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AD5405

Data Sheet

Rev. D | Page 16 of 24

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 operational amplifier and RFBA is used as the

input resistor, as shown in Figure 36, the output voltage is

inversely proportional to the digital input fraction, D.

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

(

OUT

−

VOUT

VDD

GND

VIN

IOUT2A

IOUT1A

RFBA VDD

VREFA

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 36. 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 36 can cause

the output voltage to be 16 times VIN. However, if the DAC has a

linearity specification of ±0.5 LSB, D can have a weight in the range

of 15.5/256 to 16.5/256 so that the possible output voltage is in

the range of 15.5 VIN to 16.5 VIN—an error of 3%, even though

the DAC itself has a maximum error of 0.2%.

DAC leakage current is also a potential error source in divider

circuits. The leakage current must be counterbalanced by an

opposite current supplied from the operational amplifier through

the DAC. Because only a fraction, D, of the current into the

VREF terminal 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 AD5405 and other

devices in this series of current output DACs, pay attention to the

output voltage temperature coefficient specification of the

reference. This parameter not only affects the full-scale error,

but also can affect the linearity (INL and DNL) performance. The

reference temperature coefficient must be consistent with the

system accuracy specifications. For example, an 8-bit system

required to hold its overall specification to within 1 LSB over the

temperature range 0°C to 50°C dictates that the maximum system

drift with temperature must 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, Inc., that are suitable for

use with this range of current output DACs.

AMPLIFIER SELECTION

The primary requirement for the current steering mode is an

amplifier with low input bias currents and low input offset voltage.

Because of the code dependent output resistance of the DAC, the

input offset voltage of an operational amplifier is multiplied by the

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

two adjacent digital fractions produces a step change in the output

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

voltage change is superimposed on the desired change in output

between the two codes and gives rise to a differential linearity

error, which, if large enough, could cause the DAC to be

nonmonotonic.

The input bias current of an operational amplifier also generates

an offset at the voltage output because of the bias current flowing

in the feedback resistor, RFB. Most operational amplifiers have input

bias currents low enough to prevent significant errors in 12-bit

applications.

Common-mode rejection of the operational amplifier is

important in voltage switching circuits, because it produces a

code dependent error at the voltage output of the circuit. Most

operational amplifiers have adequate common-mode rejection

for use at 12-bit resolution.

If the DAC switches are driven from true wideband, low

impedance sources (VIN and AGND), they settle quickly.

Consequently, the slew rate and settling time of a voltage-switching

DAC circuit is determined largely by the output operational

amplifier. To obtain minimum settling time in this configuration,

minimize capacitance at the VREF node (the voltage output node

in this application) of the DAC. This is done by using low input

capacitance buffer amplifiers and careful board design.

Most single-supply circuits include ground as part of the analog

signal range, which in turn requires an amplifier that can handle

rail-to-rail signals. Analog Devices offers a wide range of single-

supply amplifiers, as listed in Table 8 and Table 9.

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