AD5445 Analog Devices, AD5445 Datasheet - Page 18

• Manufacturer Part Number: AD5445
• Description: High Bandwidth CMOS 12-Bit Parallel Interface Multiplying D/A Converter
• Manufacturer: Analog Devices

Specifications of AD5445

• Resolution (bits): 12bit
• Dac Update Rate: 20.4MSPS
• Dac Settling Time: 80ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

AD5424/AD5433/AD5445
THEORY OF OPERATION
The AD5424, AD5433, and
current output DACs consisting of a standard inverting R-2R
ladder configuration. A simplified diagram for the 8-bit
shown in Figure 48. The matching feedback resistor R
value of R. The value of R is typically 10 kΩ (minimum 8 kΩ
and maximum 12 kΩ). If I
potential, a constant current flows in each ladder leg, regardless
of digital input code. Therefore, the input resistance presented
at V
The DAC output (I
resistances and capacitances. External amplifier choice should
take into account the variation in impedance generated by the
DAC on the amplifiers inverting input node.
Access is provided to the V
the DAC, making the device extremely versatile and allowing it
to be configured in several different operating modes, for example,
to provide a unipolar output, 4-quadrant multiplication in bipolar
mode or in single-supply modes of operation. Note that a matching
switch is used in series with the internal R
users attempt to measure R
achieve continuity.
CIRCUIT OPERATION
Unipolar Mode
Using a single op amp, these devices can easily be configured to
provide 2-quadrant multiplying operation or a unipolar output
voltage swing, as shown in Figure 49.
When an output amplifier is connected in unipolar mode, the
output voltage is given by
REF
V
OUT
is always constant and nominally of resistance value R.
V
REF
=
−
V
REF
2R
S1
DAC DATA LATCHES
R
OUT
×
AND DRIVERS
Figure 48. Simplified Ladder
2
D
2R
S2
) is code-dependent, producing various
n
R
OUT
REF
FB
S3
2R
AD5445
, power must be applied to V
1 and I
, R
R
FB
, I
S8
2R
OUT
OUT
are 8-, 10-, and 12-bit
1, and I
2 are kept at the same
2R
FB
R
feedback resistor. If
OUT
R
I
I
OUT
OUT
FB
2 terminals of
A
1
2
AD5424
FB
has a
DD
to
Rev. C | Page 18 of 28
is
where D is the fractional representation of the digital word loaded
to the DAC and n is the resolution of the DAC.
Note that the output voltage polarity is opposite to the V
polarity for dc reference voltages.
These DACs are designed to operate with either negative or positive
reference voltages. The V
digital logic to drive the DAC switches’ on and off states.
These DACs are also designed to accommodate ac reference
input signals in the range of –10 V to +10 V.
With a fixed 10 V reference, the circuit shown in Figure 49 gives
a unipolar 0 V to –10 V output voltage swing. When V
signal, the circuit performs 2-quadrant multiplication.
Table 7 shows the relationship between digital code and expected
output voltage for unipolar operation (AD5424, 8-bit device).
Table 7. Unipolar Code Table
Digital Input
1111 1111
1000 0000
0000 0001
0000 0000
V
REF
D = 0 to 255 (8-bit AD5424)
NOTES:
1.
2.
R1
= 0 to 1023 (10-bit AD5433)
= 0 to 4095 (12-bit AD5445)
R1 AND R2 USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
IF A1 IS A HIGH SPEED AMPLIFIER.
V
INPUTS
DATA
REF
AD5424/
AD5433/
AD5445
R/W
Figure 49. Unipolar Operation
V
V
DD
DD
CS
DD
power pin is only used by the internal
R
GND
FB
I
I
OUT
OUT
Analog Output (V)
–V
–V
V
V
1
2
REF
REF
R2
REF
REF
(1/256)
(0/256) = 0
(255/256)
(128/256) = –V
C1
AGND
A1
Data Sheet
V
0 TO –V
REF
OUT
IN
/2
=
REF
is an ac
REF