ADR293

Rev. D | Page 10 of 12

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 compen-

sated by adding a correction term generated in the same fashion

as the proportional-to-temperature (PTAT) term used to

compensate band gap references. The big advantage over a band

gap reference is that the intrinsic temperature coefficient is

some 30 times lower (therefore, less correction is needed) and

this results in much lower noise, because most of the noise of a

band gap reference comes from the temperature compensation

circuitry.

The simplified schematic in Figure 21 shows the basic topology

of the ADR293. The temperature correction term is provided by

a current source with value designed to be proportional to

absolute temperature. The general equation is

R3

I

R1

R3

R2

R1

V

V

PTAT

P

OUT

+

⎟

⎠

⎞

⎜

⎝

⎛

+

+

Δ

=

where:

is the difference in pinch-off voltage between the two FETs.

is the positive temperature coefficient correction 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 dropout voltage.

GND

R1

R2

R3

1

R1 + R2 + R3

R1

Figure 21. Simplified Schematic

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

specifications 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 junction temperature or dissipation:

JA

A

J

D

T

T

P

θ

−

=

where:

temperature, respectively.

is the device power dissipation.

is the device package thermal resistance.

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.

NC

NC

NC

NC

NC

0.1µF

0.1µF

10µF

+

NC = NO CONNECT

1

2

3

4

8

7

6

5

ADR293

Figure 22. Basic Voltage Reference Configuration

NOISE PERFORMANCE

The noise generated by the ADR293 is typically less than

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

ment is made with a band-pass 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 15 shows the typical turn-on time

for the ADR293.