AD7849
CIRCUIT DESCRIPTION
D/A CONVERSION
R
OFS
R
10kΩ
R
10kΩ
C1
G3
G1
V
OUT
DAC 3
ONE-SHOT
LOGIC
CIRCUITRY
G2
RSTIN
Figure 10 shows the D/A section of the AD7849. There are
three on-chip DACs each of which has its own buffer amplifier.
DAC1 and DAC2 are 4-bit DACs. They share a 16-resistor
string but have their own analog multiplexers. The voltage ref-
erence is applied to the resistor string. DAC3 is a 12-bit voltage
mode DAC with its own output stage.
The 4 MSBs of the 16-bit digital input code drive DAC1 and
DAC2 while the 12 LSBs control DAC3. Using DAC1 and
DAC2, the MSBs select a pair of adjacent nodes on the resistor
string and present that voltage to the positive and negative
inputs of DAC3. This DAC interpolates between these two
voltages to produce the analog output voltage.
To prevent nonmonotonicity in the DAC due to amplifier offset
voltages, DAC1 and DAC2 “leap-frog” along the resistor string.
For example, when switching from Segment 1 to Segment 2,
DAC1 switches from the bottom of Segment 1 to the top of
Segment 2 while DAC 2 remains connected to the top of Seg-
ment 1. The code driving DAC3 is automatically comple-
mented to compensate for the inversion of its inputs. This
means that any linearity effects due to amplifier offset voltages
remain unchanged when switching from one segment to the
next and 16-bit monotonicity is ensured if DAC3 is monotonic.
So, 12-bit resistor matching in DAC3 guarantees overall 16-bit
monotonicity. This is much more achievable than the 16-bit
matching which a conventional R-2R structure would have
needed.
Output Stage
LDAC
VOLTAGE
MONITOR
AGND
RSTOUT
Figure 11. AD7849 Output Stage
When the supply voltages are changing, the V
OUT
pin is clamped
to 0 V via a low impedance path . To prevent the output of A3
being shorted to 0 V during this time, transmission gate G1 is
also opened. These conditions are maintained until the power
supplies stabilize and a valid word is written to the DAC regis-
ter. At this time, G2 opens and G1 closes. Both transmission
gates are also externally controllable via the Reset In (RST
IN)
control input. For instance, if the
RST IN
input is driven from a
battery supervisor chip, then on power-off or during a brown-
out, the
RST IN
input will be driven low to open G1 and close
G2. The DAC has to be reloaded, with
RST IN
high, to re-en-
able the output. Conversely, the on-chip voltage detector out-
put (RST
OUT)
is also available to the user to control other
parts of the system.
The AD7849 output buffer is configured as a track-and-hold
amplifier. Although normally tracking its input, this amplifier is
placed in a hold mode for approximately 5
µs
after the leading
edge of
LDAC.
This short state keeps the DAC output at its
previous voltage while the AD7849 is internally changing to its
new value. So, any glitches that occur in the transition are not
seen at the output. In systems where the
LDAC
is permanently
low, the deglitching will not be in operation.
The output stage of the AD7849 is shown in Figure 11. It is ca-
pable of driving a load of 2 kΩ in parallel with 200 pF. The
feedback and offset resistors allow the output stage to be config-
ured for gains of 1 or 2. Additionally, the offset resistor may be
used to shift the output range.
The AD7849 has a special feature to ensure output stability
during power-up and power-down sequences. This is specifi-
cally available for control applications where actuators must not
be allowed to move in an uncontrolled fashion.
V
REF+
R
DAC 1
S1
R
R
DAC 2
S2
S4
A1
S3
DAC 3
10-BIT/12-BIT
DAC
OUTPUT
STAGE
S15
S17
R
R
DB15–DB12
V
REF –
R
S14
S16
10/12
DB15–DB12
A2
Figure 10. AD7849 D/A Conversion
–8–
REV. B
AD [ ANALOG DEVICES ]
