Datasheet

REF_MAX REF_MIN
6
REF RANGE
V V
Drift Error = 10 (ppm/°C)
V T
-
æ ö
´
ç ÷
´
è ø
Output Pin
Meter
Load
Force Line
Sense Line
Contact and
Trace Resistance
I
L
V
OUT
5
6
7
8
12
11
10
9
V D
OUT
V F
OUT
V H
OUT
CLR
V C
OUT
V E
OUT
V G
OUT
V /V
REFIN REFOUT
AV
DD
DAC7678
1
2
3
4
16
15
14
13
SCL
SDA
GND
V B
OUT
LDAC
ADDR0
AV
DD
V A
OUT
1 Fm
100nF
DAC7678
SBAS493B –FEBRUARY 2010–REVISED JULY 2012 www.ti.com
APPLICATION INFORMATION
INTERNAL REFERENCE Where:
The internal reference of the DAC7678 does not V
REF_MAX
= maximum reference voltage observed
require an external load capacitor for stability within temperature range T
RANGE
.
because it is stable with any capacitive load.
V
REF_MIN
= minimum reference voltage observed
However, for improved noise performance, an
within temperature range T
RANGE
.
external load capacitor of 100nF or larger connected
V
REF
= 2.5V, target value for reference output
to the V
REFIN
/V
REFOUT
output is recommended.
voltage.
Figure 94 shows the typical connections required for
The internal reference features an exceptional
operation of the DAC7678 internal reference. A
maximum drift coefficient of 25ppm/°C from –40°C to
supply bypass capacitor at the AV
DD
input is also
125°C. Temperature drift results are summarized in
recommended.
the Typical Characteristics.
Noise Performance
Typical 0.1Hz to 10Hz voltage noise can be seen in
Figure 7, Internal Reference Noise. Additional filtering
can be used to improve output noise levels, although
care should be taken to ensure the output impedance
does not degrade the ac performance. The output
noise spectrum at V
REFIN
/V
REFOUT
without any
external components is depicted in Figure 6, Internal
Reference Noise Density vs Frequency. Internal
reference noise impacts the DAC output noise; see
the DAC Noise Performance section for more details.
Load Regulation
Load regulation is defined as the change in reference
output voltage as a result of changes in load current.
Figure 94. Typical Connections for Operating the
The load regulation of the internal reference is
DAC7678 Internal Reference
measured using force and sense contacts as shown
in Figure 5. The force and sense lines reduce the
impact of contact and trace resistance, resulting in
Supply Voltage
accurate measurement of the load regulation
The internal reference features an extremely low
contributed solely by the internal reference.
dropout voltage. It can be operated with a supply of
Measurement results are summarized in the Typical
only 5mV above the reference output voltage in an
Characteristics. Force and sense lines should be
unloaded condition. For loaded conditions, refer to
used for applications that require improved load
the Load Regulation section. The stability of the
regulation.
internal reference with variations in supply voltage
(line regulation, dc PSRR) is also exceptional. Within
the specified supply voltage range of 2.7V to 5.5V,
the variation at V
REFIN
/V
REFOUT
is less than 100 µV/V;
see the Typical Characteristics.
Temperature Drift
The internal reference is designed to exhibit minimal
drift error, defined as the change in reference output
voltage over varying temperature. The drift is
calculated using the box method described by
Equation 3:
Figure 95. Accurate Load Regulation of the
DAC7678 Internal Reference
(3)
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