Hello Folks,
Thanks much for your responses. Dan's mention of the accidental application
of fairly high power CW tones without suffering apparent damage is at least
somewhat "comforting".
For the record, here's my evaluation based strictly on datasheet numbers for
the ADC, assuming that the differential input impedance is 100 ohms by
virtue
of suitable choice of the external input terminating resistors:
FS CW tone level would be -5 dBm (500 mVpp diff into 100 ohms).
Burnout CW tone level would be either +13 dBm or about +7 dBm, depending...
The datasheet max ratings section says that the maximum input on each
analog input is +/- 1V (2 Vpp) If these can be applied simultaneously,
this would correspond to a maximum of 4 Vpp differential, yielding a
burnout level of +13 dBm. But the fine print immediately below the max
ratings disclosure mentions that these limits apply to single items only
while
all others are kept within normal operating range. If this added constraint
is true for input overdrive, then some lower limit, perhaps roughly 2 Vpp
differential (+7 dBm), would seem to apply.
On top of all this is the question of the nature of the damage mechanism.
Is it merely thermal (overheating of input protection diodes), or is there
some fast damage mechanism involved (e.g. breakdown, latchup, etc)?
If thermal only, then some additional peak overdrive should be tolerable
for AC signals at all but very low frequencies and for infrequent
transients.
I really hate it when the manufacturer cannot or will not put the
information
needed by designers on their datasheets. This is a constant and major
headache in the RF systems design arena.
Cheers,
Dana
Dan Werthimer wrote:
hi dana,
matt's emails indicates full scale input of an iADC1.1 board is
-2 dBm for a 630 MHz sine wave.
you also asked about maximum input voltage before damage:
we haven't intentionally tried to destroy an ADC board, but we've
accidently injected
strong signals into these boards by accident (eg: 13 to 17 dBm sine waves
for a few minutes or more), and we haven't noticed anything different
in the iADC behavior
after these accidental tests.
to calculate the maximum voltage allowed according to the adc data sheet,
one could assume that the -2 dBm full scale input signal into the
balun corresponds
to a full scale 500mVpp differential signal into the ADC.
the data sheet says the max input at the ADC is 2Vpp differential,
which is 12 dB above full scale, or +10 dBm at the SMA input,
assuming a sine wave input.
best wishes,
dan
On 3/24/2010 12:34 PM, dana whitlow wrote:
Hello,
Can someone please tell me the following:
> FS input drive referred to the RF input connector, at perhaps about
500 MHz
> Burnout threshold, same location and similar frequency.
Thanks much,
Dana Whitlow
Microwave Receiver Specialist
Arecibo Observatory
(787) 878-2612 ext 328
