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Data Sheet

AD5444/AD5446

Rev. F | Page 17 of 28

SINGLE-SUPPLY APPLICATIONS

Voltage Switching Mode of Operation

Figure 40 shows the AD5444/AD5446 DACs operating in the

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 rather one that varies with code, so the

voltage input should be driven from a low impedance source.

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

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

IF A1 IS A HIGH SPEED AMPLIFIER.

IOUT1

GND

VOUT

VIN

RFB

VDD

VREF

VDD

Figure 40. Single-Supply Voltage Switching Mode Operation

It is important to note that, with this configuration, VIN is lim-

ited to low voltages, because the switches in the DAC ladder do

not have the same source-drain drive voltage. As a result, their

on resistance differs, which degrades the integral linearity of the

DAC. In addition, 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 full range of the

multiplying capability of the DAC is lost.

ADDING GAIN

In applications in which the output voltage is required to be

greater than VIN, gain can be added with an additional external

amplifier, or it can be achieved in a single stage. It is important

to take into consideration the effect of the temperature coeffi-

cients of the thin film resistors of the DAC. Simply placing a

resistor in series with the RFB resistor can cause mismatches in

the temperature coefficients and result in larger gain tempera-

ture coefficient errors. Instead, increase the gain of the circuit

by using the recommended configuration shown in Figure 41.

R1, R2, and R3 must all 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. Note that RFB >> R2||R3 and a gain

error percentage of 100 × (R2||R3)/RFB must be taken into

consideration.

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

RFB

IOUT1

IOUT2

VOUT

GND

VDD

VREF

GAIN =

R1 =

R2 + R3

R2R3

R2 + R3

VIN

Figure 41. Increasing Gain of Current Output DAC

DIVIDER OR PROGRAMMABLE GAIN ELEMENT

Current-steering DACs are very flexible and lend themselves to

many different applications. If this type of DAC is connected as

the feedback element of an op amp and RFB is used as the input

resistor, as shown in Figure 42, then the output voltage is

inversely proportional to the digital input fraction, D.

For D = 1 − 2−n, the output voltage is

VOUT

= −VIN/D = −VIN/(1 − 2−n)

NOTES:

1. ADDITIONAL PINS OMITTED FOR CLARITY.

IOUT1

GND

VOUT

VIN

RFB

VDD

VREF

VDD

Figure 42. Current-Steering DAC Used as a Divider

or Programmable Gain Element

As D is reduced, the output voltage increases. For small values

of the digital fraction (D), it is important to ensure that the

amplifier does not saturate and the required accuracy is met.

For example, an 8-bit DAC driven with the binary code 0x10

(0001 0000), that is, 16 decimal, in the circuit of Figure 42,

should cause the output voltage to be 16 × VIN. However, if the

DAC has a linearity specification of ±0.5 LSB, then D can, in

fact, have a weight in the range of 15.5/256 to 16.5/256, so the

possible output voltage is in the range 15.5 VIN to 16.5 VIN. This

is an error of 3%, even though the DAC itself has a maximum

error of 0.2%.

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