REV. B

ADR420/ADR421/ADR423/ADR425

–13–

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.

CONTROL

ELECTRONICS

PREAMP

AMPL

AMPL

ADR421

ADR421

ADR421

DAC

DAC

ADC

DSP

MEMS MIRROR

ACTIVATOR

RIGHT

ACTIVATOR

LEFT

GIMBAL + SENSOR

SOURCE FIBER

LASER BEAM

DESTINATION

FIBER

Figure 4. All-Optical Router Network

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

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.

GND

ADR42x

A1 = OP777, OP193

A1

4

6

2

Figure 5. Negative Reference

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.

GND

ADR42x

2.10k

2N3904

SST111

VISHAY

OP90

Figure 6. High-Voltage Floating Current Source

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 m

Ω/

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

× I

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.

GND

ADR42x

FORCE

A1

SENSE

A1 = OP191

2

6

4

Figure 7. Advantage of Kelvin Connection