AD8021 datasheet(17/32 Pages) AD

17 / 32 page

AD5429/AD5439/AD5449

Rev. 0 | Page 17 of 32

SINGLE-SUPPLY APPLICATIONS

Voltage-Switching Mode

Figure 39 shows the DACs operating in voltage-switching mode.

The reference voltage, VIN, is applied to the IOUT1 pin, IOUT2 is

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

VREF 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, but one that

varies with code. So, 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 this 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, exceeding the

maximum ratings of the device. In this type of application, the

DAC’s full range of multiplying capability 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’s 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 40.

VOUT

VDD

GND

VIN

IOUT2

IOUT1

RFB

VDD

VREF

NOTES:

1. ADDITIONAL PINS OMITTED FOR CLARITY.

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

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 39. Single-Supply Voltage-Switching Mode

VOUT = 0V TO +2.5V

VDD = +5V

GND

IOUT2

IOUT1

RFB

VDD

VREF

GND

VIN

VOUT

ADR03

+5V

–5V

1/2 AD8552

8-/10-/12-BIT

DAC

–2.5V

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

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

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 40. Positive Voltage Output with Minimum Components

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