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

[Bob Camp]

Hi

The original design with 3x75 ohm outputs would be driving a 50 ohm load rather than a 33 1/3 ohm load. That would put it "in spec" for the CLC409 as opposed to just out of spec current limit wise. 

Bob

On Aug 9, 2013, at 2:48 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:

> 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
> 
> 
> 
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