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AD5405

Rev. B | Page 16 of 24

VOUT

VDD

GND

IOUT2A

IOUT1A

VREFA

RFBA

VDD

12-BIT

DAC

VIN

R1 = R2R3

R2 + R3

GAIN = R2 + R3

NOTES

1. SIMILAR CONFIGURATION FOR DAC B.

Y BE REQUIRED

2. C1 PHASE COMPENSATION (1pF TO 2pF) MA

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 36. Increasing Gain of Current Output DAC

DIVIDER OR PROGRAMMABLE GAIN ELEMENT

Current-steering DACs are very flexible and lend themselves to

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

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

resistor, as shown in Figure 37, the output voltage is inversely

proportional to the digital input fraction, D.

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

()n

OUT

−

VOUT

VDD

GND

VIN

IOUT2A

REF

IOU

T1A

RFBA VDD

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 37. 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 that 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 37

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

weight in the range of 15.5/256 to 16.5/256 so that the possible

n error

or source in divider

ust be counterbalanced by an

the op amp through the DAC.

mperature coefficient specification. This parameter not

only affects the full-scale error, but also can affect the linearity

stem required to hold its overall

The primary requirement for the current-steering mode is an

een

This

ge in

kly.

e DAC has a linearity specification of ±0.5 LSB, D can have a

output voltage is in the range of 15.5 VIN to 16.5 VIN—a

of 3%, even though the DAC itself has a maximum error of 0.2%.

DAC leakage current is also a potential err

circuits. The leakage current m

opposite current supplied from

Because only a fraction, D, of the current into the VREF termina

is routed to the IOUT1 terminal, the output voltage changes as

follows:

Output Error Voltage Due to DAC Leakage = (Leakage × R)/D

where R is the DAC resistance at the VREF terminal.

For a DAC leakage current of 10 nA, R = 10 kΩ, and a gain (that

is, 1/D) of 16, the error voltage is 1.6 mV.

REFERENCE SELECTION

When selecting a reference for use with the AD54xx series of

current output DACs, pay attention to the reference’s output

voltage te

(INL and DNL) performance. The reference temperature coef-

ficient should be consistent with the system accuracy specifica

tions. For example, an 8-bit sy

specification to within 1 LSB over the temperature range 0°C to

50°C dictates that the maximum system drift with temperature

should be less than 78 ppm/°C. A 12-bit system with the same

temperature range to overall specification within 2 LSBs requires

a maximum drift of 10 ppm/°C. Choosing a precision reference

with low output temperature coefficient minimizes this error

source.

Table 7 lists some references available from Analog Devices that

are suitable for use with this range of current output DACs.

AMPLIFIER SELECTION

amplifier with low input bias currents and low input offset

voltage. Because of the code-dependent output resistance of th

DAC, the input offset voltage of an op amp is multiplied by the

variable gain of the circuit. A change in this noise gain betw

two adjacent digital fractions produces a step change in t

output voltage due to the amplifier’s input offset voltage.

output voltage change is superimposed on the desired chan

output between the two codes and gives rise to a differential

linearity error, which, if large enough, could cause the DAC

be nonmonotonic

The input bias current of an op amp also generates an offset at

the voltage output as a result of the bias current flowing in the

feedback resistor, RFB. Most op amps have input bias currents low

enough to prevent significant errors in 12-bit applications.

Common-mode rejection of the op amp is important in

voltage-switching circuits, because it produces a code-

dependent error at the voltage output of the circuit. Most

op amps have adequate common-mode rejection for use at

12-bit resolution.

Provided that the DAC switches are driven from true wideband,

low impedance sources (VIN and AGND), they settle quic

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