EVAL-AD5446EBZ Analog Devices Inc, EVAL-AD5446EBZ Datasheet - Page 16

• Manufacturer Part Number: EVAL-AD5446EBZ
• Description: BOARD EVALUATION FOR AD5446
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5446EBZ

• Number Of Dac's: 1
• Number Of Bits: 14
• Outputs And Type: 1, Differential
• Sampling Rate (per Second): 20.4M
• Data Interface: Serial
• Settling Time: 16ns
• Dac Type: Current
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 125°C
• Utilized Ic / Part: AD5446
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD5444/AD5446
Bipolar Operation
In some applications, it may be necessary to generate a full
4-quadrant multiplying operation, or a bipolar output swing.
This can easily be accomplished by using another external
amplifier and some external resistors, as shown in Figure 39.
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 a 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 zero ( V
( V
where:
D is the fractional representation of the digital word loaded
to the DAC:
n is the resolution of the DAC.
When V
multiplication.
OUT
D = 0 to 4095 (12-bit AD5444)
D = 0 to 16383 (14-bit AD5446)
V
− 0 V) to full scale ( V
OUT
IN
is an ac signal, the circuit performs 4-quadrant
=
⎛
⎜
⎝
V
REF
×
2
D
n
V
−1
REF
⎞
⎟
⎠
−
±10V
OUT
V
REF
= + V
OUT
= − V
REF
R1
)
REF
MICROCONTROLLER
V
SYNC
) to midscale
REF
Figure 39. Bipolar Operation (4-Quadrant Multiplication)
SCLK
AD5444/
AD5446
V
V
DD
DD
SDIN
R
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,
FB
ADJUST R1 FOR V
R3 AND R4.
IF A1/A2 IS A HIGH SPEED AMPLIFIER.
20kΩ
Rev. C | Page 16 of 28
I
I
R3
OUT
OUT
1
2
R2
AGND
C1
A1
OUT
Table 6 shows the relationship between digital code and the
expected output voltage for bipolar operation.
Table 6. Bipolar Code
Digital Input
1111 1111 1111
1000 0000 0000
0000 0000 0001
0000 0000 0000
Stability
In the current-to-voltage (I-to-V) configuration, the I
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 excessive parasitic capacitance exists 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
applications circuit.
An optional compensation capacitor (C1) can be added in
parallel with R
Figure 39. Too small a value for 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.
= 0V WITH CODE 10000000 LOADED TO DAC.
10kΩ
R4
FB
20kΩ
A2
for stability, as shown in Figure 38 and
R5
V
OUT
= –V
REF
Analog Output (V)
+V
−V
−V
0
TO +V
REF
REF
REF
(2047/2048)
(2047/2048)
(0/2048)
REF
OUT
1of the