ADR430 datasheet(16/24 Pages) AD

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ADR430/ADR431/ADR433/ADR434/ADR435/ADR439

Rev. J | Page 16 of 24

THEORY OF OPERATION

The ADR43x series of references uses a reference generation

technique known as XFET (eXtra implanted junction FET).

This technique yields a reference with low supply current, 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 can

be compensated closely by adding a correction term generated

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

term used to compensate band gap references. The primary

advantage of an XFET reference is its correction term, which is

~30 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 29 shows the basic topology of the ADR43x 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

VOUT = G (ΔVP – R1 × IPTAT)

(1)

where:

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

ΔVP is the difference in pinch-off voltage between the two JFETs.

IPTAT is the positive temperature coefficient correction current.

ADR43x devices are created by on-chip adjustment of R2 and R3 to

achieve 2.048 V or 2.500 V, respectively, at the reference output.

IPTAT

*EXTRA CHANNEL IMPLANT

VOUT = G(∆VP – R1 × IPTAT)

VIN

VOUT

GND

∆VP

ADR43x

Figure 29. Simplified Schematic Device

Power Dissipation Considerations

The ADR43x family of references is guaranteed to deliver load

currents to 10 mA with an input voltage that ranges from 4.1 V

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

currents, use the following equation to account for the

temperature effects due to the power dissipation increases:

TJ = PD × θJA + TA

(2)

where:

TJ and TA are the junction and ambient temperatures, respectively.

PD is the device power dissipation.

θJA is the device package thermal resistance.

BASIC VOLTAGE REFERENCE CONNECTIONS

Voltage references, in general, require a bypass capacitor

connected from VOUT to GND. The circuit in Figure 30

illustrates the basic configuration for the ADR43x family

of references. Other than a 0.1 μF capacitor at the output to

help improve noise suppression, a large output capacitor at

the output is not required for circuit stability.

NOTES:

1. NC = NO CONNECT

2. TP = TEST PIN (DO NOT CONNECT)

ADR43x

TOP VIEW

(Not to Scale)

COMP

VOUT

TRIM

GND

VIN

10µF

0.1µF

Figure 30. Basic Voltage Reference Configuration

NOISE PERFORMANCE

The noise generated by the ADR43x family of references is

typically less than 3.75 μV p-p over the 0.1 Hz to 10.0 Hz band

for ADR430, ADR431, and ADR433. Figure 22 shows the 0.1 Hz

to 10.0 Hz noise of the ADR431, which is only 3.5 μV p-p. 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 10.0 Hz.

HIGH FREQUENCY NOISE

The total noise generated by the ADR43x family of references is

composed of the reference noise and the op amp noise. Figure 31

shows the wideband noise from 10 Hz to 25 kHz. An internal node

of the op amp is brought out on Pin 7, and by overcompensating

the op amp, the overall noise can be reduced.

This is understood by considering that in a closed-loop

configuration, the effective output impedance of an op amp is

(3)

where:

RO is the apparent output impedance.

rO is the output resistance of the op amp.

AVO is the open-loop gain at the frequency of interest.

β is the feedback factor.

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