Datasheet

AD5429/AD5439/AD5449 Data Sheet
Rev. E | 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 V
IN
is an ac signal, the circuit performs 4-quadrant
multiplication. When connected in bipolar mode, the output
voltage is
( )
REF
n
REF
OUT
VDVV /×=
/ 1
2/
where:
D is the fractional representation of the digital word loaded to
the DAC.
D = 0 to 255 (AD5429)
= 0 to 1023 (AD5439)
= 0 to 4095 (AD5449)
n 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 +V
REF
(255/256)
1000 0000 0
0000 0001 −V
REF
(255/256)
0000 0000 −V
REF
(256/256)
Stability
In the I-to-V configuration, the I
OUT
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 R
FB
x 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.
04464-008
I
OUT
1A
I
OUT
2A
AD5429/
AD5439/
AD5449
V
REF
x
V
DD
C1
A1
V
OUT
= –V
REF
TO +V
REF
R2
V
DD
V
REF
±10V
SDIN
SCLKSYNC
MICROCONTROLLER
A2
R4
10kΩ
R5
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 V
OUT
= 0V WITH CODE 10000000 LOADED TO DAC.
R3 AND R4.
R3
20kΩ
R1
R
FB
A
AGND
GND
R1
Figure 39. Bipolar Operation