AD5726

Rev. 0 | Page 13 of 20

THEORY OF OPERATION

The AD5726 is a quad, 12-bit, serial input, unipolar/bipolar

voltage output DAC. It operates from single-supply voltages of

+5 V to +15 V or dual supply voltages of ±5 V to ±15 V. The

four outputs are buffered and capable of driving a 2 kΩ load.

Data is written to the AD5726 in a 16-bit word format via a

3-wire serial interface.

DAC ARCHITECTURE

Each of the four DACs is a voltage switched, high impedance

(50 kΩ), R-2R ladder configuration. Each 2R resistor is driven by a

OUTPUT AMPLIFIERS

The AD5726 features buffered analog voltage outputs capable of

sourcing and sinking up to 5 mA when operating from ±15 V

supplies, eliminating the need for external buffer amplifiers in

most applications while maintaining specified accuracy over the

rated operating conditions. The output amplifiers are short-

circuit protected. The designer should verify that the output

load meets the capabilities of the device, in terms of both output

current and load capacitance. The AD5726 is stable with

capacitive loads up to 2 nF typically. However, any capacitance

load increases the settling time and should be minimized if

speed is a concern.

The output stage includes a P-channel MOSFET to pull the

output voltage down to the negative supply. This is very

same potential as the negative supply. With no load, the zero-

scale output voltage in these applications are less than 500 μV

sinking current, this voltage does increase because of the finite

impedance of the output stage. The effective value of the pull-

down resistor in the output stage is typically 320 Ω. With a

100 kΩ resistor connected to 5 V, the resulting zero-scale output

voltage is 16 mV. Thus, the best single-supply operation is

obtained with the output load connected to ground, so the

output stage does not have to sink current.

Like all amplifiers, the AD5726 output buffers do generate

voltage noise, 5 nV/rtHz typically. This is easily reduced by

adding a simple RC low-pass filter on each output.

REFERENCE INPUTS

The two reference inputs of the AD5726 allow a great deal of

flexibility in circuit design. The user must take care, however, to

maintain the accuracy shown in the data sheet. These input

voltages can be set anywhere across a wide range within the

supplies, but must be a minimum of 2.5 V apart in any case (see

Figure 22). A wide output voltage range can be obtained with

±5 V references that can be provided by the AD588 as shown in

Figure 24. Many applications utilize the DACs to synthesize

symmetric bipolar waveforms, which require an accurate, low

drift bipolar reference. The AD588 provides both voltages and

needs no external components. Additionally, the part is

trimmed in production for 12-bit accuracy over the full

temperature range without user calibration.

1 LSB

0x000

2.5V MIN

–10V MIN

2.5V MIN

0xFFF

0V MIN

Figure 22. Output Voltage Range Programming

When driving the reference input, it is important to note that

of both are code dependent. Many voltage reference products

have limited current sinking capabilities and must be buffered

must always be at least 2.5 V less than the positive supply to

ensure linearity of the device. For these applications, the AD780

is an excellent low drift 2.5 V reference. It works well with the

AD5726 in a single 5 V system, as shown in Figure 26.

It is recommended that the reference inputs be bypassed with

0.2 μF capacitors when operating with ±10 V references. This

limits the reference bandwidth.

The AD5726 uses a DAC switch driver circuit that compensates

for different supply, reference voltage, and digital code inputs.

This ensures that all DAC ladder switches are always biased

equally, ensuring excellent linearity under all conditions. Thus,

requires both sourcing and sinking current capability from the

reference voltage source. Many positive voltage references are

intended as current sources only and offer little sinking capability.

The user should consider references such as the AD584, AD586,

AD587, AD588, AD780, and REF43 for such an application.