Building downconverter with low IF, 15...300kHz, next schematic is planned:
Unclear moment from datasheet:
1. Is it correct, that there are internal 125 Ohm resistors, so adding 50 Ohm bias makes differential output impedance 2x(50||125) = 2x35,7 Ohm?
1a. What are the reasons for that 50 Ohm resistor value? They can provide 3.75V com.-mode voltage for <25mA only.
2. There is data about consumption current (80-140mA at -30...+5dBm RF when VSET driven from DETO due to fig.91), but no separate data about IF current. Is it correct that almost all shown current flows equally into IF pins, with minor contribution of other supply inputs? So I must consider 40...70mA per each IF bias resistor? Some users state typical 25mA per pin, which is far from that.
For that currents, 50 Ohm at 5V allows no 3.75V common-mode voltage. Rising output supply looks unreliable (if IC under some conditions "doesn't want" that 40mA, voltage at IF pin rises above abs. max.), so I need quite large inductor/transformer (undesired by other design issues), or maybe some active bias circuit may be fine?
Or it is ok to use (5-3.75)V/70mA_max = 18 Ohm bias, like that weird-looking 10 Ohm in AD8342 datasheet? Signal voltage will be dropped.
Some users advice not to use VSET-DETO connection for resistive IF bias, but to set fixed VSET. Anyway, what exact value? Can't find IF currents to select resistors; tables 4,5 give only "mixer supply current", and if it is our bias current, still too large for 50 Ohm.
