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|Description||Ultralow Noise XFET® Voltage References with Current Sink and Source Capability|
|ADR433ARZ Datasheet PDF : 24 Pages |
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)
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.
*EXTRA CHANNEL IMPLANT
VOUT = G(∆VP – R1 × IPTAT)
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
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.
2 ADR43x 7
(Not to Scale)
1. NC = NO CONNECT
2. TP = TEST PIN (DO NOT CONNECT)
Figure 30. Basic Voltage Reference Configuration
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
1 + AVOβ
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.
Rev. J | Page 16 of 24
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