AD9951/AD9859 temperature-depended stability issue

Hello everyone,

I'm working on a project containing the AD9951 as frequency generator for 145 MHz. Previously to that, the AD9859 was used, but was swapped for the pin-compatible AD9951 because of availability reasons. The issue described in the following post applied to both devices.

The DDS chip is used to generate an approx. 145 MHz carrier (HAM-radio band). When turned on, it works nicely and generates a clean signal on the output transformer when viewed on the spectrum analyzer.

This trace, however, suddenly becomes much broader and unusable when the chip and the board heats up. And by heating I do not mean hundreds of degrees, it is not at all uncomfortable to touch by hand.

It is important to state that the whole setup was not touched at all between good and bad signal. The behaviour can be reverted to the first picture by placing a small heatsink on the AD9951. If however, a hot soldering iron is placed approx. 1cm above the heatsink (without touching, just to raise the temperature by a couple of degrees), the issue re-emerges.

First I was suspecting some oscillation issues with the 145MHz power amplifier on the same board, but switching off the power and disconnecting the amplifier from the DDS does not change the behaviour at all. The issue seems to be coming from the DDS itself.

The used schematic and board layout are shown here:

The spectrum analyzer is connected directly to the output of the transformer with a 4n7 coupling capacitor and a 20dB attenuator (for safety). Attempts were made to amplitude-modulate the DDS via the DAC_RSET pin, but this feature is currently disabled, so the 3k9 resistor is just connected to ground.

Furthermore, the issue occured on two different PCBs, one with the AD9951 and one with the AD9859. The layout, however, is almost the same.

Now it is clear to me that it is probably hard to tell exactly what is happening on my board from a distance via a couple of pictures, but I just wanted to ask if this has been observed bevore in some situations or maybe I've left something obvious out. In case more information helps, I could also upload more design data, Settings, microcontroller code etc.

Thank you very much for your time in advance!

Best regards,

Josef

Parents
  • Hmm. I am almost sure that the solder connection is good because the EPAD as well as the mating pad on the PCB have been tinned before. It was therefore not possible to push the chip flat on the PCB. Then, the underside was heated until the tin would melt and only then it was possible to even push the chip flat on the PCB. Much flux was added, so I can hardly imagine it did not flow together.  I was more concerned to cause a short by excess tin. But anyhow, I will strip it off, examine and solder back on for comparison.

    The second thing that is very unclear is how it is even possible to exceed the devices temperature range. The maximum power dissipation is stated around 170mW, and the worst thermal resistance mentioned is 38K/W. This would only result in about 6.5°C difference from ambient. I measured the top of the case at 42 degrees and the PCB under the EPAD to 46°C. How can I possibly exceed the 150°C junction temperature?

    And in some sense, red and blue are both GND-Planes, but the main one (connected through the 9 vias) is the blue one.

Reply
  • Hmm. I am almost sure that the solder connection is good because the EPAD as well as the mating pad on the PCB have been tinned before. It was therefore not possible to push the chip flat on the PCB. Then, the underside was heated until the tin would melt and only then it was possible to even push the chip flat on the PCB. Much flux was added, so I can hardly imagine it did not flow together.  I was more concerned to cause a short by excess tin. But anyhow, I will strip it off, examine and solder back on for comparison.

    The second thing that is very unclear is how it is even possible to exceed the devices temperature range. The maximum power dissipation is stated around 170mW, and the worst thermal resistance mentioned is 38K/W. This would only result in about 6.5°C difference from ambient. I measured the top of the case at 42 degrees and the PCB under the EPAD to 46°C. How can I possibly exceed the 150°C junction temperature?

    And in some sense, red and blue are both GND-Planes, but the main one (connected through the 9 vias) is the blue one.

Children
  • +1
    •  Analog Employees 
    on Aug 25, 2020 1:31 PM in reply to Thegon

    From what you describe, it sounds like the EPAD solder connection is OK. Given "red" and "blue" constitute GND connections, I'm less concerned about adequate heat sinking via the PCB.

    Perhaps the problem has something to do with the system clock. You might try checking SYNC_CLK for both good and bad conditions. SYNC_CLK should be a clock signal of constant frequency (1/4 of the system clock frequency). If this signal shows "bad" behavior along with the output signal, you might start looking at the circuitry associated with the system clock PLL.