AD5764CSUZ datasheet(24/28 Pages) AD

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AD5764

Data Sheet

Rev. F | Page 24 of 28

DESIGN 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 VOUTx pins are 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 (see Figure 35). These condi-

tions are maintained until the power supplies stabilize and a

valid word is written to the data register. At this time, G2 opens and

G1 closes. Both transmission gates are also externally controllable

via the reset logic (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 upon power-down 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

Figure 35

RSTOUT

RSTIN

VOUTA

AGNDA

VOLTAGE

MONITOR

AND

CONTROL

Figure 35. Analog Output Control Circuitry

DIGITAL OFFSET AND GAIN CONTROL

The AD5764 incorporates a digital offset adjust function with a

±16 LSB adjust range and 0.125 LSB resolution. The coarse gain

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

scale ranges of ±10 V, ±10.2564 V, and ±10.5263 V. A fine gain

trim is also provided.

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 PGND. 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:

If the ISCC pin is left unconnected, the short-circuit current

limit defaults to 5 mA. Note 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 the

programmed short circuit should take into account the size of

the capacitive load being driven.

DIGITAL I/O PORT

The AD5764 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 control 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 can, for example, be applied to D0

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

LOCAL GROUND OFFSET ADJUST

The AD5764 incorporates a local ground offset adjust feature

that, when enabled in the function register, adjusts the DAC

outputs for voltage differences between the individual DAC ground

pins, AGNDx, 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 +5 mV with respect to the

REFGND pin and VOUTA is measured with respect to AGNDA,

a −5 mV error results, enabling the local ground offset adjust

feature to adjust VOUTA by +5 mV, eliminating the error.

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