EVAL-AD5425EBZ Analog Devices Inc, EVAL-AD5425EBZ Datasheet - Page 14

• Manufacturer Part Number: EVAL-AD5425EBZ
• Description: BOARD EVALUATION FOR AD5425
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5425EBZ

• Number Of Dac's: 1
• Number Of Bits: 8
• Outputs And Type: 1, Differential
• Sampling Rate (per Second): 2.47M
• Data Interface: Serial
• Settling Time: 15ns
• Dac Type: Current
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 125°C
• Utilized Ic / Part: AD5425
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD5425
THEORY OF OPERATION
The AD5425 is an 8-bit current output DAC consisting of a
standard inverting R-2R ladder configuration. A simplified
diagram is shown in Figure 29. The 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
output, I
and capacitances. When choosing the external amplifier, 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 exam-
ple, to provide a unipolar output, bipolar output, or in single-
supply modes of operation in unipolar mode or 4-quadrant
multiplication in bipolar mode. Note that a matching switch is
used in series with the internal R
attempt to measure R
achieve continuity.
CIRCUIT OPERATION
Unipolar Mode
Using a single op amp, this device can easily be configured to
provide 2-quadrant multiplying operation or a unipolar output
voltage swing, as shown in Figure 30.
When an output amplifier is connected in unipolar mode, the
output voltage is given by
REF
V
V
REF
OUT
is always constant and nominally of value R. The DAC
OUT
=
, is code-dependent, producing various resistances
−
S1
2R
V
R
DAC DATA LATCHES
REF
AND DRIVERS
S2
Figure 29. Simplified Ladder
2R
×
FB
R
2
, power must be applied to V
D
n
OUT
S3
2R
REF
1 and I
, R
R
FB
FB
, I
feedback resistor. If users
S8
2R
OUT
OUT
1, and I
2 are kept at the same
2R
R
OUT
2 terminals of
R
I
I
OUT
OUT
DD
FB
1
2
to
FB
, has a
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where D is the fractional representation of the digital word
loaded to the DAC, in this case 0 to 255, and n is the number
of bits.
Note that the output voltage polarity is opposite to the V
polarity for dc reference voltages.
This DAC is designed to operate with either negative or positive
reference voltages. The V
nal digital logic to drive the DAC switches’ on and off states.
This DAC is 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 30 gives
a unipolar 0 V to −10 V output voltage swing. When V
signal, the circuit performs 2-quadrant multiplication.
Table 5 shows the relationship between digital code and the
expected output voltage for unipolar operation.
Table 5. Unipolar Code Table
Digital Input
1111 1111
1000 0000
0000 0001
0000 0000
V
REF
NOTES:
1. R1 AND R2 USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
IF A1 IS A HIGH SPEED AMPLIFIER.
R1
MICROCONTROLLER
SYNC
V
REF
SCLK SDIN GND
Figure 30. Unipolar Operation
AD5425
V
V
Analog Output (V)
−V
−V
−V
−V
DD
DD
DD
REF
REF
REF
REF
power pin is used by only the inter-
(255/256)
(128/256) = −V
(1/256)
(0/256) = 0
R
FB
I
I
OUT
OUT
R2
1
2
C1
AGND
REF
/2
A1
A1
V
TO –V
OUT
IN
= 0
REF
REF
is an ac