REV. 0

–16–

AD5424/AD5433/AD5445

GND

10V

NOTES

1. R1 AND R2 ARE USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.

2. MATCHING AND TRACKING IS ESSENTIAL FOR RESISTOR PAIRS R3 AND R4.

3. C1 PHASE COMPENSATION (1pF–2pF) MAY BE REQUIRED IF A1/A2 IS

A HIGH SPEED AMPLIFIER.

AGND

R3

10k

AD5424/

AD5433/AD5445

R5

20k

R4

10k

A2

R1

R2

C1

CS

R/

W

DATA

INPUTS

A1

Figure 5. Bipolar Operation (4-Quadrant Multiplication)

Bipolar Operation

In some applications, it may be necessary to generate full

4-quadrant multiplying operation or a bipolar output swing. This

can be easily accomplished by using another external amplifier and

some external resistors as shown in Figure 5. In this circuit, the

second amplifier A2 provides a gain of 2. Biasing the external

amplifier with an offset from the reference voltage results in full

4-quadrant multiplying operation. The transfer function of this

circuit shows that both negative and positive output voltages

are created as the input data (D) is incremented from code

VV

D

V

OUT

REF

n

REF

=×

−

2

1

where D is the fractional representation of the digital word loaded

to the DAC and n is the resolution of the DAC.

D= 0 to 255 (8-Bit AD5424)

= 0 to 1023 (10-Bit AD5433)

= 0 to 4095 (12-Bit AD5445)

multiplication.

Table II shows the relationship between digital code and the

expected output voltage for bipolar operation (AD5426, 8-bit

device).

Table II. Bipolar Code Table

Digital Input

Analog Output (V)

1111 1111

1000 0000

0

0000 0001

0000 0000

Stability

ing node of the op amp must be connected as close as possible,

and proper PCB layout techniques must be employed. Since

every code change corresponds to a step function, gain peaking

may occur if the op amp has limited GBP and there is excessive

parasitic capacitance at the inverting node. This parasitic capaci-

tance introduces a pole into the open-loop response, which can

cause ringing or instability in closed-loop applications.

An optional compensation capacitor, C1, can be added in parallel

value of C1 can produce ringing at the output, while too large

a value can adversely affect the settling time. C1 should be

found empirically but 1 pF to 2 pF is generally adequate

for compensation.