Q
I have carefully looked through the data book on AD9952. Can't understand how
should I connect DACBP pin (DAC biasline decoupling pin) for AD9952. Would you
clarify this please.
A
1) DACBP is normally connected to ground via a series 1uF capacitor.
You can either tie a 1uF capacitor to ground, or possibly even leave it
disconnected.
It's a bypass for the DAC reference and we haven't seen a great impact in the
performance of the device with placing a capacitor or leaving it open.
2) The AD9951 reference input is normally driven with a sine wave.
It is AC coupled (with a capacitor) and if a single-ended drive is used the
other clock input is decoupled with another capacitor.
The drive level must not be over 1.8 V pk-pk (0.64 V r.m.s.) with a 1.8V
supply, but may be as low as 100 mV r.m.s.
However it is better to use as large a drive as possible to minimise phase
noise.
If you drive the reference input with a square wave it must have a duty cycle
of close to 50% and logic levels (for 1.8V supply) of logic 0<.4V
and logic 1>1.25V.
If you use the PLL clock multiplier with logic inputs the duty cycle may be
slightly worse - between 35% and 65%.
3) If CLKMODESELECT is connected to ground, the crystal out pin will be high
impedance.
Looking at the functional block diagram on page 1 of the datasheet, the
clock out pin is driven from the output of the XTAL oscillator circuit, while
there is a separate path for an external reference clock via the MUX.
4) CLKMODESELECT can be tied directly to DVDDIO level (or through a small
pullup resistor, perhaps 1Kohm).
5) Comparator inputs . These could be left open if you have disabled the
comparators using bit 6 of register CFR 1.
You could also terminate the inputs by putting a resistor (e.g. 50 ohms) across
them.
6) The DAC is designed to give a differential output – which results in better
performance.
If you need to convert this to a single-ended output, you should use a
diiferential to single-ended conversion circuit.
The easy way is to use a Balun (balance to unbalance transformer) ; you could
also use a suitable high speed amplifier .
(www.analog.com/highspeedamps)
