AD7801 Analog Devices, AD7801 Datasheet - Page 9

• Manufacturer Part Number: AD7801
• Description: +2.7 V to +5.5 V, Parallel Input, Voltage Output 8-Bit DAC
• Manufacturer: Analog Devices

Specifications of AD7801

• Resolution (bits): 8bit
• Dac Update Rate: 833kSPS
• Dac Settling Time: 1.2µs
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Voltage Out
• Dac Input Format: Par

POWER-ON RESET
The AD7801 has a power-on reset circuit designed to allow
output stability during power up. This circuit holds the DAC in
a reset state until a write takes place to the DAC. In the reset
state all zeros are latched into the input register of the DAC and
the DAC register is in transparent mode thus the output of the
DAC is held at ground potential until a write takes place to the
DAC. The power-on reset circuitry generates a PON STRB
signal which is a gating signal used within the logic to identify
a power-on condition.
POWER-DOWN FEATURES
The AD7801 has a power-down feature implemented by
exercising the external PD pin. An active low signal puts the
complete DAC into power-down mode. When in power-down,
the current consumption of the device is reduced to less than
1 A max at +25 C or 2 A max over temperature, making the
device suitable for use in portable battery powered equipment.
The internal reference resistors, the reference bias servo loop,
the output amplifier and associated linear circuitry are all shut
down when the power-down is activated. The output terminal
sees a load of
shown in Figure 25. The contents of the data register are
unaffected when in power-down mode. The device typically
comes out of power-down in 13 s (see Figure 10).
Analog Outputs
The AD7801 contains a voltage output DAC with 8-bit resolution
and rail-to-rail operation. The output buffer provides a gain of
two at the output. Figures 2, 3 and 4 show the source and sink
capabilities of the output amplifier. The slew rate of the output
amplifier is typically 7.5 V/ s and has a full-scale settling to
eight bits with a 100 pF capacitive load in typically 1.2 s.
The input coding to the DAC is straight binary. Table I shows
the binary transfer function for the AD7801. Figure 26 shows
the DAC transfer function for binary coding. Any DAC output
voltage can be expressed as:
REV. 0
Figure 25. Output Stage During Power-Down
23 k to GND when in power-down mode as
I
DAC
V
REF
11.7k
11.7k
V
DD
–9–
where:
N
V
REF
is the decimal equivalent of the binary input
code. N ranges from 0 to 255.
is the voltage applied to the external REFIN pin
when the external reference is selected and is V
if the internal reference is used.
Table I. Output Voltage for Selected Input Codes
Digital
MSB . . . LSB
1111 1111
1111 1110
1000 0001
1000 0000
0111 1111
0000 0001
0000 0000
DAC INPUT CODE
Figure 26. DAC Transfer Function
2V
V
REF
REF
V
0
O UT
00 01
2 V
REF
Analog Output
V
0 V
2
2
2
2
2
7F 80 81
REF
V
256
255
256
254
256
129
256
127
256
256
N
V
REF
V
V
V
V
V
REF
REF
REF
REF
AD7801
V
V
V
V
FE FF
DD
/2