Spark Logic

AD9851 direct digital synthesizer connection guide

AD9851 direct digital synthesizer SSOP-28 (RS-28) wiring guide with power, signal assignments, and bench checks.

SSOP-28 (RS-28)

28 contacts Current package

Analog Devices · 28-lead shrink small-outline package.

RS-28 pinout and package. Check the manufacturer’s ordering guide for packing and grade.

Supply power and reference clock

Required

Use a 3.3 V supply and an external 25 MHz 3.3 V CMOS oscillator. Place a bypass capacitor near each supply pin.

100 nF at pin 6 between 3.3 V and Ground3.3 V100 nF at pin 6Ground
100 nF at pin 11 between 3.3 V and Ground3.3 V100 nF at pin 11Ground
100 nF at pin 18 between 3.3 V and Ground3.3 V100 nF at pin 18Ground
100 nF at pin 23 between 3.3 V and Ground3.3 V100 nF at pin 23Ground

Bench checks

  • Check all power domains at the package and verify the reference clock waveform relative to the common ground.
  • Hold the controller outputs low until the device and clock are powered. Perform the hardware reset before the first update.

Analog Devices AD9851 Rev. D, Specifications and PCB layout information. The 25 MHz source and bypass values are fixture choices.

Connection scenarios

Load a 100 kHz sine in serial mode

Hold D2 low and D1 and D0 high for the entry word. With the reference clock running, hold RESET high for 1 µs, release it and wait 1 µs. Pulse W_CLK, then FQ_UD. Shift the tuning word 0x010624DD least-significant bit first followed by eight zero control/phase bits. Pulse FQ_UD to update.

100 nF at pin 6 between 3.3 V and Ground3.3 V100 nF at pin 6Ground
100 nF at pin 11 between 3.3 V and Ground3.3 V100 nF at pin 11Ground
100 nF at pin 18 between 3.3 V and Ground3.3 V100 nF at pin 18Ground
100 nF at pin 23 between 3.3 V and Ground3.3 V100 nF at pin 23Ground
RSET 3.92 kΩ between DAC current set and GroundDAC current setRSET 3.92 kΩGround
RA 49.9 Ω between True DAC output and GroundTrue DAC outputRA 49.9 ΩGround
RB 49.9 Ω between Complementary DAC output and GroundComplementary DAC outputRB 49.9 ΩGround
RBIAS1 10 kΩ between 3.3 V and Unused comparator bias3.3 VRBIAS1 10 kΩUnused comparator bias
RBIAS2 10 kΩ between Unused comparator bias and GroundUnused comparator biasRBIAS2 10 kΩGround

Bench checks

  • Verify 40 rising W_CLK edges after entry and a separate rising FQ_UD edge after the final bit.
  • The 32-bit word is round(100,000 × 2³² ÷ 25,000,000). Measure near 100 kHz at IOUT and approximately 0–0.5 V across the 49.9 Ω load.
  • The zero control byte keeps the 6× multiplier disabled.

Analog Devices AD9851 Rev. D, Figures 14 and 17–19. The resistor loads, comparator bias and tuning word are fixture choices.

SSOP-28 (RS-28) wiring map

Highlighted contacts are used in the required step or selected circuit. Select a contact to inspect it.

Top view. Locate the package index before counting counterclockwise from pin 1.
Selected pin details

7 · W_CLK

Input · Clock inputs / oscillator

Rising edge loads a byte in parallel mode or one bit in serial mode.

Analog Devices AD9851 Rev. D, Pin Function Descriptions

Electrical limits · Pin in the reference

Models and documents

This is the bare RS-28 IC. Breakout modules carry external oscillators, terminations and filter networks that vary by board.

AD9851BRS
RS-28 pinout and package. Check the manufacturer’s ordering guide for packing and grade.
AD9851BRSRL
RS-28 pinout and package. Check the manufacturer’s ordering guide for packing and grade.
AD9851BRSZ
RS-28 pinout and package. Check the manufacturer’s ordering guide for packing and grade.
AD9851BRSZRL
RS-28 pinout and package. Check the manufacturer’s ordering guide for packing and grade.