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ISL21007
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, along with a ground plane underneath, which will
effectively shield it 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
4.5μV P-P . 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 40μV P-P with no capacitance on the
output. This noise measurement is 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.
Load capacitance up to 1000pF can be added but will result in
only marginal improvements in output noise and transient
Turn-On Time
The ISL21007 devices have low supply current and thus the time
to bias up internal circuitry to final values will be longer than with
higher power references. Normal turn-on time is typically 120μs.
This is shown in Figure 10. Circuit design must take this into
account when looking at power-up delays or sequencing.
Temperature Coefficient
The limits stated for temperature coefficient (tempco) are governed
by the method of measurement. The overwhelming standard for
specifying the temperature drift of a reference is to measure the
reference voltage at two temperatures, take the total variation,
(V HIGH – V LOW ), and divide by the temperature extremes of
measurement (T HIGH – T LOW ). The result is divided by the nominal
reference voltage (at T = +25°C) and multiplied by 10 6 to yield
ppm/°C. This is the “Box” method for specifying temperature
coefficient.
Output Voltage Adjustment
The output voltage can be adjusted up or down by 2.5% by placing a
potentiometer from V OUT to ground, and connecting the wiper to the
TRIM pin. The TRIM input is high impedance, so no series resistance
is needed. The resistor in the potentiometer should be a low tempco
(<50ppm/°C) and the resulting voltage divider should have very low
tempco <5ppm/°C. A digital potentiometer such as the ISL95810
provides a low tempco resistance and excellent resistor and tempco
matching for trim applications. See Figure 59 and TB473 for further
information.
V IN = 5.0V
response. The output stage of the ISL21007 is not designed to
10μF
V IN
drive heavily capacitive loads, so for load capacitances above
0.001μF, the noise reduction network shown in Figure 55 is
0.1μF
ISL21007
V O
recommended. This network reduces noise significantly over the
full bandwidth. Noise is reduced to less than 20μV P-P from 1Hz to
1MHz using this network with a 0.01μF capacitor and a 2k Ω
resistor in series with a 10μF capacitor. Also, transient response is
improved with higher value output capacitor. The 0.01μF value can
be increased for better load transient response with little sacrifice
in output stability.
GND
0.01μF
FIGURE 55. HANDLING HIGH LOAD CAPACITANCE
2k Ω
10μF
17
FN6326.10
March 27, 2012
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