ADR431B AD [Analog Devices], ADR431B Datasheet - Page 16

• Manufacturer Part Number: ADR431B
• Description: Ultralow Noise XFET Voltage References with Current Sink and Source Capability
• Manufacturer: AD [Analog Devices]

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 the upcoming high capacity, all-optical router network,
Figure 32 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 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, because total
noise within the system can be multiplied by the number of
converters employed. As a result, to maintain the stability of the
control loop for this application, the ADR43x is necessary due
to its exceptionally low noise.
ADR43x
INPUT
GND
V
IN
TRIM
V
O
Figure 31. Output Trim Adjustment
470kΩ
R1
R2
R
10kΩ
10kΩ (ADR420)
15kΩ (ADR421)
OUTPUT
P
V
O
= ±0.5%
Rev. B | 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
reference voltage, it is often required to reconfigure a current-
switching DAC into a voltage-switching DAC through the use
of a 1.25 V reference, an op amp, and a pair of resistors. Using a
current-switching DAC directly requires an additional opera-
tional amplifier at the output to re-invert the signal. A negative
voltage reference is then desirable from the standpoint that an
additional operational amplifier is not required for either
re-inversion (current-switching mode) or amplification
(voltage-switching mode) of the DAC output voltage. In
general, any positive voltage reference can be converted into a
negative voltage reference through the use of an operational
amplifier and a pair of matched resistors in an inverting
configuration. The disadvantage to 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
precision op amp and configuring it as shown in Figure 33.
V
can be taken directly from the output of the op amp. The op
amp must be dual supply, have low offset and rail-to-rail
capability, if negative supply voltage is close to the reference
output.
OUT
CONTROL
is at virtual ground and, therefore, the negative reference
LASER BEAM
ACTIVATOR
LEFT
SOURCE FIBER
Figure 32. All-Optical Router Network
AMPL
DAC
GIMBAL + SENSOR
MEMS MIRROR
PREAMP
GND
ADC
DSP
AMPL
DAC
ACTIVATOR
RIGHT
DESTINATION
FIBER
ADR431
ADR431
ADR431