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AD5346/AD5347/AD5348

Rev. 0 | Page 21 of 24

COARSE AND FINE ADJUSTMENT USING

THE AD5346/AD5347/AD5348

Two of the DACs in the AD5346/AD5347/AD5348 can be

paired together to form a coarse and fine adjustment function,

as shown in Figure 47. As with the window comparator

previously described, the description refers to DACs A and B.

DAC A provides the coarse adjustment, while DAC B provides

the fine adjustment. Varying the ratio of R1 and R2 changes the

relative effect of the coarse and fine adjustments. With the

resistor values shown, the output amplifier has unity gain for

the DAC A output, so the output range is 0 V to (VREF – 1 LSB).

For DAC B, the amplifier has a gain of 7.6 × 10–3, giving DAC B

a range equal to 2 LSBs of DAC A.

The circuit is shown with a 2.5 V reference, but reference

voltages up to VDD may be used. The op amps indicated allow a

rail-to-rail output swing.

AD780/ADR381/REF192

WITH VDD = 5V

VDD = 5V

VOUT

EXT

REF

VOUT

VIN

GND

AD5346/AD5347/

AD5348

VOUTB

VOUTA

VDD

VREFAB

390

Ω

390

Ω

51.2k

Ω

51.2k

Ω

0.1

µF

0.1

µF

Figure 47. Coarse and Fine Adjustment

POWER SUPPLY BYPASSING AND GROUNDING

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance.

The printed circuit board on which the AD5346/AD5347/

AD5348 is mounted should be designed so that the analog and

digital sections are separated and are confined to certain areas

of the board. This facilitates the use of ground planes that can

be separated easily. A minimum etch technique is generally best

for ground planes because it gives the best shielding. Digital and

analog ground planes should be joined in one place only. If the

AD5346/AD5347/AD5348 is the only device requiring an

AGND-to-DGND connection, then the ground planes should

be connected at the AGND and DGND pins of the AD5346/

AD5347/AD5348. If the AD5346/AD5347/AD5348 is in a

system where multiple devices require AGND-to-DGND

connections, the connection should be made at one point only, a

star ground point that should be established as close as possible

to the AD5346/AD5347/AD5348.

The AD5346/AD5347/AD5348 should have ample supply

bypassing of 10 µF in parallel with 0.1 µF on the supply located

as close to the package as possible, ideally right up against the

device. The 10 µF capacitors are the tantalum bead type. The

0.1 µF capacitor should have low effective series resistance

(ESR) and effective series inductance (ESI), such as the

common ceramic types that provide a low impedance path to

ground at high frequencies to handle transient currents due to

internal logic switching.

The power supply lines of the device should use the largest trace

possible to provide low impedance paths and to reduce the

effects of glitches on the power supply line. Fast switching

signals such as clocks should be shielded with digital ground to

avoid radiating noise to other parts of the board, and should

never be run near the reference inputs. Avoid crossover of

digital and analog signals. Traces on opposite sides of the board

should run at right angles to each other to reduce the effects of

feedthrough through the board. A microstrip technique is by far

the best, but not always possible with a double-sided board. In

this technique, the component side of the board is dedicated to

ground plane, while signal traces are placed on the solder side.

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