Datasheet

V
REF
R
1
1 Q
2
NQ
1
R
2
Reference
Disable
DAC7568
DAC8168
DAC8568
SBAS430D –JANUARY 2009–REVISED MAY 2012 www.ti.com
Enable/Disable Internal Reference
INTERNAL REFERENCE
The internal reference in the DAC7568, DAC8168,
The DAC7568, DAC8168, and DAC8568 include a
and DAC8568 is disabled by default for debugging,
2.5V internal reference that is disabled by default.
evaluation purposes, or when using an external
The internal reference is externally available at the
reference. The internal reference can be powered up
V
REF
IN/V
REF
OUT pin. A minimum 100nF capacitor is
and powered down using a serial command that
recommended between the reference output and
requires a 32-bit write sequence (see the Serial
GND for noise filtering.
Interface section), as shown in Table 2 and Table 4.
The internal reference of the DAC7568, DAC8168,
During the time that the internal reference is disabled,
and DAC8568 is a bipolar, transistor-based, precision
the DAC functions normally using an external
bandgap voltage reference. Figure 122 shows the
reference. At this point, the internal reference is
basic bandgap topology. Transistors Q
1
and Q
2
are
disconnected from the V
REF
IN/V
REF
OUT pin (3-state
biased such that the current density of Q
1
is greater
output). Do not attempt to drive the V
REF
IN/V
REF
OUT
than that of Q
2
. The difference of the two base-
pin externally and internally at the same time
emitter voltages (V
BE1
– V
BE2
) has a positive
indefinitely.
temperature coefficient and is forced across resistor
There are two modes that allow communication with
R
1
. This voltage is gained up and added to the base-
the internal reference: Static and Flexible. In Flexible
emitter voltage of Q
2
, which has a negative
mode, DB19 must be set to '1'.
temperature coefficient. The resulting output voltage
is virtually independent of temperature. The short-
Static Mode (see Table 2 and Table 3)
circuit current is limited by design to approximately
100mA.
Enabling Internal Reference:
To enable the internal reference, write the 32-bit
serial command shown in Table 2. When performing
a power cycle to reset the device, the internal
reference is switched off (default mode). In the
default mode, the internal reference is powered down
until a valid write sequence is applied to power up the
internal reference. If the internal reference is powered
up, it automatically powers down when all DACs
power down in any of the power-down modes (see
the Power Down Modes section). The internal
reference automatically powers up when any DAC is
powered up.
Disabling Internal Reference:
To disable the internal reference, write the 32-bit
serial command shown in Table 3. When performing
a power cycle to reset the device, the internal
reference is put back into its default mode and
Figure 122. Bandgap Reference Simplified
switched off (default mode).
Schematic
Table 2. Write Sequence for Enabling Internal Reference (Static Mode)
(Internal Reference Powered On—08000001h)
DB31 DB27 DB23 DB19 DB4 DB0
0 X X X
C3 C2 C1 C0 A3 A2 A1 A0 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 F3 F2 F1 F0
0 X X X 1 0 0 0 X X X X X X X X X X X X X X X X X X X X X X X 1
|-- Prefix Bits --| |- Control Bits -| | Address Bits | |-------------------------------------- Data Bits --------------------------------------| | Feature Bits |
Table 3. Write Sequence for Disabling Internal Reference (Static Mode)
(Internal Reference Powered On—08000000h)
DB31 DB27 DB23 DB19 DB4 DB0
0 X X X
C3 C2 C1 C0 A3 A2 A1 A0 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 F3 F2 F1 F0
0 X X X 1 0 0 0 X X X X X X X X X X X X X X X X X X X X X X X 0
|-- Prefix Bits --| |- Control Bits -| | Address Bits | |-------------------------------------- Data Bits --------------------------------------| | Feature Bits |
34 Submit Documentation Feedback Copyright © 2009–2012, Texas Instruments Incorporated
Product Folder Link(s): DAC7568 DAC8168 DAC8568