ADR423 Analog Devices, ADR423 Datasheet - Page 13

• Manufacturer Part Number: ADR423
• Description: Ultraprecision Low Noise/ 2.048 V/2.500 V/ 3.00 V/5.00 V XFET Voltage References
• Manufacturer: Analog Devices

Reference for Converters in Optical Network Control Circuits
In the upcoming high-capacity, all-optical router network, Figure 4
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 information 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 numbers of converters employed. As a result,
the ADR42x is necessary for this application for its exceptional
low noise to maintain the stability of the control loop.
A 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 the need for an additional
operational amplifier at the output to reinvert the signal. A
negative voltage reference is then desirable from the point that
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 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 that 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 as in Figure 5. V
and, therefore, the negative reference can be taken directly from
the output of the op amp. The op amp must be dual supply, low
offset, and have rail-to-rail capability if negative supply voltage
is close to the reference output.
ELECTRONICS
CONTROL
LASER BEAM
ACTIVATOR
LEFT
SOURCE FIBER
AMPL
DAC
GIMBAL + SENSOR
MEMS MIRROR
PREAMP
ADC
DSP
AMPL
DAC
OUT
ACTIVATOR
RIGHT
DESTINATION
FIBER
is at virtual ground
ADR421
ADR421
ADR421
High-Voltage Floating Current Source
The circuit of Figure 6 can be used to generate a floating current
source with minimal self-heating. This particular configuration
can operate on high supply voltages determined by the breakdown
voltage of the N-channel JFET.
Kelvin Connections
In many portable instrumentation applications, where PC board
cost and area go hand-in-hand, circuit interconnects are very
often of dimensionally minimum width. These narrow lines can
cause large voltage drops if the voltage reference is required to
provide load currents to various functions. In fact, a circuit’s
interconnects can exhibit a typical line resistance of 0.45 mΩ/
square (1 oz. Cu, for example). Force and sense connections,
also referred to as Kelvin connections, offer a convenient method
of eliminating the effects of voltage drops in circuit wires. Load
currents flowing through wiring resistance produce an error
(V
of Figure 7 overcomes the problem by including the wiring
resistance within the forcing loop of the op amp. Since the op
amp senses the load voltage, op amp loop control forces the
output to compensate for the wiring error and to produce the
correct voltage at the load.
ERROR
ADR420/ADR421/ADR423/ADR425
= R × I
ADR42x
ADR42x
L
GND
–V
) at the load. However, the Kelvin connection
GND
V
A1
V
IN
4
2
IN
V
DD
V
OUT
OUT
6
6
A1 = OP777, OP193
V
OUT
OP90
ADR42x
+V
GND
A1 = OP191
A1
V
2
4
DD
IN
V
IN
R
LW
R
LW
+V
S
–V
–V
2N3904
SST111
VISHAY
SENSE
R
V
V
FORCE
S
REF
2.10k
R
OUT
OUT
L
L