AD±±70
APPLICATIONS INFORMATION
TYPICAL OPERATING CIRCUIT
1
2
3
4
5
6
7
8
V
V
REFGND 16
REFIN 15
SS
DD
Figure 38 shows the typical operating circuit for the AD5570.
The only external component needed for this precision 16-bit
DAC is a single external positive reference. Because the device
incorporates reference buffers, it eliminates the need for a
negative reference, external inverters, precision amplifiers, and
resistors. This leads to an overall saving in both cost and board
space.
CLR
REFGND 14
LDAC
SYNC
SCLK
SDIN
SDO
V
13
OUT
AD5570
AGNDS 12
AGND 11
PD 10
2
3
6
OP177*
DGND
9
In the circuit below, VDD and VSS are both connected to 15 V,
but VDD and VSS can operate supplies from +11.ꢁ V to +16.5 V.
In Figure 38, AGNDS is connected to AGND, but the option of
Force/Sense is included on this device, if required by the user.
(OTHER CONNECTIONS OMITTED
FOR CLARITY)
*FOR OPTIMUM SETTLING TIME PERFORMANCE,
THE AD845 IS RECOMMENDED.
Figure 39. Driving AGND and AGNDS Using a Force/Sense Amplifier
0.1µF
–15V
10µF
1
2
3
4
5
6
7
8
V
V
REFGND 16
REFIN 15
There are four possible sources of error to consider when
choosing a voltage reference for high accuracy applications:
initial accuracy, temperature coefficient of the output voltage,
long term drift, and output voltage noise.
SS
DD
+15V
0.1µF
ADR435
10µF
CLR
REFGND 14
LDAC
SYNC
SCLK
SDIN
SDO
V
13
V
LDAC
SYNC
SCLK
SDIN
SDO
OUT
OUT
AD5570
AGNDS 12
AGND 11
PD 10
Initial accuracy on the output voltage of an external reference
could lead to a full-scale error in the DAC. Therefore, to
minimize these errors, a reference with low initial accuracy
specification is preferred. Also, choosing a reference with an
output trim adjustment, such as the ADRꢁꢀ5, allows a system
designer to trim system errors out by setting the reference
voltage to a voltage other than the nominal. The trim adjust-
ment can also be used at temperature to trim out any error.
DGND
9
5kΩ
5V
Figure 38. Typical Operating Circuit
Long term drift (LTD) is a measure of how much the reference
drifts over time. A reference with a tight long-term drift
specification ensures that the overall solution remains relatively
stable over its entire lifetime.
Force/Sense of AGND
Because of the extremely high accuracy of this device, system
design issues such as grounding and contact resistance are very
important. The AD5570, with 10 V output, has an LSB size of
305 µV. Therefore, series wiring and connector resistances of
very small values could cause voltage drops of an LSB. For this
reason, the AD5570 offers a Force/Sense output configuration.
The temperature coefficient of a reference’s output voltage
affects INL, DNL, and TUE. A reference with a tight tempera-
ture coefficient specification should be chosen to reduce the
dependence of the DAC output voltage on ambient conditions.
Figure 39 shows how to connect the AD5570 to the Force/Sense
amplifier. Where accuracy of the output is important, an ampli-
fier such as the OP177 is ideal. The OP177 is ultraprecise with
offset voltages of 10 µV maximum at room temperature, and
offset drift of 0.1 µV/°C maximum. Alternative recommended
amplifiers are the OP1177 and the OP77. For applications where
optimization of the circuit for settling time is needed, the
AD8ꢁ5 is recommended.
In high accuracy applications, which have a relatively low noise
budget, reference output voltage noise needs to be considered.
Choosing a reference with as low an output noise voltage as
practical for the system resolution required is important. Preci-
sion voltage references such as the ADRꢁ35 (XFET design)
produce low output noise in the 0.1 Hz to 10 Hz region.
However, as the circuit bandwidth increases, filtering the output
of the reference may be required to minimize the output noise.
Precision Voltage Reference Selection
To achieve the optimum performance from the AD5570,
thought should be given to the selection of a precision voltage
reference. The AD5570 has just one reference input, REFIN.
This voltage on REFIN is used to provide a buffered positive
and negative reference for the DAC core. Therefore, any error in
the voltage reference is reflected in the output of the device.
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