APPLICATIONS NOTES:
The ALD1801 is an 8 bit multiplying D/A converter. It has
been designed to operate with standard single or split
power supplies of 5V or
±5V.
Functionality extends down
to 3V or
±1.5V
power supply, making it ideal for lithium
battery or rechargeable battery operated systems where
power efficiency and performance are important design
parameters.
The ALD1801 features ultra low quiescent bias current
which depends on the operating current in the internal
current steering circuit. A high gain internal operational
amplifier is fully compensated for stable operation without
any need for external capacitor compensation. The
operational amplifier is placed in a negative feedback loop
to generate the reference current for ratio matched
transistors. It is trimmed to provide nonlinearity as
specified. In some applications this feature can be used
along with the multiplying nature of the ALD1801 to obtain
9 or 10 bit digital to analog conversion with external
switches.
The ALD1801 is a multiplying D/A converter in which the
output is a current that is a function of an 8 bit digital input
word multiplied by the reference current. The reference
current may be a fixed reference current or a varying input
current. If a fixed reference voltage or a varying low
impedance voltage output source is used, a fixed value
resistor (referred to as RREF) can be used to convert
the voltage into a reference current for most applications.
R
REF
is a precision fixed resistor without trimming.
Trimming can be accomplished by adjusting R
REF
through
either connecting a potentiometer in series with a fixed
resistor or by substituting the fixed resistor with a multi-
turn potentiometer. The recommended nominal value of
R
REF
is 25KΩ.
The ALD1801 can operate from single power supply or
dual supplies up to
±6.6V.
Symmetrical supplies are not
required in the case of dual supply operation. However,
V
REF-
input voltage must be set at 1V or higher above the
V- voltage. Due to the internal operational amplifier
feedback, V
REF+
input terminal would be set at the same
potential as V
REF
- input terminal, with the input reference
current determined by: Input reference current
I
REF
= { (V
REF
+
)- (V
REF
-
) } / R
REF
The ALD1801 is fabricated in CMOS technology and
provides direct logic interface to all CMOS logic families
including logic levels of the CD4000 and 74C logic families
operating at logic voltage levels other than 5V. It is
primarily designed to operate in a power supply
environment where interface takes place between digital
logic circuitry and analog circuitry. Both types of circuitry
operate with the same power supplies.
The nonlinearity and monotonicity of the ALD1801 applies
over the full rated operating temperature. The device has
been developed with minimal temperature drift, typically
at
±10ppm/°C.
A single array of matched and tracked
resistor networks of nominal 50KΩ can be used for all the
resistors required for the D/A converter. As an example,
R
REF
can be set to 25KΩ by connecting two 50KΩ resistors
in parallel where output resistors are set at 50KΩ.
Due to the very low quiescent and reference currents
used, layout of the circuit board to minimize any parasitic
capacitance is important in getting high frequency
operation, as the settling time is due in large part to the
output currents charging and discharging the node
capacitances at the output. As the internal bias current
levels is adaptive to the reference current, the switching
speed of the device depends on the reference current
selected. Characterization of the settling time has been
performed at full scale current set at 100µA and device
operation at single 5V supply. Faster operation can be
obtained by using short leads, minimizing output
capacitance, load resistor values, and by adequate bypass
capacitors placed on the board at the supply and reference
nodes.
Both
I
o
and
I
o
outputs can be used simultaneously. If one
of the outputs is not used, then it must be connected to
ground or a potential where the sum of
I
o
and
I
o
can
maintain a constant value. Due to internal leakage currents
that vary with temperature, it is recommended that I
FS
of
50µA to 100µA be used so that 1/2LSB bit current is set at
as high a level as possible. The temperature coefficient
of the reference resistor R
REF
and output resistors R
OUT
and R
OUT
should match to minimize temperature drift.
ALD1801A/ALD1801B
ALD1801
Advanced Linear Devices
5
ALD [ ADVANCED LINEAR DEVICES ]
