EVAL-AD5405EB View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

EVAL-AD5405EB

Dual 12-Bit, High Bandwidth, Multiplying DAC with 4-Quadrant Resistors and Parallel Interface

Analog Devices 

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AD5405

VDD

VDD

C1

RFBA

R1

VIN

VREFA 12-BIT

DAC

IOUT1A

IOUT2A

GND

VOUT

R3

R2 GAIN = R2 + R3

R2

NOTES

R1 = R2R3

1. SIMILAR CONFIGURATION FOR DAC B.

R2 + R3

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

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

( ) VOUT = − VIN D = − VIN 1 − 2−n

VDD

VIN

RFBA VDD

IOUT1A

IOUT2A

GND

VREFA

VOUT

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

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 op amp 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 AD54xx series of

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

voltage temperature coefficient specification. This parameter not

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

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

ficient should be consistent with the system accuracy specifica-

tions. 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

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

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 op amp 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 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 quickly.

Rev. B | Page 16 of 24

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