REV. 0

–18–

AD5424/AD5433/AD5445

ADDING GAIN

In applications where the output voltage is required to be greater

it can also be achieved in a single stage. It is important to consider

the effect of temperature coefficients of the thin film resistors of

the DAC. Simply placing a resistor in series with the RFB resistor

will cause mismatches in the temperature coefficients resulting in

larger gain temperature coefficient errors. Instead, the circuit of

Figure 9 is a recommended method of increasing the gain of the

circuit. R1, R2, and R3 should all have similar temperature coef-

ficients, but they need not match the temperature coefficients of the

DAC. This approach is recommended in circuits where gains of

great than 1 are required.

GND

C1

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY

2. C1 PHASE COMPENSATION (1pF– 2pF) MAY BE

REQUIRED IF A1 IS A HIGH SPEED AMPLIFIER.

R3

R2

R1

R1 = R2R3

R2 + R3

GAIN = R2 + R3

R2

8-/10-/12-BIT DAC

Figure 9. Increasing Gain of Current Output DAC

USING DACS AS A DIVIDER OR A PROGRAMMABLE

GAIN ELEMENT

Current steering DACs are very flexible and lend themselves to

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

resistor as shown in Figure 10, then the output voltage is inversely

proportional to the digital input fraction D. For D = 1 – 2

output voltage is

VV

D

V

OUT

IN

IN

n

=−

=−

−

12

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 also that the required accuracy

is met. For example, an 8-bit DAC driven with the binary code

10H (00010000), i.e., 16 decimal, in the circuit of Figure 10

should cause the output voltage to be 16

DAC has a linearity specification of

±0.5 LSB then D can in

fact have the weight anywhere in the range 15.5/256 to 16.5/256

maximum error of 0.2%.

GND

NOTE

ADDITIONAL PINS OMITTED FOR CLARITY

Figure 10. Current Steering DAC Used as a Divider

or Programmable Gain Element

Table III. Suitable ADI Precision References Recommended for Use with AD5424/AD5433/AD5445 DACs

Part No.

Output Voltage

Initial Tolerance

Temperature Drift

0.1 Hz to 10 Hz Noise

Package

ADR01

10 V

0.1%

3 ppm/

°C

20

V p-p

SC70, TSOT, SOIC

ADR02

5 V

0.1%

3 ppm/

°C

10

V p-p

SC70, TSOT, SOIC

ADR03

2.5 V

0.2%

3 ppm/

°C

10

V p-p

SC70, TSOT, SOIC

ADR425

5 V

0.04%

3 ppm/

°C

3.4

V p-p

MSOP, SOIC

Table IV. Some Precision ADI Op Amps Suitable for Use with AD5424/AD5433/AD5445 DACs

Part No.

Max Supply Voltage (V)

GBP (MHz)

Slew Rate (V/ s)

OP97

±20

25

0.1

0.9

0.2

OP1177

±18

60

2

1.3

0.7

AD8551

±65

0.05

1.5

0.4

Table V. Some High Speed ADI Op Amps Suitable for Use with AD5424/AD5433/AD5445 DACs

Max Supply Voltage

Slew Rate

Part No.

(V)

(MHz)

(V/ s)

( V)

(nA)

AD8065

±12

145

180

1500

0.01

AD8021

±12

200

100

1000

1000

AD8038

±5

350

425

3000

0.75

AD9631

±5

320

1300

10000

7000