ISL21007_11 Datasheet, PDF (16/20 Pages) Intersil Corporation – Precision, Low Noise FGA™ Voltage References

ISL21007
Typical Performance Curves (ISL21007-30) (R_EXT = 100kΩ) (Continued)
+7mA
-7mA
100µs/DIV
FIGURE 54. LOAD TRANSIENT RESPONSE
Applications Information
FGA Technology
The ISL21007 voltage reference uses floating gate technology to
create references with very low drift and supply current.
Essentially, the charge stored on a floating gate cell is set
precisely in manufacturing. The reference voltage output itself is
a buffered version of the floating gate voltage. The resulting
reference device has excellent characteristics which are unique
in the industry: very low temperature drift, high initial accuracy,
and almost zero supply current. Also, the reference voltage itself
is not limited by voltage bandgaps or zener settings, so a wide
range of reference voltages can be programmed (standard
voltage settings are provided, but customer-specific voltages are
available).
The process used for these reference devices is a floating gate
CMOS process, and the amplifier circuitry uses CMOS transistors
for amplifier and output transistor circuitry. While providing
excellent accuracy, there are limitations in output noise level and
load regulation due to the MOS device characteristics. These
limitations are addressed with circuit techniques discussed in
other sections.
Micropower Operation
The ISL21007 consumes extremely low supply current due to the
proprietary FGA technology. Low noise performance is achieved
using optimized biasing techniques. Supply current is typically
75µA and noise is 4.5µV_P-P benefitting precision, low noise
portable applications such as handheld meters and instruments.
Data Converters in particular can utilize the ISL21007 as an
external voltage reference. Low power DAC and ADC circuits will
realize maximum resolution with lowest noise.
Handling and Board Mounting
FGA references provide excellent initial accuracy and low
temperature drift at the expense of very little power drain. There
are some precautions to take to insure this accuracy is not
compromised. Excessive heat during solder reflow can cause
excessive initial accuracy drift, so the recommended +260°C
max temperature profile should not be exceeded. Expect up to
1mV drift from the solder reflow process.
FGA references are susceptible to excessive X-radiation like that
used in PC board manufacturing. Initial accuracy can change
10mV or more under extreme radiation. If an assembled board
needs to be X-rayed, care should be taken to shield the FGA
reference device.
Board Mounting Considerations
For applications requiring the highest accuracy, board mounting
location should be reviewed. Placing the device in areas subject to
slight twisting can cause degradation of the accuracy of the
reference voltage due to die stresses. It is normally best to place the
device near the edge of a board, or the shortest side, as the axis of
bending is most limited at that location. Obviously, mounting the
device on flexprint or extremely thin PC material will likewise cause
loss of reference accuracy.
Board Assembly Considerations
FGA references provide high accuracy and low temperature drift
but some PC board assembly precautions are necessary. Normal
Output voltage shifts of 100µV to 1mV can be expected with Pb-
free reflow profiles or wave solder on multi-layer FR4 PC boards.
Precautions should be taken to avoid excessive heat or extended
exposure to high reflow or wave solder temperatures, this may
reduce device initial accuracy.
Post-assembly x-ray inspection may also lead to permanent
changes in device output voltage and should be minimized or
avoided. If x-ray inspection is required, it is advisable to monitor
the reference output voltage to verify excessive shift has not
occurred. If large amounts of shift are observed, it is best to add
an X-ray shield consisting of thin zinc (300µm) sheeting to allow
clear imaging, yet block x-ray energy that affects the FGA
reference.
Special Applications Considerations
In addition to post-assembly examination, there are also other X-
ray sources that may affect the FGA reference long term
accuracy. Airport screening machines contain X-rays and will
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FN6326.8
July 14, 2011