AD9854
Rev. E | Page 31 of 52
INVERSE SINC FUNCTION
The inverse sinc function precompensates input data to both
DACs for the sin(x)/x roll-off characteristic inherent in the
DAC’s output spectrum. This allows wide bandwidth signals
(such as QPSK) to be output from the DACs without
appreciable amplitude variations as a function of frequency. The
inverse sinc function can be bypassed to reduce power
consumption significantly, especially at higher clock speeds.
When the Q DAC is configured as a control DAC, the inverse
sinc function does not apply to the Q path.
Inverse sinc is engaged by default and is bypassed by bringing
the bypass inverse sinc bit high in Control Register 20 hex, as
noted in Table 8.
FREQUENCY NORMALIZED TO SAMPLE RATE
4.0
0
0.1
–0.5
0
M
AG
NI
T
UDE
(
d
B)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
0.2
0.3
0.4
0.5
SYSTEM
ISF
SINC
00
63
6-
0
51
Figure 51. Inverse Sinc Filter Response
REFCLK MULTIPLIER
The REFCLK multiplier is a programmable PLL-based
reference clock multiplier that allows the user to select an
integer clock multiplying value over the range of 4× to 20×.
With this function, users can input as little as 15 MHz at the
REFCLK input to produce a 300 MHz internal system clock.
Five bits in Control Register 1E hex set the multiplier value, as
detailed in Table 7.
The REFCLK multiplier function can be bypassed to allow
direct clocking of the AD9854 from an external clock source.
The system clock for the AD9854 is either the output of the
REFCLK multiplier (if it is engaged) or the REFCLK inputs.
REFCLK can be either a single-ended or differential input by
setting Pin 64, DIFF CLK ENABLE, low or high, respectively.
PLL Range Bit
The PLL range bit selects the frequency range of the REFCLK
multiplier PLL. For operation from 200 MHz to 300 MHz
(internal system clock rate), the PLL range bit should be set to
Logic 1. For operation below 200 MHz, the PLL range bit
should be set to Logic 0. The PLL range bit adjusts the PLL loop
parameters for best phase noise performance within each range.
PLL Filter
The PLL FILTER pin (Pin 61) provides the connection for the
external zero-compensation network of the PLL loop filter. The
zero-compensation network consists of a 1.3 kΩ resistor in
series with a 0.01 μF capacitor. The other side of the network
should be connected as close as possible to Pin 60, AVDD. For
optimum phase noise performance, the clock multiplier can be
bypassed by setting the bypass PLL bit in Control Register
Address 1E hex.
Differential REFCLK Enable
A high level on the DIFF CLK ENABLE pin enables the differ-
ential clock inputs, REFCLK and REFCLK (Pin 69 and Pin 68,
respectively). The minimum differential signal amplitude required
is 400 mV p-p at the REFCLK input pins. The center point or
common-mode range of the differential signal can range from
1.6 V to 1.9 V.
When Pin 64 (DIFF CLK ENABLE) is tied low, REFCLK
(Pin 69) is the only active clock input. This is referred to as
single-ended mode. In this mode, Pin 68 (REFCLK) should
be tied low or high.
High Speed Comparator
The comparator is optimized for high speed and has a toggle rate
greater than 300 MHz, low jitter, sensitive input, and built-in
hysteresis. It also has an output level of 1 V p-p minimum into 50 Ω
or CMOS logic levels into high impedance loads. The comparator
can be powered down separately to conserve power. This com-
parator is used in clock-generator applications to square up the
filtered sine wave generated by the DDS.
Power-Down
The programming registers allow several individual stages to be
powered down to reduce power consumption while maintaining
the functionality of the desired stages. These stages are identified
in Table 8, Address 1D hex. Power-down is achieved by setting
the specified bits to logic high. A logic low indicates that the
stages are powered up.
Furthermore, and perhaps most significantly, the inverse sinc
filters and the digital multiplier stages can be bypassed to achieve
significant power reduction by programming the control registers
in Address 20 hex. Again, logic high causes the stage to be bypassed.
Of particular importance is the inverse sinc filter; this stage
consumes a significant amount of power.
A full power-down occurs when all four PD bits in Control
Register 1D hex are set to logic high. This reduces power
consumption to approximately 10 mW (3 mA).
