ADR430 Analog Devices, Inc., ADR430 Datasheet - Page 16

• Manufacturer Part Number: ADR430
• Description: Ultralow Noise Xfet Voltage References With Current Sink And Source Capability
• Manufacturer: Analog Devices, Inc.

ADR430/ADR431/ADR433/ADR434/ADR435/ADR439
APPLICATIONS
OUTPUT ADJUSTMENT
The ADR43x trim terminal can be used to adjust the output
voltage over a ±0.5% range. This feature allows the system
designer to trim system errors out by setting the reference to a
voltage other than the nominal. This is also helpful if the part is
used in a system at temperature to trim out any error. Adjustment
of the output has negligible effect on the temperature perform-
ance of the device. To avoid degrading temperature coefficients,
both the trimming potentiometer and the two resistors need to
be low temperature coefficient types, preferably <100 ppm/°C.
REFERENCE FOR CONVERTERS IN OPTICAL
NETWORK CONTROL CIRCUITS
In Figure 32, the high capacity, all-optical router network
employs arrays of micromirrors to direct and route optical
signals from fiber to fiber without first converting them to
electrical form, which reduces the communication speed. The
tiny micromechanical mirrors are positioned so that each is
illuminated by a single wavelength that carries unique informa-
tion and can be passed to any desired input and output fiber.
The mirrors are tilted by the dual-axis actuators, which are
controlled by precision ADCs and DACs within the system.
Due to the microscopic movement of the mirrors, not only is
the precision of the converters important but the noise
associated with these controlling converters is also extremely
critical. Total noise within the system can be multiplied by the
number of converters employed. Therefore, to maintain the
stability of the control loop for this application, the ADR43x,
with its exceptionally low noise, is necessary.
ADR43x
INPUT
GND
V
IN
V
TRIM
OUT
Figure 31. Output Trim Adjustment
470kΩ
R1
R2
R
10kΩ
10kΩ (ADR430)
15kΩ (ADR431)
OUTPUT
P
V
O
= ±0.5%
Rev. D | Page 16 of 24
ELECTRONICS
NEGATIVE PRECISION REFERENCE WITHOUT
PRECISION RESISTORS
In many current-output CMOS DAC applications, where the
output signal voltage must be of the same polarity as the refer-
ence voltage, it is required to reconfigure a current-switching
DAC into a voltage-switching DAC through the use of a 1.25 V
reference, an operational amplifier, and a pair of resistors.
Using a current-switching DAC directly requires an additional
operational amplifier at the output to reinvert the signal. A
negative voltage reference is desirable because an additional
operational amplifier is not required for either reinversion
(current-switching mode) or amplification (voltage-switching
mode) of the DAC output voltage. In general, any positive
voltage reference can be converted to a negative voltage
reference through the use of an operational amplifier and a
pair of matched resistors in an inverting configuration. The
disadvantage of this approach is that the largest single source of
error in the circuit is the relative matching of the resistors used.
A negative reference can easily be generated by adding a preci-
sion operational amplifier, such as the OP777 or the OP193, and
configuring it as shown in Figure 33. V
therefore, the negative reference can be taken directly from the
output of the amplifier. The operational amplifier must be dual
supply and have low offset and rail-to-rail capability if negative
supply voltage is close to the reference output.
CONTROL
LASER BEAM
ACTIVATOR
LEFT
SOURCE FIBER
Figure 32. All-Optical Router Network
AMPL
DAC
GIMBAL + SENSOR
MEMS MIRROR
PREAMP
ADC
GND
DSP
AMPL
DAC
OUT
is at virtual ground;
ACTIVATOR
RIGHT
DESTINATION
FIBER
ADR431
ADR431
ADR431