Datasheet

AD5760 Data Sheet
Rev. D | Page 24 of 28
AD5760 FEATURES
POWER-ON TO 0 V
The AD5760 contains a power-on reset circuit that, as well as
resetting all registers to their default values, controls the output
voltage during power-up. Upon power-on, the DAC is placed in
tristate (its reference inputs are disconnected), and its output is
clamped to AGND through a ~6 kΩ resistor. The DAC remains
in this state until programmed otherwise via the control register.
This is a useful feature in applications where it is important to
know the state of the DAC output while it is in the process of
powering up.
CONFIGURING THE AD5760
After power-on, the AD5760 must be configured to put it into
normal operating mode before programming the output. To
do this, the control register must be programmed. The DAC
is removed from tristate by clearing the DACTRI bit, and the
output clamp is removed by clearing the OPGND bit. At this
point, the output goes to V
REFN
unless an alternative value is
first programmed to the DAC register.
DAC OUTPUT STATE
The DAC output can be placed in one of three states, controlled
by the DACTRI and OPGND bits of the control register, as
shown in Table 15.
Table 15. Output State Truth Table
DACTRI OPGND Output State
0 0 Normal operating mode.
0 1 Output is clamped via ~6 kΩ to AGND.
1 0 Output is in tristate.
1 1 Output is clamped via ~6 kΩ to AGND.
OUTPUT AMPLIFIER CONFIGURATION
There are a number of different ways that an output amplifier
can be connected to the AD5760, depending on the voltage
references applied and the desired output voltage span.
Unity-Gain Configuration
Figure 50 shows an output amplifier configured for unity gain.
In this configuration, the output spans from V
REFN
to V
REFP
.
A1
6.8kΩ
6.8kΩ
R1 R
FB
V
REFP
R
FB
INV
V
OUT
V
OUT
16-BIT
DAC
V
REFN
AD5760
AD8675
ADA4898-1
ADA4004-1
09650-052
Figure 50. Output Amplifier in Unity-Gain Configuration
A second unity-gain configuration for the output amplifier is
one that removes an offset from the input bias currents of the
amplifier. It does this by inserting a resistance in the feedback
path of the amplifier that is equal to the output resistance of the
DAC. The DAC output resistance is 3.4 kΩ. By connecting R1
and R
FB
in parallel, a resistance equal to the DAC resistance is
available on chip. Because the resistors are all on one piece of
silicon, they are temperature coefficient matched. To enable this
mode of operation, the RBUF bit of the control register must be
set to Logic 1. Figure 51 shows how the output amplifier is
connected to the AD5760. In this configuration, the output
amplifier is in unity gain, and the output spans from V
REFN
to
V
REFP
. This unity-gain configuration allows a capacitor to be
placed in the amplifier feedback path to improve dynamic
performance.
V
REFP
R
FB
INV
V
OUT
V
OUT
10pF
16-BIT
DAC
V
REFN
AD5760
AD8675
ADA4898-1
ADA4004-1
R
FB
6.8kΩ
R1
6.8kΩ
09650-053
Figure 51. Output Amplifier in Unity-Gain with Amplifier Input Bias Current
Compensation