X60003

FN8137 Rev 5.00

Page 13 of 17

September 1, 2015

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, although 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, so 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 typically

on page 8 and 9. 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

output, as shown in Figure 34. 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 10x the center

frequency. Figure 34 also shows the noise in the 10kHz to 1MHz band

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 due to instability of the very low power amplifier with a

0.1µF capacitance load. For load capacitances above 0.001µF, the

noise reduction network shown in Figure 35 is recommended. This

network reduces noise significantly over the full bandwidth. Figure 35

using this network with a 0.01µF capacitor and a 2k

 resistor in

series with a 10µF capacitor.

Turn-On Time

The X60003 device has ultra-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

7ms. This is shown in the graph, Figure 32. Since devices can

vary in supply current down to 300nA, turn-on time can last up to

about 12ms. Care should be taken in system design to include

this delay before measurements or conversions are started.

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

ppm/°C. This is the “Box” method for determining temperature

coefficient.

400

350

300

250

200

150

100

50

0

1

10

100

1k

10k

100k

FIGURE 34. X60003 NOISE REDUCTION

CL = 0

CL = 0.001µF

CL = 0.1µF

CL = 0.01µF AND 10µF + 2k



GND

X60003

0.01µF

10µF

2k



0.1µF

10µF

FIGURE 35. NOISE REDUCTION NETWORK