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
  • The EPAD on the underside of the device is not only a GND connection, but a heat sink, as well. That said, make sure the EPAD is adequately soldered to the GND plane, and that the GND plane is sufficiently large to draw heat away from the device.

    The two most common causes of heat-related signal quality issues:

    1. poor (or inadequate) solder connection to the EPAD
    2. a GND plane not substantial enough to transfer heat away from the device
Reply
  • The EPAD on the underside of the device is not only a GND connection, but a heat sink, as well. That said, make sure the EPAD is adequately soldered to the GND plane, and that the GND plane is sufficiently large to draw heat away from the device.

    The two most common causes of heat-related signal quality issues:

    1. poor (or inadequate) solder connection to the EPAD
    2. a GND plane not substantial enough to transfer heat away from the device
Children
  • Thank you very much for your quick reply!

    1. In terms of soldering: The part is hand-soldered on this prototype PCB, but the EPAD was tinned manually before assembly and the 9 vias in the board were flooded with tin. Then a broad tip was used to melt the tin in the vias from the underside and the chip could be gently pressed down, with everything flowing together.

    2. The board is standard 35um copper, only two layers for cost reasons. On one side, there are a couple of traces, but other than that the GND plane is not cut. Do you think it is too small as it is?

    I mean putting a small heatsink also increases this "threshold" a little bit, but I felt this was not really a reliable solution because the chip does not even get hot when this happens. Also, in the final design, it could easily get 50-60 degrees if located in the sun.