Can AD8318 CLPF pin be shorted to ground indefinately?

Can AD8318 CLPF pin be shorted to ground indefinately?  We are making a passive intermodulation tester which uses an automatic level control loop.  The loop incorporates a 50dBm power amplifier, an ADL5330 VGA, and an AD8318 Log Amp as described on Page 16 of the ADL5330 datasheet.  We are experiencing PA burnouts which have been traced to a nasty transient when the PA is energized.  This is pretty much the last thing which happens.  Essentially, since the PA was initially off, the ALC loop is railed out, commanding the VGA to produce maximum power when the PA comes on.

The PA we're buying isn't the most robust thing in the world, and it seems to have a damage threshold near (or perhaps below) the the level that would be required to produce maximum output if the PA turned out to have the minimum specified gain.  It other words, the minimum gain spec of the PA is 45dB, but they typically run 55dB or so.  There is no maximum damage spec, but we have seen them damaged at +6dBm of input power.  So, while we could add enough padding to the output of the ADL5330 to guarantee the PA wouldn't get damaged by the turn on transient, we might not have enough power to drive a PA that actually had a 45dB gain.  Sadly, for a while, we are simply going to have to live with this amplifier as it is.

To solve this problem, the first thing I tried was to turn off the AD8318 Log Amp via its ENBL pin, which is supposed to put the amp into a low current state.  However, to my dismay I've discovered the loop capacitor continues to charge--even when ENBL is held low.  So, I still get the same transient when the ENBL pin is brought high again.  Dang!

The next thing I tried was to short out CLPF on the AD8318.  I actually tried this with a pair of tweezers, and it seems to do exactly what I want:  shorting out the capacitor drives the VGA gain to its minimum value, effecting a level dip and allowing the PA to be turned on gracefully.  This effectively closes the loop--except that the loop capacitor is now being held at ground.  Once the PA is steady-state, the tweezers are removed, unshorting the capacitor, and allowing the power level to nicely climb to the desired value.  This is much better for the PA than slamming it to the maximum value, and declining to the desired level.

BUT, it raised one last question:  Can the current source feeding CLPF in the AD8318 tolerate being shorted to ground indefinately without damage?  My guess is that, since we're talking about a controlled current source there is no restriction on the size of CLPF, it probably can.

I'm also curious why the ENBL pin does not perform as one might expect, and discharge CLPF to zero when ENBL is brought low.  Surely other people are bothered by this type of turn-on transient, yes?

Thanks for your help.

--andy robertson

design engineer

Anritsu

  • 0
    •  Analog Employees 
    on Feb 1, 2011 3:29 AM

    Moved this discussion related to the AD8318 to the RF Components community.  Please continue the discussion here.

    AndyR

    EngineerZone Community Manager

  • 0
    •  Analog Employees 
    on Feb 1, 2011 7:52 PM

    Tying the CLPF node directly to ground is not advised. Doing would exceed the breakdown voltage of a transistor in the attached circuitry. It is recommended to use a 1 k ohm resistor to ground at this node if you must take the node to ground.

    The reason CFPL is  not brought to 0 V when the AD8318 is in power down is to reduce the charging time and ultimately  the turn on time when the device is powered up.

  • Hi Jim,

     

    Thanks for the quick resopnse. 

     

    I'm not sure I completely understand what you're telling me.  First, the

    log amp appears to be a curent source, so I would think the voltage

    would be lowest when CLPF is shorted to ground.  Can you be more

    specific about what might break down?  Second, it is important to note

    that we can leave the log amp enabled all the time (as is done in the

    datasheet).  So, under these circumstances, shorting CLPF would seem to

    be equivalent to connecting it to an infinately large

    capacitor--something not specifically prohibited in the datasheet, no?

    In any case, the potential to break down that transistor would seem to

    exist for any loop capacitor that ever was completely discharged, yes?

    Third, assuming we did use a 1K resistor, would we not still see a a

    turn-on spike? (though it would 13 times smaller, we don't want to

    overshoot at all.)

     

    My original concern was that there was some potential to overload the

    current source with a steady-state short.  However it appears that there

    may be more to it than that.  What do you suggest we do to tame this

    nasty little spike?  As far as I can tell, nothing in the datasheet even

    suggests it can happen--much less what to do about it.  We can't be the

    only customers who don't want over power transients in our leveling

    loops.

     

    In the short term, a breadboard version of this circuit has worked very,

    very well in the lab.  It will be hard to walk away from that.  Would

    you mind looking at the attached circuit one more time?  Below is the

    version we actually breadboarded up.  If I have to tell my boss the fix

    I found doesn't work after all, I'd like to be able to say exactly why

    with some confidence, and if possible offer a better solution.

     

    Regards,

     

    --Andy Robertson

  • 0
    •  Analog Employees 
    on Feb 4, 2011 10:35 PM

    The transistor in question is part of a current mirror circuit. By connecting the CLPF node directly to ground overtime the transistor will degrade and eventually fail. The problem is the Vbceo of the transistor is exceeded. So yes, your original concern is correct.

    During the power up sequence you could force the VGA to minimum gain  during initialization. Enable / disable the circuit using the enable pin on the ADL5330. Before enabling the VGA bring the VSET voltage on the AD8318 to 2.5 V. This translates to a power level well below the minimum detectable power of the AD8318. As a result the gain of the ADL5330 will be set to its minimum eliminating the severe overshoot.

    84f02dd5a6d0905e66265c623ffdf0e2.emf

  • 0
    •  Analog Employees 
    on Feb 4, 2011 10:54 PM

    Hi Andy,

    The transistor in question is part of a current mirror circuit. By connecting the CLPF node directly to ground overtime the transistor will degrade and eventually fail. The problem is the Vbceo of the transistor is exceeded. So yes, your original concern is correct.

    During the power up sequence you could force the VGA to minimum gain  during initialization. Enable / disable the circuit using the enable pin on the ADL5330. Before enabling the VGA bring the VSET voltage on the AD8318 to 2.5 V. This translates to a power level well below the minimum detectable power of the AD8318. As a result the gain of the ADL5330 will be set to its minimum eliminating the severe overshoot.

    cid:image003.png@01CBC46A.9DEB6BA0<http://ez.analog.com/servlet/JiveServlet/showImage/2-19082-4454/Enable-VSET.emf>

    Regards,

    Jim Bedrosian

    From: andyrobertson analog@sgaur.hosted.jivesoftware.com

    Sent: Tuesday, February 01, 2011 2:54 PM

    To: Bedrosian, Jim

    Subject: New message: "Can AD8318 CLPF pin be shorted to ground indefinately?"

    Analog Devices EngineerZone<http://ez.analog.com/index.jspa>

    Can AD8318 CLPF pin be shorted to ground indefinately?

    reply from andyrobertson<http://ez.analog.com/people/andyrobertson> in RF Components - View the full discussion<http://ez.analog.com/message/18894#18894