AD5440 datasheet(19/32 Pages) AD

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Data Sheet

AD5428/AD5440/AD5447

Rev. C | Page 19 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 IOUTA pin,

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 constant impedance (the

DAC ladder resistance). Therefore, an op amp is necessary to

buffer the output voltage. The reference input no longer sees

constant input impedance, but 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.

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

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

IF A1 IS A HIGH SPEED AMPLIFIER.

VDD

VIN

VREFA

VDD

RFBA

GND

VOUT

IOUTA

AGND

Figure 40. Single-Supply Voltage-Switching Mode

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 41.

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

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

IF A1 IS A HIGH SPEED AMPLIFIER.

VDD = 5V

VDD

VIN

VREFA

RFBA

8-/10-/12-BIT

DAC

ADR03

GND

VOUT

VOUT =

0V to 2.5V

IOUTA

AGND

+5V

–5V

–2.5V

Figure 41. Positive Voltage Output with Minimum Components

ADDING GAIN

In applications where the output voltage must be greater than

VIN, gain can be added with an additional external amplifier, or

it can be achieved in a single stage. Consider the effect of temper-

ature coefficients of the thin film resistors of the DAC. Simply

placing a resistor in series with the RFB resistor causes mismatches

in the temperature coefficients, resulting in larger gain temper-

ature coefficient errors. Instead, the circuit in Figure 42 shows

the recommended method for increasing the gain of the circuit.

R1, R2, and R3 should have similar temperature coefficients,

but they need not match the temperature coefficients of the

DAC. This approach is recommended in circuits where gains of

greater than 1 are required.

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

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

IF A1 IS A HIGH SPEED AMPLIFIER.

VDD

VIN

VREFA

RFBA

8-/10-/12-BIT

DAC

GND

VOUT

IOUTA

AGND

R2 + R3

GAIN =

R1 =

R2R3

R2 + R3

Figure 42. Increasing Gain of Current Output DAC

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