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Circuit review: Fast DAC driver / pulse generator for 50 ohm loads

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Weston:
A few more comments:

Looking at your schematic the ground symbol for the feedback resistor is not actually connected. Did you correct that at some later point? It could cause issues.

If you are seeing oscillations on the opamp input it could be the DAC oscillating. Another possibility is that for some opamps there is a path between the + and - inputs that is biased off during correct operation but can conduct when the + and - inputs are not close. An antiparallel diode pair is one example of this. If its oscillating its a possibility that its coupling back to the other input.

Looking through the DAC datasheet there is a maximum capacitive load of 500pF. So more capacitance on the DAC output could cause it to oscillate. If you removed that capacitor the DAC itself should not oscillate. The nominal loading of the opamp should be pretty high, so also verify the DAC itself without the opamp connected?

In regards to the output loading, another advantage of putting a 50 ohm resistor in between the opamp output / connection to the feedback network and the load is that it reduces the impact of output capacitance on stability. As previously mentioned, this cuts your output signal by half, but it may be needed to get the opamp to be stable with a capacitive load. You could also try a smaller output resistor and increasing the resistance of the feedback network. Both of those would increase phase margin with a capacitive load.

Joakim:
Having worked on the circuit now for two days I am happy to see that the amplifier itself seems to be working decently now. Great news!


--- Quote from: Weston on June 13, 2022, 09:59:35 pm ---A few more comments:

Looking at your schematic the ground symbol for the feedback resistor is not actually connected. Did you correct that at some later point? It could cause issues.

If you are seeing oscillations on the opamp input it could be the DAC oscillating. Another possibility is that for some opamps there is a path between the + and - inputs that is biased off during correct operation but can conduct when the + and - inputs are not close. An antiparallel diode pair is one example of this. If its oscillating its a possibility that its coupling back to the other input.

Looking through the DAC datasheet there is a maximum capacitive load of 500pF. So more capacitance on the DAC output could cause it to oscillate. If you removed that capacitor the DAC itself should not oscillate. The nominal loading of the opamp should be pretty high, so also verify the DAC itself without the opamp connected?

In regards to the output loading, another advantage of putting a 50 ohm resistor in between the opamp output / connection to the feedback network and the load is that it reduces the impact of output capacitance on stability. As previously mentioned, this cuts your output signal by half, but it may be needed to get the opamp to be stable with a capacitive load. You could also try a smaller output resistor and increasing the resistance of the feedback network. Both of those would increase phase margin with a capacitive load.

--- End quote ---

You are correct about GND in the first version of the schematic. Unfortunately I attached the revised schematic as a pdf so it was not immediately obvious which one the PCB was based on. Probing the net confirms it was properly grounded.

After confirming that the power rails all are stable and noise-free I desoldered the switch in order to isolate the input of the op-amp, and I soldered a 47 ohm resistor inline with the output of the op amp. Small pads in combination with a large ground plane made desoldering the SMA connector impossible without ripping the through-hole plating. No big deal really as I still have a few boards to spare, and I still need to have rev 2 manufactured. Anyway, the amplifier still did not work and after pulling my hair out I had to call it a day. What I did notice however was that the issue seemed to be with the input signal, as I was able to get somewhat decent output when I poked the input signal into my finger. The LT app note mentioned that any bandwidth limiting capacitors should be on the positive input, not in the feedback net, so I suspected that the capacitive coupling to my finger which made the circuit work implied that I needed a cap on the input.

When returning to the lab today I realized that I hadn't terminated the input of the signal generator. I had simply connected the output of the signal generator to both the scope and the DUT with a T-coupler. Of course this worked for low frequencies, but when I increased the frequency of the stimulus signal I started seeing oscillation and the current draw increased beyond comfortable levels. It turned out that the output load also made a poor connection to the coax/T-coupler, so I had intermittent issues here. Now with new connectors and a 50 ohm load in the input signal path I am starting to get results.

Unfortunately I am not able to reach the full output swing that I would like when halving the voltage with a ~50 ohm inline with the output of the op amp. The output of the op amp would have to swing swing between 0-12V +-Voffset, so eliminating that resistor would be ideal.

So, with properly terminated in-/output I am starting to make progress. I'm feeding a 40ns/100ns pulse directly into the amplifier and I get the results in the attached captures. The rise time is ~15ns and the fall time ~25ns, which is decent enough for me at this point. However I am seeing some noise on top of the signal, plus something which I suspect is a reflection.

In the tests above I am supplying only VCC, while VEE is turned off as the amplifier becomes very noisy when supplying +8/-2 volts. I cannot figure out why this is the case here, but if the circuit works with a single supply it is more than fine. At the moment I am not concerned about the exact gain as I am able to saturate the amplifier, as I could easily just calibrate the DAC appropriately. I can confirm that the output indeed is adjustable by adjusting the trimmer pot.

What worries me now is how the output of the switch would operate when not terminated, just like yesterday's tests when the output of the function generator was not terminated.

The next steps now are to confirm that the DAC still is working after having reworked the amplifier stage, re-connect the switch and test the board in its entirety. Judging by the LT app note, in a new version of the board I should swap the fixed resistor and the pot in the feedback network.

Edit: Added photograph of the board in its current state. It's sticky with flux residue, mangled and not pretty - but I'm making progress :)

Marco:
On second thought the concept is fine, it's the choice of components which is the problem.

The THS4271 can handle the necessary supply voltage (~16V from negative to positive) to handle source termination while still being able to supply 6.6V to the modulator, the AD8009 simply doesn't, taking into account the necessary margins around the rails voltages.

There is a faster analogue switch, the SD5400CY. Though it's a little harder to work with, also unobtainium at the moment through normal channels, so would have to play ebay roulette.

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