AD5547 Просмотр технического описания (PDF) - Analog Devices

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AD5547

Dual-Current Output, Parallel Input, 16-/14-Bit Multiplying DACs with 4-Quadrant Resistors

Analog Devices 

Data Sheet

DIGITAL SECTION

The AD5547/AD5557 have 16-/14-bit parallel inputs. The devices

are double buffered with 16-/14-bit registers. The double buffered

feature allows the simultaneous update of several AD5547s/

AD5557s. For the AD5547, the input register is loaded directly

from a 16-bit controller bus when WR is brought low. The DAC

register is updated with data from the input register when LDAC

is brought high. Updating the DAC register updates the DAC

output with the new data (see Figure 18). To make both registers

transparent, tie WR low and LDAC high. The asynchronous RS

pin resets the part to zero scale if MSB = 0 and to midscale if

MSB = 1.

ESD Protection Circuits

All logic input pins contain back-biased ESD protection Zeners

connected to ground (DGND) and VDD, as shown in Figure 19.

As a result, the voltage level of the logic input should not be

greater than the supply voltage.

VDD

DIGITAL

INPUTS 5kΩ

DGND

Figure 19. Equivalent ESD Protection Circuits

Amplifier Selection

In addition to offset voltage, the bias current is important in

op amp selection for precision current output DACs. A 30 nA

input bias current in the op amp contributes to 1 LSB in the

full-scale error of the AD5547. The OP1177 and AD8628 op

amps are good candidates for the I-to-V conversion.

Reference Selection

The initial accuracy and rated output of the voltage reference

determine the full-span adjustment. The initial accuracy of

the reference is usually a secondary concern because it can be

trimmed. Figure 25 shows an example of a trimming circuit.

The zero-scale error can also be minimized by standard op amp

nulling techniques.

AD5547/AD5557

The voltage reference temperature coefficient (TC) and long-

term drift are primary considerations. For example, a 5 V

reference with a TC of 5 ppm/°C means the output changes by

25 µV/°C. As a result, a reference operating at 55°C contributes

an additional 750 µV full-scale error.

Similarly, the same 5 V reference with a ±50 ppm long-term

drift means the output may change by ±250 µV over time.

Therefore, it is practical to calibrate a system periodically to

maintain its optimum precision.

PCB LAYOUT, POWER SUPPLY BYPASSING, AND

GROUND CONNECTIONS

It is a good practice to employ a compact, minimum lead length,

PCB layout design. The leads to the input should be as short as

possible to minimize IR drop and stray inductance.

The PCB metal traces between VREF and RFB should also be

matched to minimize gain error.

It is also essential to bypass the power supply with quality

capacitors for optimum stability. Supply leads to the device

should be bypassed with 0.01 µF to 0.1 µF disc or chip ceramic

capacitors. Low ESR 1 µF to 10 µF tantalum or electrolytic

capacitors should also be applied at the supply in parallel with

the ceramic capacitor to minimize transient disturbance and

filter out low frequency ripple.

To minimize the digital ground bounce, the AD5547/AD5557

DGND terminal should be joined with the AGND terminal at

a single point. Figure 20 illustrates the basic supply bypassing

configuration and AGND/DGND connection for the

AD5547/AD5557.

+ C2

5V

–

C1

1µF

0.1µF

VDD

AD5547/AD5557

AGND

DGND

Figure 20. Power Supply Bypassing

Rev. D | Page 13 of 20

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