ADR430/ADR431/ADR433/ADR434/ADR435/ADR439

Rev. B | Page 16 of 24

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.

INPUT

OUTPUT

TRIM

GND

R1

470k

Ω

R2

10k

Ω (ADR420)

15k

Ω (ADR421)

10k

Ω

ADR43x

Figure 31. Output Trim Adjustment

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.

GND

SOURCE FIBER

GIMBAL + SENSOR

DESTINATION

FIBER

ACTIVATOR

RIGHT

MEMS MIRROR

LASER BEAM

ACTIVATOR

LEFT

AMPL

PREAMP

AMPL

CONTROL

ELECTRONICS

DAC

ADC

DAC

DSP

ADR431

ADR431

ADR431

Figure 32. All-Optical Router Network

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.

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.