AD5751 datasheet(23/32 Pages) AD

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Data Sheet

AD5751

Rev. D | Page 23 of 32

CURRRENT OUTPUT ARCHITECTURE

The voltage input from the analog input VIN core (0 V to 4.096 V)

is either converted to a current (see Figure 52), which is then

mirrored to the supply rail so that the application simply sees

a current source output with respect to an internal reference

voltage, or it is buffered and scaled to output a software-selectable

unipolar voltage range (see Figure 53). The reference is used to

provide internal offsets for range and gain scaling. The selecta-

ble output range is programmable through the digital interface

(software mode) or via the range pins (R0 to R3) (hardware mode).

AVDD

VIN

VREF

RANGE DECODE

FROM INTERFACE

VOUT RANGE

SCALING

IOUT

Figure 52. Current Output Configuration

VIN

(0V TO 4.096V)

VREF

VOUT SHORT FAULT

VOUT

VSENSE+

GND

RANGE DECODE

FROM INTERFACE

VOUT RANGE

SCALING

Figure 53. Voltage Output

DRIVING INDUCTIVE LOADS

When driving inductive or poorly defined loads, connect a 0.01 µF

capacitor between IOUT and GND. This ensures stability with

loads beyond 50 mH. There is no maximum capacitance limit.

The capacitive component of the load may cause slower settling.

Voltage Output Amplifier

The voltage output amplifier is capable of driving a load of 1 kΩ

(for 0 V to 5 V and 0 V to 10 V ranges) and a load of 5 kΩ (for

0 V to 40 V range) and capacitive loads up to 2 µF (with an

external compensation capacitor on the COMP1 and COMP2

pins). The source and sink capabilities of the output amplifier

can be seen in Figure 15. The slew rate is 2 V/µs.

Internal to the device, there is a 2.5 MΩ resistor connected

between VOUT and VSENSE+. If a fault condition occurs,

these resistors act to protect the AD5751 by ensuring that the

amplifier loop is closed so that the part does not enter into an

open-loop condition.

The current and voltage are output on separate pins and cannot

be output simultaneously. This allows the user to tie both the

current and voltage output pins together and configure the end

system as a single-channel output.

Driving Large Capacitive Loads

The voltage output amplifier is capable of driving capacitive loads

of up to 1 µF with the addition of a nonpolarized compensation

capacitor between the COMP1 and COMP2 pins.

Without the compensation capacitor, up to 20 nF capacitive loads

can be driven. Care should be taken to choose an appropriate

value for the CCOMP capacitor. This capacitor, while allowing the

AD5751 to drive higher capacitive loads and reduce overshoot,

increases the settling time of the part and therefore affects the

bandwidth of the system. Considered values of this capacitor

should be in the range of 0 nF to 4 nF depending on the trade-off

required between settling time, overshoot, and bandwidth.

POWER-ON STATE OF THE AD5751

On power-up, the AD5751 senses whether hardware or software

mode is loaded and sets the power-up conditions accordingly.

In software SPI mode, the power-up state of the output is

dependent on the state of the CLEAR pin. If the CLEAR pin is

pulled high, the part powers up, driving an active 0 V on the

output. If the CLEAR pin is pulled low, the part powers up with

the voltage output channel in tristate mode. In both cases, the

current output channel powers up in the tristate condition (0

mA). This allows the voltage and current outputs to be

connected together if desired.

To put the part into normal operation, the user must set the

OUTEN bit in the control register to enable the output and, in

the same write, set the output range configuration using the R3

to R0 range bits. If the CLEAR pin is still high (active) during

this write, the part automatically clears to its normal clear state

as defined by the programmed range and by the CLRSEL pin or

the CLRSEL bit (see the Asynchronous Clear (CLEAR) section

for more details). The CLEAR pin must be taken low to operate

the part in normal mode.

The CLEAR pin is typically driven directly from a microcontroller.

In cases where the power supply for the AD5751 supply is

independent of the microcontroller power supply, the user can

connect a weak pull-up resistor to DVCC or a pull-down resistor

to ground to ensure that the correct power-up condition is

achieved independent of the microcontroller. A 10 kΩ pull-up/

pull-down resistor on the CLEAR pin should be sufficient for

most applications.

If hardware mode is selected, the part powers up to the conditions

defined by the R3 to R0 range bits and the status of the OUTEN

or CLEAR pin. It is recommended to keep the output disabled

when powering up the part in hardware mode.

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