[time-nuts] new op amp for distribution amplifiers

[John Miles]

> Has anybody tried the AD8003 (triple video line driver amplifier) for use
as
> distribution amplifiers?  It looks to me like this part might be a decent
> choice...

I don't have any NF or residual PN numbers handy for the triple-amp
versions, but the single-amplifier part from the same family (AD8000) is
somewhat noisier than the LMH6702, with an NF around 22.9 dB for the former
versus 17.5 dB for the latter when both are tested with Av=+2.   

The AD8000 has the advantage of an enable pin, and it's also available in a
QFN package that may be a bit more convenient to use on a PCB.  The AD8000
is also good for about 0.5 dB more output power than the LMH6702 can deliver
at 10 MHz.  But that's about it; the LMH6702 is preferable in all other
cases from what I've seen.  

Being a triple opamp, the AD8003 would be comparable to the LMH6733, which
also includes enable/disable pins.  I haven't done any benchmarking with the
AD8003 but would expect comparable results, with the LMH6733 a bit quieter.

Re: feedback resistors, the data sheet for a given CFB amp will recommend an
optimum value, often tied to a particular Av, and it's best not to deviate
too far from that.  I've always found that 237 ohms works well with the
LMH6702 regardless of gain, but as noted, the THS3491 wants a much larger
resistor value.  

When tested with Av=+5 -- meaning overall gain of +8 dB with 6 more dB lost
in the series termination -- the LMH6702 measures 15 dB NF and the THS3491
17.3 dB.  These figures are similar when observed with either a traditional
NF meter or a residual AM/PM noise measurement.  Neither of these parts
contributes enough 1/f noise to degrade the performance of an oscillator, so
they are both excellent choices for distribution amplifiers in the -165
dBc/Hz neighborhood.  The nod goes to the THS3491 for such applications,
IMHO.  It's not that much noisier than the LMH6702 and the additional output
power is very welcome.  

I don't see a good use case for the AD800x family unless you need a
single-amplifier package with an enable pin.  There's nothing really wrong
with them, but they are noisier than the competition and they don't excel in
any other areas AFAIK.

> On concern I have is that there *might* be some difficulty with driving a
> non-
> terminated transmission line (by mistake, of course).

Not an issue, they will happily drive anything (or nothing) through a 50R
series termination.  

> One other thing is that this part uses an "exposed paddle", which needs
> to be soldered down to a well-grounded pad beneath the device for both
> electrical and thermal reasons.  For those in possession of a suitably-
> modified toaster oven, this should not be a big issue.

True, and all of these parts can also be mounted dead-bug style for
experimenting and prototyping.  I keep several LMH6702 amps around in
Hammond boxes, mounted on bare copper with small cermet gain control
trimpots and powered from NiMH AA cells.  They are dirt cheap, rock stable,
and very handy.  If I build any more of these amps I will probably switch to
THS3491s and small lithium batteries.

> > Is there anything else that can be done, besides increasing Rf/Rg for
more
> > stability?
> > Anything obviously wrong with the layout? It does work fine with LMH6702
> > and LMH6609.

I can 't tell much from your photo but it sounds like you may have some
stray C near the inverting input terminal.  It's a good idea to open up the
ground plane near the - input with all amps in this class.  Watch the output
capacitance upstream of the termination resistor as well.   These parts are
all well-behaved, but the data sheet guidelines have to be followed more or
less religiously.  

-- john, KE5FX
Miles Design LLC