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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