AD5544/AD5554

Data Sheet

Rev. H | Page 14 of 24

THEORY OF OPERATION

The AD5544 and the AD5554 contain four 16-bit and 14-bit,

current output DACs, respectively. Each DAC has its own inde-

pendent multiplying reference input. Both the AD5544 and the

AD5554 use a 3-wire, SPI-compatible serial data interface, with

a configurable asynchronous RS pin for half-scale (MSB = 1) or

zero-scale (MSB = 0) preset. In addition, an LDAC strobe enables

4-channel, simultaneous updates for hardware synchronized

output voltage changes.

DIGITAL-TO-ANALOG CONVERTER (DAC)

Each part contains four current-steering R-R ladder DACs.

Figure 20 shows a typical equivalent DAC. Each DAC contains

a matching feedback resistor for use with an external I-to-V

connected to the load point, requiring full 16-bit accuracy. These

DACs are designed to operate with both negative and positive

reference voltage.

switches on and off. Note that a matching switch is used in

series with the internal 5 kΩ feedback resistor. If users attempt

substrate during high temperature applications, minimizing zero-

scale leakage currents that double every 10°C. The DAC output

following equations:

536

,

65

D

V

V

REF

OUT

×

−

=

(for the AD5544)

(1)

384

,

16

D

V

V

REF

OUT

×

−

=

(for the AD5554)

(2)

reference voltages.

DGND

R

R

R

2R

2R

2R

5k

Ω

S1

S2

R

DIGITAL INTERFACE CONNECTIONS OMITTED FOR CLARITY.

Figure 20. Typical Equivalent DAC Channel

These DACs are also designed to accommodate ac reference input

signals. Both the AD5544 and the AD5554 accommodate input

reference voltages in the range of −15 V to +15 V. The reference

voltage inputs exhibit a constant nominal input resistance of 5 kΩ ±

DAC outputs are code dependent and produce various output

resistances and capacitances. The choice of external amplifier

should take into account the variation in impedance generated by

the AD5544/AD5554 on the inverting input node of the amplifier.

The feedback resistance, in parallel with the DAC ladder resistance,

dominates output voltage noise. For multiplying mode applications,

to provide a critically damped output response for step changes in

reference input voltages. Figure 21 shows the gain vs. frequency

performance at various attenuation settings using a 23 pF external

for the AD5544 and the AD5554, respectively. To maintain good

analog performance, power supply bypassing of 0.01 µF, in parallel

with 1 µF, is recommended. Under these conditions, a clean power

supply with low ripple voltage capability should be used. Switching

power supplies is usually not suitable for this application due to

the higher ripple voltage and PSS frequency-dependent characteris-

tics. It is best to derive the supply of the AD5544/AD5554 from

system analog supply voltages. Do not use the digital supply

(see Figure 22).

FREQUENCY (Hz)

2

0

–2

–4

–6

–8

100k

1M

10M

100M

Figure 21. AD5554 Reference Multiplying Bandwidth vs. Code