ISL21080 Datasheet, PDF (7/10 Pages) Intersil Corporation – 300nA NanoPower Voltage References

ISL21080
not in use can remain powered up between conversions as
shown in Figure 17. Data acquisition circuits providing
12 bits to 24 bits of accuracy can operate with the reference
device continuously biased with no power penalty, providing
the highest accuracy and lowest possible long term drift.
Other reference devices consuming higher supply currents
will need to be disabled in between conversions to conserve
battery capacity. Absolute accuracy will suffer as the device is
biased and requires time to settle to its final value, or, may not
actually settle to a final value as power on time may be short.
Table 1 shows an example of battery life in years for ISL21080
in various power on condition with 1.5µA maximum current
consumption.
TABLE 1. EXAMPLE OF BATTERY LIFE IN YEARS FOR
ISL21080 IN VARIOUS POWER ON CONDITIONS
WITH 1.5µA MAX CURRENT
BATTERY
50% DUTY
RATING (mAH) CONTINUOUS CYCLE
10% DUTY
CYCLE
40
3
6
30*
225
16.3*
32.6*
163*
NOTE: *Typical Li-Ion battery has a shelf life of up to 10 years.
V_IN = +3.0V
10µF
0.01µF
V_{IN VOUT}
ISL21080
GND
0.001µF TO 0.01µF
SERIAL
BUS
REF IN
ENABLE
SCK
SDAT
12 TO 24-BIT
A/D CONVERTER
FIGURE 17.
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
I_SET
=
V_OUT
R_SET
I_L = I_{SET +} IR_SET
0.01µF
V_{IN VOUT}
ISL21080-1.5
V_OUT = 1.5V
Z_{OUT > 100MΩ}
GND
R_SET
10kΩ
0.1%
10ppm/°C
I_SY ~ 0.31µA
I_SET
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. Precautions
should be taken to avoid excessive heat or extended
exposure to high reflow temperatures, which 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 a shield of thin zinc (300µm) to allow imaging but
block x-rays that affect the FGA reference.
Noise Performance and Reduction
The output noise voltage in a 0.1Hz to 10Hz bandwidth is
typically 30µV_P-P. This is shown in the plot in the “Typical
Performance Characteristics Curves” which begin on
page 4. 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 400µV_P-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
50µV_P-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
7
FN6934.0
July 28, 2009