ADR293

REV. 0

–8–

Device Power Dissipation Considerations

The ADR293 is guaranteed to deliver load currents to 5 mA

with an input voltage that ranges from 5.5 V to 15 V. When this

device is used in applications with large input voltages, care

should be exercised to avoid exceeding the published specifica-

tions for maximum power dissipation or junction temperature

that could result in premature device failure. The following

formula should be used to calculate a device’s maximum junc-

tion temperature or dissipation:

P

TT

D

A

A

=

−

J

J

θ

θ

Basic Voltage Reference Connections

References, in general, require a bypass capacitor connected

illustrates the basic configuration for the ADR293. Note that

the decoupling capacitors are not required for circuit stability.

ADR293

1

2

3

4

8

7

6

5

NC

NC

NC

NC

NC

OUTPUT

0.1 F

10 F

0.1 F

+

INPUT

NC = NO CONNECT

Figure 19. Basic Voltage Reference Configuration

Noise Performance

The noise generated by the ADR293 is typically less than

15

µVp-p over the 0.1 Hz to 10 Hz band. The noise measure-

ment is made with a bandpass filter made of a 2-pole high-pass

filter with a corner frequency at 0.1 Hz and a 2-pole low-pass

filter with a corner frequency at 10 Hz.

Turn-On Time

Upon application of power (cold start), the time required for the

output voltage to reach its final value within a specified error

band is defined as the turn-on settling time. Two components

normally associated with this are; the time for the active circuits

to settle, and the time for the thermal gradients on the chip to

stabilize. Figure 13 shows the typical turn-on time for the

ADR293.

THEORY OF OPERATION

The ADR293 uses a new reference generation technique known

as XFET, which yields a reference with low noise, low supply

current and very low thermal hysteresis.

The core of the XFET reference consists of two junction field-

effect transistors one of which has an extra channel implant to

raise its pinch-off voltage. By running the two JFETS at the

same drain current, the difference in pinch-off voltage can be

amplified and used to form a highly stable voltage reference.

The intrinsic reference voltage is around 0.5 V with a negative

temperature coefficient of about –120 ppm/K. This slope is

essentially locked to the dielectric constant of silicon and can be

closely compensated by adding a correction term generated in

the same fashion as the proportional-to-temperature (PTAT)

term used to compensate bandgap references. The big advan-

tage over a bandgap reference is that the intrinsic temperature

coefficient is some thirty times lower (therefore less correction is

needed) and this results in much lower noise since most of the

noise of a bandgap reference comes from the temperature com-

pensation circuitry.

The simplified schematic below shows the basic topology of the

ADR293. The temperature correction term is provided by a

current source with value designed to be proportional to abso-

lute temperature. The general equation is:

VV

RR

R

R

IR

OUT

P

PTAT

=

++









+

()( )

∆

12

3

1

3

where

∆V

current.

The process used for the XFET reference also features vertical

NPN and PNP transistors, the latter of which are used as output

devices to provide a very low drop-out voltage.

R1

R3

R2

R1

R3

R1

R2

R3

*

GND

Figure 18. Simplified Schematic