EVAL-AD5449EBZ datasheet(17/32 Pages) AD

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AD5429/AD5439/AD5449

Rev. C | Page 17 of 32

SINGLE-SUPPLY APPLICATIONS

Voltage-Switching Mode

Figure 40 shows the DACs operating in voltage-switching mode.

The reference voltage, VIN, is applied to the IOUT1A pin; IOUT2A

is connected to AGND; and the output voltage is available at the

VREFA terminal. In this configuration, a positive reference voltage

results in a positive output voltage, making single-supply operation

possible. The output from the DAC is voltage at a constant

impedance (the DAC ladder resistance). Therefore, an op amp

is necessary to buffer the output voltage. The reference input

no longer sees a constant input impedance; instead, it sees one

that varies with code. Therefore, the voltage input should be

driven from a low impedance source.

Note that VIN is limited to low voltages because the switches

in the DAC ladder no longer have the same source-drain drive

voltage. As a result, their on resistance differs and degrades the

integral linearity of the DAC. Also, VIN must not go negative by

more than 0.3 V, or an internal diode turns on, causing the device

to exceed the maximum ratings. In this type of application, the

full range of multiplying capability of the DAC is lost.

Positive Output Voltage

The output voltage polarity is opposite to the VREF polarity for

dc reference voltages. To achieve a positive voltage output, an

applied negative reference to the input of the DAC is preferred

over the output inversion through an inverting amplifier because

of the resistor tolerance errors. To generate a negative reference,

the reference can be level-shifted by an op amp such that the VOUT

and GND pins of the reference become the virtual ground and

−2.5 V, respectively, as shown in Figure 41.

VOUT

VDD

GND

VIN

IOUT2A

IOUT1A

RFBA VDD

VREFA

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED

IF A1 IS A HIGH SPEED AMPLIFIER.

8-/10-/12-BIT

DAC

Figure 40. Single-Supply Voltage-Switching Mode

VOUT = 0V TO +2.5V

VDD = +5V

GND

IOUT2A

IOUT1A

RFBA

VDD

VREFA

GND

VIN

VOUT

ADR03

+5V

–5V

8-/10-/12-BIT

DAC

–2.5V

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 41. Positive Voltage Output with Minimum Components

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