ADR291/ADR292

Rev. F | Page 13 of 20

THEORY OF OPERATION

The ADR291/ADR292 series of references uses a reference

generation technique known as XFET (eXtra implanted junc-

tion FET). This technique 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 having 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 band gap references. Because most of the noise

of a band gap reference comes from the compensation circuitry,

the intrinsic temperature coefficient offers a significant advan-

tage (being about 30 times lower), and therefore, requiring less

correction resulting in much lower noise.

The simplified schematic in Figure 31 shows the basic topology

of the ADR291/ADR292 series. The temperature correction

term is provided by a current source with a value designed to be

proportional to absolute temperature. The general equation is

1

3

2

1

R

I

R

R

R

R

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 various versions of the ADR291/ADR292 family are created

by on-chip adjustment of R1 and R3 to achieve 2.500 V or

4.096 V at the reference output.

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 31. ADR291/ADR292 Simplified Schematic

DEVICE POWER DISSIPATION CONSIDERATIONS

The ADR291/ADR292 family of references is guaranteed to

deliver load currents to 5 mA with an input voltage that ranges

from 2.7 V to 15 V (minimum supply voltage depends on the

output voltage chosen). When these devices are 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. Use the following formula to calculate

maximum junction temperature or dissipation of a device:

JA

A

J

D

T

T

P

θ

−

=

where

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 ADR291/ADR292

family of references. 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

ADR291/

ADR292

Figure 32. Basic Voltage Reference Configuration

NOISE PERFORMANCE

The noise generated by the ADR291/ADR292 family of refer-

ences is typically less than 12 μV p-p over the 0.1 Hz to 10 Hz

band. The noise measurement 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 com-

ponents normally associated with this are the time it takes for

the active circuits to settle and for the thermal gradients on the

chip to stabilize. Figure 28 shows the turn-on settling time for

the ADR291.