AD5554/AD5554

Rev. A | Page 14 of 20

CIRCUIT OPERATION

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

rent-output, digital-to-analog converters, respectively. Each

DAC has its own independent 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 four channel simultaneous updates for

hardware synchronized output voltage changes.

D/A CONVERTER

Each part contains four current-steering R-2R ladder DACs.

Figure 25 shows a typical equivalent DAC. Each DAC contains a

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

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

DACs are designed to operate with both negative or positive

drive the DAC switches on and off. Note that a matching switch

is used in series with the internal 5 kΩ feedback resistor. If users

used to guard the substrate during high temperature applica-

tions, minimizing zero-scale leakage currents that double every

digital data (D) in the following equations:

(

)

5544

AD

For

65536

D

V

V

REF

OUT

×

−

=

(1)

(

5554

AD

For

1638

)

4

D

V

V

REF

OUT

×

−

=

(2)

dc reference voltages.

DGND

RR

R

2R

2R

2R

5k

Ω

S1

S2

DIGITAL INTERFACE CONNECTIONS OMITTED FOR CLARITY.

R

Figure 25. 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 −12 V to +12 V. The

reference voltage inputs exhibit a constant nominal input

resistance of 5 kΩ, ±30%. On the other hand, the DAC outputs

IOUTA, B, C, D are code-dependent and produce various out-

put resistances and capacitances. The choice of external ampli-

fier should take into account the variation in impedance

generated by the AD5544/AD5554 on the amplifiers’ inverting

input node. The feedback resistance, in parallel with the DAC

ladder resistance, dominates output voltage noise. For multi-

plying mode applications, an external feedback compensation

output response for step changes in reference input voltages.

Figure 26 and Figure 27 show the gain vs. frequency perfor-

mance at various attenuation settings using a 23 pF external

minals for AD5544 and AD5554, respectively. In order to main-

tain 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 capa-

bility should be used. Switching power supplies is usually not

suitable for this application due to the higher ripple voltage and

PSS frequency-dependent characteristics. It is best to derive the

AD5544/AD5554’s 5 V supply from the system’s analog supply

voltages. Do not use the digital 5 V supply (see Figure 28).

FREQUENCY (Hz)

1k

10k

100k

1M

10M

100

B15

B14

B13

B12

B11

B10

B9

B8

B7

B6

B5

B4

B3

B2

ZS

B1

B0

Figure 26. AD5554 Reference Multiplying Bandwidth vs. Code

FREQUENCY (Hz)

1k

10k

100k

1M

10M

100

B13

B12

B11

B10

B9

B8

B7

B6

B5

B4

B3

B2

B1

B0

ZS

Figure 27. AD5554 Reference Multiplying Bandwidth vs. Code