AD9851
PIN FUNCTION DESCRIPTIONS
Pin
No.
4–1,
28–25
5
6
7
8
Mnemonic
D0–D7
PGND
PVCC
W_CLK
FQ_UD
Function
8-Bit Data Input. The data port for loading the 32-bit frequency and 8-bit phase/control words. D7 = MSB;
D0 = LSB. D7, Pin 25, also serves as the input pin for 40-bit serial data word.
6× REFCLK Multiplier Ground Connection.
6× REFCLK Multiplier Positive Supply Voltage Pin.
Word Load Clock. Rising edge loads the parallel or serial frequency/phase/control words asynchronously
into the 40-bit input register.
Frequency Update. A rising edge asynchronously transfers the contents of the 40-bit input register to be
acted upon by the DDS core. FQ_UD should be issued when the contents of the input register are known
to contain only valid, allowable data.
Reference Clock Input. CMOS/TTL-level pulse train, direct or via the 6× REFCLK Multiplier. In direct
mode, this is also the SYSTEM CLOCK. If the 6× REFCLK Multiplier is engaged, then the output of the
multiplier is the SYSTEM CLOCK. The rising edge of the SYSTEM CLOCK initiates operations.
Analog Ground. The ground return for the analog circuitry (DAC and Comparator).
Positive supply voltage for analog circuitry (DAC and Comparator, Pin 18) and bandgap voltage reference,
Pin 11.
The DAC’s external R
SET
connection—nominally a 3.92 kΩ resistor to ground for 10 mA out. This sets
the DAC full-scale output current available from IOUT and IOUTB. R
SET
= 39.93/IOUT
Voltage Output Negative. The comparator’s “complementary” CMOS logic level output.
Voltage Output Positive. The comparator’s “true” CMOS logic level output.
Voltage Input Negative. The comparator’s inverting input.
Voltage Input Positive. The comparator’s noninverting input.
DAC Bypass Connection. This is the DAC voltage reference bypass connection normally NC (NO
CONNECT) for optimum SFDR performance.
The “complementary” DAC output with same characteristics as IOUT except that
IOUTB = (full-scale
output–IOUT).
Output load should equal that of IOUT for best SFDR performance.
The “true” output of the balanced DAC. Current is “sourcing” and requires current-to-voltage
conversion, usually a resistor or transformer referenced to GND.
IOUT = (full-scale output–IOUTB)
Master Reset pin; active high; clears DDS accumulator and phase offset register to achieve 0 Hz and 0°
output phase. Sets programming to parallel mode and disengages the 6× REFCLK Multiplier. Reset does
not clear the 40-bit input register. On power-up, asserting RESET should be the first priority before pro-
gramming commences.
Positive supply voltage pin for digital circuitry.
Digital Ground. The ground return pin for the digital circuitry.
PIN CONFIGURATION
D3
1
D2
2
D1
3
LSB D0
4
PGND
5
PVCC
6
W CLK
7
28
D4
27
D5
26
D6
25
D7 MSB/SERIAL LOAD
24
DGND
9
REFCLOCK
10, 19
11, 18
12
13
14
15
16
17
20
21
22
AGND
AVDD
R
SET
VOUTN
VOUTP
VINN
VINP
DACBP
IOUTB
IOUT
RESET
23
24
DVDD
DGND
AD9851
23
DVDD
TOP VIEW
22
RESET
(Not to Scale)
21
IOUT
FQ UD
8
20
IOUTB
19
AGND
18
AVDD
17
DACBP
16
VINP
15
VINN
REFCLOCK
9
AGND
10
AVDD
11
R
SET 12
VOUTN
13
VOUTP
14
REV. C
–5–
AD [ ANALOG DEVICES ]
