AD5744CSU datasheet(22/27 Pages) AD

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Preliminary Technical Data

Rev. PrA 15-Nov-04| Page 22 of 27

AD5744/64 FEATURES

ANALOG OUTPUT CONTROL

In many industrial process control applications, it is vital that

the output voltage be controlled during power up and during

brownout conditions. When the supply voltages are changing,

the VOUT pin is clamped to 0 V via a low impedance path. To

prevent the output amp being shorted to 0 V during this time,

transmission gate G1 is also opened. These conditions are

maintained until the power supplies stabilize and a valid word is

written to the DAC register. At this time, G2 opens and G1

closes. Both transmission gates are also externally controllable

via the Reset In (RSTIN) control input. For instance, if RSTIN is

driven from a battery supervisor chip, the RSTIN input is

driven low to open G1 and close G2 on power-off or during a

brownout. Conversely, the on-chip voltage detector output

(RSTOUT) is also available to the user to control other parts of

the system. The basic transmission gate functionality is shown

in Figure 10.

VOLTAGE

MONITOR

AND

CONTROL

AGNDA

VOUTA

RSTOUT

RSTIN

Figure 10. Analog Output Control Circuitry

DIGITAL OFFSET AND GAIN CONTROL

The AD5744/64 incorporates a digital offset adjust function

with a ±16 LSB adjust range and 0.125 LSB resolution. The gain

output range. The full-scale output can be programmed to

achieve full-scale ranges of ±10 V, ±10.25 V, and ±10.5 V. A fine

gain trim is also available, allowing a trim range of ±16 LSB in 1

LSB steps.

PROGRAMMABLE SHORT-CIRCUIT PROTECTION

The short-circuit current of the output amplifiers can be pro-

grammed by inserting an external resistor between the ISCC

pin and AGND. The programmable range for the current is

500 µA to 10 mA, corresponding to a resistor range of 120 kΩ

to 6 kΩ.

The resistor value is calculated as follows;

Isc

If the ISCC pin is left unconnected the short circuit current

limit defaults to 5 mA. It should be noted that limiting the short

circuit current to a small value can affect the slew rate of the

output when driving into a capacitive load, therefore the value

of short-circuit current programmed should take into account

the size of the capacitive load being driven.

DIGITAL I/O PORT

The AD5744/64 contains a 2-bit digital I/O port (D1 and D0);

these bits can be configured as inputs or outputs independently,

and can be driven or have their values read back via the serial

interface. The I/O port signals are referenced to DVCC and

DGND. When configured as outputs, they can be used as con-

trol signals to multiplexers or can be used to control calibration

circuitry elsewhere in the system. When configured as inputs,

the logic signals from limit switches, for example can be applied

to D0 and D1 and can be read back via the digital interface.

TEMPERATURE SENSOR

The on-chip temperature sensor provides a voltage output that

is linearly proportional to the Centigrade temperature scale.

The typical accuracy of the temperature sensor is ±1°C at +25°C

and ±5°C over the −40°C to +105°C range. Its nominal output

voltage is 1.5V at +25°C, varying at 5 mV/°C, giving a typical

output range of 1.175V to 1.9 V over the full temperature range.

Its low output impedance, low self heating, and linear output

simplify interfacing to temperature control circuitry and A/D

converters.

LOCAL GROUND OFFSET ADJUST

The AD5744/64 incorporates a Local Ground Offset Adjust

feature which when enabled in the Function Register adjusts the

DAC outputs for voltage differences between The individual

DAC ground pins and the REFGND pin ensuring that the DAC

output voltages are always with respect to the local DAC ground

pin. For instance if pin AGNDA is at +5mV with respect to the

REFGND pin and VOUTA is measured with respect to

AGNDA then a +5mV error will result, enabling the Local

Ground Offset Adjust feature will offset VOUTA by +5mV

eliminating the error.

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