AD5415 datasheet(16/28 Pages) AD

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AD5415

Rev. 0 | Page 16 of 28

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 the on chip 4-quadrant resistors, as shown in

Figure 34.

When in bipolar mode, the output voltage is given by

VOUT = VREF × D/2n − 1 −VREF

where D is the fractional representation of the digital word

loaded to the DAC, in the range of 0 to 4095.

n is the number of bits.

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

multiplication.

Table 6 shows the relationship between digital code and the

expected output voltage for bipolar operation.

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 close 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 that can cause ringing or instability in the closed loop

application’s circuit.

An optional compensation capacitor, C1, can be added in

parallel with RFB for stability, as shown in Figure 33 and

Figure 34. 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

RFBA

RFB

AD5415

12-BIT DAC A

GND

SDIN

VREFA

SCLK

SYNC

R2A

R2_3A

R3A

VDD

VOUT =–VIN TO +VIN

AGND

uCONTROLLER

R1A

AGND

VIN

NOTES:

1DAC B OMITTED FOR CLARITY.

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

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 34. Bipolar Operation

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