AD7390(Rev0) View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

AD7390
(Rev.:Rev0)

+3 Volt Serial-Input Micropower 10-Bit & 12-Bit DACs

Analog Devices 

AD7390/AD7391

RESET (CLR) PIN

Forcing the CLR pin low will set the DAC register to all zeros

and the DAC output voltage will be zero volts. The reset func-

tion is useful for setting the DAC outputs to zero at power-up or

after a power supply interruption. Test systems and motor con-

trollers are two of many applications which benefit from power-

ing up to a known state. The external reset pulse can be

generated by the microprocessor’s power-on RESET signal, by

an output from the microprocessor, or by an external resistor

and capacitor. CLR has a Schmitt trigger input which results in

a clean reset function when using external resistor/capacitor

generated pulses. The CLR input overrides other logic inputs,

specifically LD. However, LD should be set high before CLR

goes high. If CLR is kept low, then the contents of the shift reg-

ister will be transferred to the DAC register as soon as CLR re-

turns high. See the Control-Logic Truth Table I.

UNIPOLAR OUTPUT OPERATION

This is the basic mode of operation for the AD7390. As shown

in Figure 26, the AD7390 has been designed to drive loads as

low as 5 kΩ in parallel with 100 pF. The code table for this op-

eration is shown in Table IV.

0.01µF

EXT

REF

µC

RS

+2.7V to +5.5V

R

7

REF VDD

0.1µF ؉ 10µF

LD 3

CLK 2

SDI 1

CLR 4

AD7390

VOUT 6

GND

5

RL

≥ 5kΩ

CL

≤ 100pF

Figure 26. AD7390 Unipolar Output Operation

Table IV. AD7390 Unipolar Code Table

Hexadecimal

Number

in DAC Register

FFF

801

800

7FF

000

Decimal

Number

in DAC Register

4095

2049

2048

2047

0

Output

Voltage (V)

VREF = 2.5 V

2.4994

1.2506

1.2500

1.2494

0

The circuit can be configured with an external reference plus

power supply, or powered from a single dedicated regulator or ref-

erence depending on the application performance requirements.

BIPOLAR OUTPUT OPERATION

Although the AD7391 has been designed for single-supply op-

eration, the output can be easily configured for bipolar opera-

tion. A typical circuit is shown in Figure 27. This circuit uses a

clean regulated ϩ5 V supply for power, which also provides the

circuit’s reference voltage. Since the AD7391 output span

swings from ground to very near ϩ5 V, it is necessary to choose

an external amplifier with a common-mode input voltage range

that extends to its positive supply rail. The micropower con-

sumption OP196 has been designed just for this purpose and re-

sults in only 50 microamps of maximum current consumption.

Connection of the equal valued 470 kΩ resistors results in a dif-

ferential amplifier mode of operation with a voltage gain of two,

which results in a circuit output span of ten volts, that is, Ϫ5 V

to ϩ5 V. As the DAC is programmed with zero-code 000H to

midscale 200H to full-scale 3FFH, the circuit output voltage VO

is set at Ϫ5 V, 0 V and ϩ5 V (minus 1 LSB). The output volt-

age VO is coded in offset binary according to Equation 4.

΄ ΂ ΃ ΅ VO =

D

512

Ϫ1

ϫ5

Equation 4

where D is the decimal code loaded in the AD7391 DAC regis-

ter. Note that the LSB step size is 10/1024 = 10 mV. This cir-

cuit has been optimized for micropower consumption including

the 470 kΩ gain setting resistors, which should have low tem-

perature coefficients to maintain accuracy and matching (prefer-

ably the same material, such as metal film). If better stability is

required the power supply could be substituted with a precision

reference voltage such as the low dropout REF195, which can

easily supply the circuit’s 162 µA of current, and still provide

additional power for the load connected to VO. The micropower

REF195 is guaranteed to source 10 mA output drive current,

but only consumes 50 µA internally. If higher resolution is re-

quired, the AD7390 can be used with the addition of two more

bits of data inserted into the software coding, which would re-

sult in a 2.5 mV LSB step size. Table V shows examples of

nominal output voltages VO provided by the Bipolar Operation

circuit application.

ISY < 162µA

؉5V

470kΩ

470kΩ

< 100µA

REF VDD

C

AD7391

VOUT

GND

< 50µA

OP196

؉

؉5V

VO

BIPOLAR

OUTPUT

SWING

؊5V

؊5V

DIGITAL INTERFACE CIRCUITRY OMITTED FOR CLARITY

Figure 27. Bipolar Output Operation

Table V. Bipolar Code Table

Hexadecimal

Number

In DAC Register

Decimal

Number

in DAC Register

Analog

Output

Voltage (V)

3FF

1023

4.9902

201

513

0.0097

200

512

0.0000

1FF

511

-0.0097

000

0

-5.0000

–10–

REV. 0

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