Q
Can you tell me what source impedance should I present
at AD9432's input to obtain best SFDR and linearity ?
Datasheet suggest 50ohm 22pF, is it the optimum ?
A
-In order to obtain the optimum SFDR and distortion performance from the
AD9432, the most important factors are
-Drive the Analog inputs differentially
-Use matched impedances at the AIN and /AIN inputs
-Use a low-jitter clock source
-Set the absolute common mode voltage to 3V
The two 50 Ohm resistor and the capacitor (shown in figure 26 on page 10 of the
datasheet) constitute a low pass filter (f0 = 72.3MHz) at the input of the ADC
(NOT an impedance matching circuit to obtain optimum SFDR etc). The impedance
level was chosen to be 100 Ohms (50 Ohms + 50 Ohms symmetrical load on each
output of the AD8138) in order to prevent instability and ringing at high
frequencies due to the capacitive load. I imagine that there is a trade off (a
lower resistor value would have required a higher capacitor value. Result: more
ringing at high frequencies; a higher resistance value would result in a
greater voltage drop as caused by the AD9432's 3k input resistance and would
also increase the input noise)
Some high speed ADCs have an unbuffered input, so that the amplifier which
drives the input has to charge/discharge the sampling capacitor (very quickly).
To minimize the effects of the switched capacitor front end, it can sometimes
be helpful to place series resistors between the output of the driving
amplifier and the ADC inputs. In the case of AD9432, the inputs are buffered;
so this is not the reason for the 50 Ohm resistors.
Figure 29 and fig 33a show the transformer-coupled input structure. In this
case, a termination resistance of 50 Ohms (25 Ohms + 25 Ohms) has been provided
to match the impedance of external signal generators. Again this has nothing to
do with the ADC. It is precaution against reflections on the input cable.
So, how should you drive the ADC?
1. If you are using an ADC driver similar to the AD8138 (differential output,
dc coupled) then you can probably just connect it directly to the AIN terminals
of the ADC, although you might want to include an antialiasing filter between
the Amplifier and the ADC, as shown in fig 26 on the AD9432 datasheet.
2. If you have a single ended signal, you can either use the AD8138 (although
it's performance will degrade above 20MHz, as shown in fig. 27 (page 10) of the
AD9432 datasheet if you need a dc coupled signal path or you can use a single
or double transformer arrangement shown in fig 33a and fig 29 of the AD9432
datasheet if you are ac coupling the signal.
3. In all cases, the impedances at AIN and /AIN should match and should be much
smaller than the 3k input impedance of the ADC itself, to reduce voltage drop
and noise.