[time-nuts] Modified Extron DA [WAS: Rb video]

[Charles Steinmetz]

Bob wrote:

>Ok, each op-amp drives three loads. Each load 
>has a 50 ohm resistor in series with it. Each 
>output could be terminated in 50 ohms. If they 
>all are terminated, the op amp is driving three 
>100 ohm loads in parallel. The op amp sees 33 
>1/3 ohm at it's output in that case. That's way 
>below the 100 ohms that the manufacturer seems to be recommending.

The circuit is basically the same as the Extron 
ADA 6 Component that I posted about back in May.  I noted:

>Several units modified as above have been tested 
>and each amplifier drives 3x 50 ohm loads to ~ 
>+13dBm (~ 1 Vrms) with harmonics <-65dBc and 
>about 1 dB of headroom.  Note that this requires 
>peak currents of ±85 mA from the CLC409s, which 
>are rated for output currents of ±60 mA “for 
>maximum reliability” (also note that the 
>guaranteed minimum current limit value of the 
>CLC409 is ±50 mA).  Thus, for optimum 
>performance, 50 ohm loads should be distributed 
>one or two per amplifier.  Note that the 
>reference inputs of many devices is higher than 
>50 ohms (often ~ 1k ohm), so this may not be an issue in many applications.

(see 
<http://www.febo.com/pipermail/time-nuts/attachments/20130522/45ae59f8/attachment-0001.pdf>)

I do not know for how long a CLC409 will drive 3 
parallel 50 ohm back-terminated loads to +13 dBm 
before failure.  My test mule did it continuously 
for several weeks, but I would not recommend 
it.  Of course, some CLC409s (with current limits 
closer to the minimum spec) may not drive 3 
parallel 50 ohm back-terminated loads to +13 dBm at all.

The CLC409 is within its +/- 60 mA "maximum 
reliability" spec driving 2 parallel 50 ohm 
back-terminated loads to +13 dBm (peak current 
+/- 56 mA), although parts that are right at the 
50 mA minimum current limit spec may be starting to current limit.

Someone asked how the modified DA performs.  In 
addition to the distortion data above, I posted:

>Phase noise of < -155dBc at 10 MHz was observed on the modified units.
>Adjacent-output isolation (same CLC409) was > 
>30dB. Non-adjacent-output isolation was > 90dB.

Best regards,

Charles