AD5429 datasheet(16/28 Pages) AD

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AD5429/AD5439/AD5449

Data Sheet

Rev. F | Page 16 of 28

Bipolar Operation

In some applications, it may be necessary to generate full

4-quadrant multiplying operation or a bipolar output swing.

This can easily be accomplished by using another external

amplifier and three external resistors, as shown in Figure 39.

When VIN is an ac signal, the circuit performs 4-quadrant

multiplication. When connected in bipolar mode, the output

voltage is



REF

OUT







 1

where:

is the fractional representation of the digital word loaded to

the DAC.

= 0 to 255 (AD5429)

= 0 to 1023 (AD5439)

= 0 to 4095 (AD5449)

is the number of bits.

Table 6 shows the relationship between digital code and the

expected output voltage for bipolar operation with the AD5429.

Table 6. Bipolar Code

Digital Input

Analog Output (V)

1111 1111

+VREF (255/256)

1000 0000

0000 0001

−VREF (255/256)

0000 0000

−VREF (256/256)

Stability

In the I-to-V configuration, the IOUT of the DAC and the inverting

node of the op amp must be connected as closely as possible, and

proper PCB layout techniques must be used. Because every code

change corresponds to a step function, gain peaking may occur

if the op amp has limited gain bandwidth product (GBP) and

there is excessive parasitic capacitance at the inverting node.

This parasitic capacitance introduces a pole into the open-loop

response, which can cause ringing or instability in the closed-

loop applications circuit.

As shown in Figure 38 and Figure 39, an optional compensation

capacitor, C1, can be added in parallel with RFBx for stability.

Too small a value of C1 can produce ringing at the output,

whereas 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 the compensation.

IOUT1A

IOUT2A

AD5429/

AD5439/

AD5449

VREFx

VDD

VOUT = –VREF TO +VREF

VDD

VREF ±10V

SDIN

SCLK

SYNC

MICROCONTROLLER

10kΩ

20kΩ

NOTES

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

2. MATCHING AND TRACKING IS ESSENTIAL FOR RESISTOR PAIRS

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

4. DAC B AND ADDITIONAL PINS OMITTED FOR CLARITY.

IF A1/A2 IS A HIGH SPEED AMPLIFIER.

ADJUST R1 FOR VOUT = 0V WITH CODE 10000000 LOADED TO DAC.

R3 AND R4.

20kΩ

RFBA

AGND

GND

Figure 39. Bipolar Operation

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