Taka Kamiya wrote:
I've been playing around with Clifton amplifier as well.  Mine, input is 
terminated with 50 ohm register, and rest is unmodified, so it has 6dB gain.  I 
have a 10dB pad on input side.  I, too, noticed there will be a severe clipping 
with driving it too hard.    *  *  *   I with there was a little more room 
there....

Bob wrote:
Maybe there's some noise in those resistors
Well..... maybe not so much.
>>
If you drive this board so it has a couple db more output, it goes into 
clipping. When that happens .... yuck.
>> Noise and ADEV both are massively impacted.
You very much do *not* want to overdrive this board.   *  *  *
I would stick with 12 dbm or less

You can buy some headroom by raising the supply voltage. There is an on-card LM78L09 9v voltage regulator (U902) that can safely be raised to 12v (LM78L12). This will get you cleanly to and a bit past the traditional +13 dBm (1v rms) standard reference level. Of course, you will need to make sure the raw supply voltage is >15v.

As to noise, the 200 ohm resistor on the opamp's noninverting input (R901) accounts for nearly 6dB of the amplifier noise (assuming an effective source impedance of 50 ohms). Reducing this to, say, 33 ohms will lower the noise floor a few dB.

Finally, the state of CFB video amplifier development has advanced dramatically in the 20 years since the AD8007 was introduced. New amplifiers with supply voltage ratings up to 36v are available (allowing about 10dB greater headroom than the 8007), and many of the newer opamps clip much more gracefully than the 8007 when you do hit the limit (but you really want not to do that in any measurement application).

Many of these new CFB amplifiers have been discussed here on the list, and each has its own fans. One I like that doesn't get mentioned much is the LME49713. It is discontinued, but still available from Rochester Electronics and others. But there are lots of good choices.

Best regards,

Charles



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