ISL21070

8

FN7599.2

July 1, 2011

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

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 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

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.

Wideband noise is reduced by adding capacitor to the output, but

the value should be limited to 1nF or less to insure stability.

Temperature Drift

The limits stated for output accuracy over-temperature are

governed by the method of measurement. For the -40°C to 85°C

temperature range, measurements are made at +25°C and the

two extremes. This measurement method combined with the

fact that FGA references have a fairly linear temperature drift

characteristic insures that the limits stated will not be exceeded

over the temperature range.

ISL21070 Used as a Low Cost Precision

Current Source

Using an N-JET and the ISL21070, 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.

FIGURE 17. ISL21070 USED AS A LOW COST PRECISION CURRENT

SOURCE

+8V TO 28V

0.01μF

1kΩ

0.1%

10ppm/°C

IL AT 0.1% ACCURACY

~2.5011mA

ISL21070-2.5

GND