Question
1. I need to know what is the Base Band frequency range of the AD8347 ?
[AE] 60MHz.
2. My required base band frequency is 100Hz - 5MHz
can I use the AD8347 for this frequency band ?
[AE] Yes
3. Base Band LPF: On page 17 of the data sheet capacitors C2, C4 are tied to
VRef but on page 18 the LPF capacitors are tied to GND.
What is the correct way to connect the base band LPF ?
Answer
There are two specifications of interest relating to your question.
The -3db bandwidth of the quadrature demodulator stage, from RFIP/RFIN to
IMXO/QMXO is 90MHz.
There are two uncommitted baseband amplifiers on the AD8347, whose -3dB
bandwidth is 65MHz.
Whether you use the baseband amplifiers or not, the devices can easily
accommodate your desired 100Hz to 5MHz bandwidth.
It is relatively unimportant where the capacitors are terminated to, since VREF
is at DC and is therefore an AC ground: the critical thing is that the
TERMINATION RESISTOR is connected to VREF, so that the dc bias level at the
amplifier input is correct.
In figure 8, RL is connected between IAIN and VREF.
In figure 11, R39 is connected between IAIN and VREF.
So there is no real contradiction between figure 8 and figure 11. In general, I
would recommend connecting the capacitors to ground, unless there is a specific
reason to connect them elsewhere, because the ground plane can be relied on to
have a very low impedance at high frequencies; this is not necessarily the case
for an output pin of the AD8347.