AD8021 datasheet(16/32 Pages) AD

16 / 32 page

AD5429/AD5439/AD5449

Rev. 0 | Page 16 of 32

BIPOLAR OPERATION

In some applications, it might 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 three external resistors, as shown in Figure 38.

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 D is the fractional representation of the digital word

loaded to the DAC, and n is the number of bits.

D = 0 to 255 (AD5429)

= 0 to 1023 (AD5439)

= 0 to 4095 (AD5449)

Table 6 shows the relationship between digital code and the

expected output voltage for bipolar operation with the AD5429.

Table 6. Bipolar Code Table

Digital Input

Analog Output (V)

1111 1111

+VREF (2047/2048)

1000 0000

0000 0001

−VREF (2047/2048)

0000 0000

−VREF (2048/2048)

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 employed.

Because every code change corresponds to a step function, gain

peaking can occur, if the op amp has limited 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 37 and Figure 38, an optional compensation

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

small a 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 the

compensation.

IOUT1A

IOUT2A

AD5429/

AD5439/

AD5449

VREF

VDD

VOUT =–VREF TO +VREF

VDD

VREF ±10V

SDIN

SCLK

SYNC

µCONTROLLER

10k

Ω

20k

Ω

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.

20k

Ω

RFBA

AGND

GND

Figure 38. Bipolar Operation

AD8021 Datasheet(PDF) 16 Page - Analog Devices

AD8021 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL