ISL21080-09 View Datasheet(PDF) - Intersil
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ISL21080-09 Datasheet PDF : 17 Pages
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ISL21080
VIN = +3.0V
10µF
VIN VOUT
ISL21080
GND
0.001µF TO 0.01µF
SERIAL
BUS
0.01µF
REF IN
ENABLE
SCK
SDAT
12 TO 24-BIT
A/D CONVERTER
FIGURE 17. REFERENCE INPUT FOR ADC CONVERTER
ISL21080 Used as a Low Cost Precision
Current Source
Using an N-JET and a Nanopower voltage reference,
ISL21080, a precision, low cost, high impedance current
source can be created. The precision of the current
source is largely dependent on the tempco and accuracy
of the reference. The current setting resistor contributes
less than 20% of the error.
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.
+8V TO 28V
ISET
VOUT
= RSET
IL = ISET + IRSET
0.01µF
VIN VOUT
ISL21080-1.5
VOUT = 1.5V ZOUT > 100MΩ
GND
RSET
10kΩ
0.1%
10ppm/°C
ISY ~ 0.31µA
ISET
IL AT 0.1% ACCURACY
~150.3µA
FIGURE 18. ISL21080 USED AS A LOW COST
PRECISION CURRENT SOURCE
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 have a cumulative effect on the voltage
reference output accuracy. Carry-on luggage screening
uses low level X-rays and is not a major source of output
voltage shift, however, if a product is expected to pass
through that type of screening over 100 times, it may
need to consider shielding with copper or aluminum.
Checked luggage X-rays are higher intensity and can
cause output voltage shift in much fewer passes, thus
devices expected to go through those machines should
definitely consider shielding. Note that just two layers of
1/2 ounce copper planes will reduce the received dose by
over 90%. The leadframe for the device which is on the
bottom also provides similar shielding.
If a device is expected to pass through luggage X-ray
machines numerous times, it is advised to mount a
2-layer (minimum) PC board on the top, and along with a
ground plane underneath will effectively shield it from
from 50 to 100 passes through the machine. Since these
machines vary in X-ray dose delivered, it is difficult to
produce an accurate maximum pass recommendation.
Noise Performance and Reduction
The output noise voltage in a 0.1Hz to 10Hz bandwidth is
typically 30µVP-P. This is shown in the plot in the “Typical
Performance Characteristics Curves” which begin on
page 8. The noise measurement is made with a
bandpass filter made of a 1 pole high-pass filter with a
corner frequency at 0.1Hz and a 2-pole low-pass filter
with a corner frequency at 12.6Hz to create a filter with a
9.9Hz bandwidth. Noise in the 10kHz to 1MHz bandwidth
is approximately 400µVP-P with no capacitance on the
output, as shown in Figure 19. These noise
measurements are made with a 2 decade bandpass filter
made of a 1-pole high-pass filter with a corner frequency
at 1/10 of the center frequency and 1-pole low-pass filter
with a corner frequency at 10 times the center frequency.
Figure 19 also shows the noise in the 10kHz to 1MHz
band can be reduced to about 50µVP-P using a 0.001µF
capacitor on the output. Noise in the 1kHz to 100kHz
band can be further reduced using a 0.1µF capacitor on
the output, but noise in the 1Hz to 100Hz band increases
12
FN6934.2
October 14, 2009
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