Datasheet

AD5425 Data Sheet
Rev. C | Page 14 of 24
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
FB
, has a
value of R. The value of R is typically 10 kΩ� (minimum 8 kΩ
and maximum 12 kΩ). If I
OUT
1 and I
OUT
2 are kept at the same
potential, a constant current flows in each ladder leg, regardless
of digital input code. Therefore, the input resistance presented
at V
REF
is always constant and nominally of value R. The DAC
output, I
OUT
, is code-dependent, producing various resistances
and capacitances. When choosing the external amplifier, take
into account the variation in impedance generated by the DAC
on the amplifiers inverting input node.
Figure 29. Simplified Ladder
Access is provided to the V
REF
, R
FB
, I
OUT
1, and I
OUT
2 terminals
of 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, 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
FB
feedback resistor. If users attempt to measure R
FB
, power must
be applied to V
DD
to 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
n
REF
OUT
D
VV
2
×−=
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
REF
polarity for dc reference voltages.
This DAC is designed to operate with either negative or positive
reference voltages. The V
DD
power pin is used by only the inter-
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
IN
is an ac
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 Analog Output (V)
1111 1111 −V
REF
(255/256)
1000 0000
−V
REF
(128/256) = −V
REF
/2
0000 0001 −V
REF
(1/256)
0000 0000 −V
REF
(0/256) = 0
Figure 30. Unipolar Operation
V
REF
I
OUT
2
DAC DATA LATCHES
AND DRIVERS
I
OUT
1
R
FB
2R
S1
2R
S2
2R
S3
2R 2R
S8
R
RRR
03161-029
V
OUT
= 0
TO –V
REF
SCLK SDIN GND
V
REF
SYNC
I
OUT
2
I
OUT
1
R
FB
AGND
AD5425
R1
R2
A1
V
REF
V
DD
V
DD
C1
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.
MICROCONTROLLER
03161-030
A1