Datasheet

AD5735 Data Sheet
Rev. C | Page 28 of 48
THEORY OF OPERATION
The AD5735 is a quad, precision digital-to-current loop and
voltage output converter designed to meet the requirements of
industrial process control applications. It provides a high precision,
fully integrated, low cost, single-chip solution for generating
current loop and unipolar/bipolar voltage outputs.
The current ranges available are 0 mA to 20 mA, 4 mA to 20 mA,
and 0 mA to 24 mA. The voltage ranges available are 0 V t o 5 V,
±5 V, 0 V to 10 V, and ±10 V. The current and voltage outputs
are available on separate pins, and only one output is active at
any one time. The output configuration is user-selectable via the
DAC control register.
On-chip dynamic power control minimizes package power
dissipation in current mode (see the Dynamic Power Control
section).
DAC ARCHITECTURE
The DAC core architecture of the AD5735 consists of two
matched DAC sections. A simplified circuit diagram is shown
in Figure 69. The four MSBs of the 12-bit data-word are decoded
to drive 15 switches, E1 to E15. Each switch connects one of
15 matched resistors either to ground or to the reference buffer
output. The remaining eight bits of the data-word drive Switch S0
to Switch S7 of an 8-bit voltage mode R-2R ladder network.
8-BIT R-2R LADDER FOUR MSBs DECODED INTO
15 EQUAL SEGMENTS
2R
2R
S0 S1
S7 E1 E2 E15
V
OUT
2R 2R 2R 2R 2R
09961-069
Figure 69. DAC Ladder Structure
The voltage output from the DAC core can be
• Buffered and scaled to output a software selectable
unipolar or bipolar voltage range (see Figure 70)
• Converted to a current, which is then mirrored to the
supply rail so that the application sees only a current
source output (see Figure 71)
Both the voltage and current outputs are supplied by V
BOOST_x
.
The current and voltage are output on separate pins and cannot
be output simultaneously. The current and voltage output pins
of a channel can be tied together (see the Voltage and Current
Output Pins on the Same Terminal section).
RANGE
SCALING
12-BIT
DAC
V
OUT_X
SHORT FAULT
+V
SENSE_X
–V
SENSE_X
V
OUT_X
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Figure 70. Voltage Output
12-BIT
DAC
V
BOOST_x
R2
T2
T1
R3
I
OUT_x
R
SET
A1
A2
09961-071
Figure 71. Voltage-to-Current Conversion Circuitry
Voltage Output Amplifier
The voltage output amplifier is capable of generating both
unipolar and bipolar output voltages. It is capable of driving a
load of 1 kΩ in parallel with 1 µF (with an external compen-
sation capacitor) to AGND. The source and sink capabilities
of the output amplifier are shown in Figure 22. The slew rate is
1.9 V/µs with a full-scale settling time of 18 µs max (10 V step).
If remote sensing of the load is not required, connect +V
SENSE_x
directly to V
OUT_x
, and connect −V
SENSE_x
directly to AGND.
−V
SENSE_x
must stay within ±3.0 V of AGND for specified opera-
tion. The difference in voltage between +V
SENSE_x
and V
OUT_x
should be added directly to the headroom requirement.
Driving Large Capacitive Loads
The voltage output amplifier is capable of driving capacitive
loads of up to 2 µF with the addition of a 220 pF, nonpolarized
compensation capacitor on each channel. The 220 pF capacitor
is connected between the COMP
LV_x
pin and the V
OUT_x
pin.
Care should be taken to choose an appropriate value of com-
pensation capacitor. This capacitor, while allowing the AD5735
to drive higher capacitive loads and reduce overshoot, increases
the settling time of the part and, therefore, affects the bandwidth
of the system. Without the compensation capacitor, capacitive
loads of up to 10 nF can be driven.
Reference Buffers
The AD5735 can operate with either an external or internal
reference. The reference input requires a 5 V reference for
specified performance. This input voltage is then buffered
before it is applied to the DAC.