ADR440/ADR441/ADR443/ADR444/ADR445

Rev. A | Page 14 of 20

THEORY OF OPERATION

The ADR44x series of references uses a new reference generation

technique known as XFET (eXtra implanted junction FET).

This technique yields a reference with low dropout, good

thermal hysteresis, and exceptionally low noise. The core of the

XFET reference consists of two junction field-effect transistors

(JFETs), 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/°C. This slope is

essentially constant to the dielectric constant of silicon, and it can

be closely compensated for by adding a correction term generated

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

term used to compensate band gap references. The advantage

of an XFET reference is its correction term, which is approx-

imately 20 times lower and requires less correction than that of a

band gap reference. Because most of the noise of a band gap

reference comes from the temperature compensation circuitry,

the XFET results in much lower noise.

Figure 33 shows the basic topology of the ADR44x 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

(

)

PTAT

P

OUT

I

R1

V

G

V

×

−

=

Δ

(1)

where:

G is the gain of the reciprocal of the divider ratio.

ADR44x devices are created by on-chip adjustment of R2

and R3 to achieve the different voltage option at the

reference output.

*

*EXTRA CHANNEL IMPLANT

R2

GND

R3

R1

ADR44x

Figure 33. Simplified Schematic Device

POWER DISSIPATION CONSIDERATIONS

The ADR44x family of references is guaranteed to deliver load

currents to 10 mA with an input voltage that ranges from 3 V to

18 V. When these devices are used in applications at higher

currents, users should use the following equation to account for

the temperature effects of increases in power dissipation:

A

JA

D

J

T

P

T

+

θ

×

=

(2)

where:

respectively.

BASIC VOLTAGE REFERENCE CONNECTIONS

The ADR44x family requires a 0.1 μF capacitor on the input

and the output for stability. While not required for operation,

a 10 μF capacitor at the input can help with line voltage

transient performance.

NOTES

1. NC = NO CONNECT

2. TP = TEST PIN (DO NOT CONNECT)

6

0.1µF

+

10µF

0.1µF

4

TP

8

NC

7

TRIM

5

ADR440/

ADR441/

ADR443/

ADR444/

ADR445

TOP VIEW

(Not to Scale)

2

GND

Figure 34. Basic Voltage Reference Configuration

NOISE PERFORMANCE

The noise generated by the ADR44x family of references is

typically less than 1.4 μV p-p over the 0.1 Hz to 10.0 Hz band

for ADR440, ADR441, and ADR443. Figure 26 shows the 0.1 Hz

to 10 Hz noise of the ADR441, which is only 1.2 μV p-p. The

noise measurement is made with a band-pass filter made of a

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

2­pole low-pass filter with a corner frequency at 10.0 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 compo-

nents 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 20 and Figure 21 show the turn-on and

turn-off settling times for the ADR441.