AD586 datasheet(28/48 Pages) AD

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AD5735

Data Sheet

Rev. B | Page 28 of 48

THEORY OF OPERATION

The AD5735 is a quad, precision digital-to-current loop and

voltage output converter designed to meet the requirements of

industrial process control applications. It provides a high precision,

fully integrated, low cost, single-chip solution for generating

current loop and unipolar/bipolar voltage outputs.

The current ranges available are 0 mA to 20 mA, 4 mA to 20 mA,

and 0 mA to 24 mA. The voltage ranges available are 0 V to 5 V,

±5 V, 0 V to 10 V, and ±10 V. The current and voltage outputs

are available on separate pins, and only one output is active at

any one time. The output configuration is user-selectable via the

DAC control register.

On-chip dynamic power control minimizes package power

dissipation in current mode (see the Dynamic Power Control

section).

DAC ARCHITECTURE

The DAC core architecture of the AD5735 consists of two

matched DAC sections. A simplified circuit diagram is shown

in Figure 69. The four MSBs of the 12-bit data-word are decoded

to drive 15 switches, E1 to E15. Each switch connects one of

15 matched resistors either to ground or to the reference buffer

output. The remaining eight bits of the data-word drive Switch S0

to Switch S7 of an 8-bit voltage mode R-2R ladder network.

8-BIT R-2R LADDER

FOUR MSBs DECODED INTO

15 EQUAL SEGMENTS

E15

VOUT

Figure 69. DAC Ladder Structure

The voltage output from the DAC core can be

•

Buffered and scaled to output a software selectable

unipolar or bipolar voltage range (see Figure 70)

•

Converted to a current, which is then mirrored to the

supply rail so that the application sees only a current

source output (see Figure 71)

Both the voltage and current outputs are supplied by VBOOST_x.

The current and voltage are output on separate pins and cannot

be output simultaneously. The current and voltage output pins

of a channel can be tied together (see the Voltage and Current

Output Pins on the Same Terminal section).

RANGE

SCALING

12-BIT

DAC

VOUT_X SHORT FAULT

+VSENSE_X

–VSENSE_X

VOUT_X

Figure 70. Voltage Output

12-BIT

DAC

VBOOST_x

IOUT_x

RSET

Figure 71. Voltage-to-Current Conversion Circuitry

Voltage Output Amplifier

The voltage output amplifier is capable of generating both

unipolar and bipolar output voltages. It is capable of driving a

load of 1 kΩ in parallel with 1 µF (with an external compen-

sation capacitor) to AGND. The source and sink capabilities

of the output amplifier are shown in Figure 22. The slew rate is

1.9 V/µs with a full-scale settling time of 18 µs max (10 V step).

If remote sensing of the load is not required, connect +VSENSE_x

directly to VOUT_x, and connect −VSENSE_x directly to AGND.

−VSENSE_x must stay within ±3.0 V of AGND for specified opera-

tion. The difference in voltage between +VSENSE_x and VOUT_x

should be added directly to the headroom requirement.

Driving Large Capacitive Loads

The voltage output amplifier is capable of driving capacitive

loads of up to 2 µF with the addition of a 220 pF, nonpolarized

compensation capacitor on each channel. The 220 pF capacitor

is connected between the COMPLV_x pin and the VOUT_x pin.

Care should be taken to choose an appropriate value of com-

pensation capacitor. This capacitor, while allowing the AD5735

to drive higher capacitive loads and reduce overshoot, increases

the settling time of the part and, therefore, affects the bandwidth

of the system. Without the compensation capacitor, capacitive

loads of up to 10 nF can be driven.

Reference Buffers

The AD5735 can operate with either an external or internal

reference. The reference input requires a 5 V reference for

specified performance. This input voltage is then buffered

before it is applied to the DAC.

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