuit I placed 22 ohm resistors to isolate the DAC output capacitance (5pF in data sheet). This seems to fix the oscillation and then I bumped up RF to get the TIA gain you had originally. This should also help distortion because the feedback path is lower load on the output.

    TI-Tina is free. For more information and download link go to  http://www.ti.com/tool/tina-ti 

    3223.ISink DAC to THS4509 stability test Original.TSC6758.ISink DAC to THS4509 stability test New 1.TSC

  • Dear Jim,

    Thank you very much for your interest in my problem.

    I have prototyped your circuit suggestion and oscillates at 1.9 GHz.

    In the mean time I have been doing more experiments. I have come to the surprising conclusion that the device does not like to see feedback resistor of 100 R or lower. The lowest value in which I tested absence of oscillation was 200 R. I have no idea of what can be the reason for that, although figure 39 of data sheet (CM input impedance vs frq) makes me thing this amplifier can not be simply considered as an ideal voltage feedback amplifier. The experiment consisted in mounting an amplifier with gain of 4 with Rf=390R and Rin=100R. The circuit does not oscillate, and does not either if I use Rf=200R and Rin=50R. But when I use Rf=100R and Rin=25R, does. I cannot understand why.

    Hower, your topology suggestion, combined with my experiments have leaded to a nice solution: just use R1 and R2 of 50R and R11 and R13 of 200R. Just an slight increase in voltage gain and more importantly, increase value of feedback resistor. I can see linearity behaviour is quite good despite the fact it is not a transimpedance design.

    Best regards

    Luis Miguel

    PS: In my humble oppinion, simulation seems to me not to be an adequate tool for this kind of problems. For example, it seems unrealistic to have a 3 GHz GBWP device oscillating at 6 GHz. I think it would be a real mistake to have application note circuits just based on simulations and not tested in the lab.

  • Luis Miguel,

    I am glad to hear you are able to solve the problem. The larger value resistors sounds like the way to go, but for better assurance you should also test the THS4513 in your design. It is the same basic op amp as the THS4509, but compensated for unity gain operation. So it will be even more stable and still gives very similar noise and distortion performance.

    I built the basic circuit with RF=100, RG=22, and R to 3.3V = 22 on the THS4509 EVM. I could not find an oscillation, but the network analyzer showed it was very unstable.  I then changed to RF=200, RG=50, and kept R to 3.3V = 22; this looked much better on the network analyzer.