9

FN8141.0

March 14, 2005

APPLICATIONS INFORMATION

FGA Technology

The X60008 series of voltage references use the float-

ing gate technology to create references with very low

drift and supply current. Essentially the charge stored

on a floating gate cell is set precisely in manufacturing.

The reference voltage output itself is a buffered ver-

sion of the floating gate voltage. The resulting refer-

ence device has excellent characteristics which are

unique in the industry: very low temperature drift, high

initial accuracy, and almost zero supply current. Also,

the reference voltage itself is not limited by voltage

bandgaps or zener settings, so a wide range of refer-

ence voltages can be programmed (standard voltage

settings are provided, but customer-specific voltages

are available).

The process used for these reference devices is a

floating gate CMOS process, and the amplifier circuitry

uses CMOS transistors for amplifier and output tran-

sistor circuitry. While providing excellent accuracy,

there are limitations in output noise level and load reg-

ulation due to the MOS device characteristics. These

limitations are addressed with circuit techniques dis-

cussed in other sections.

Nanopower Operation

Reference devices achieve their highest accuracy

when powered up continuously, and after initial stabili-

zation has taken place. For example, power-up drift on

a high accuracy reference can reach 20ppm or more

in the first 30 seconds, and generally will settle to a

stable value in 100 hours or so. This drift can be elimi-

nated by leaving the power-on continuously.

The X60008 is the first high precision voltage reference

with ultra low power consumption that makes it possible

to leave power-on continuously in battery operated cir-

cuits. The X60008 consumes extremely low supply cur-

rent due to the proprietary FGA technology. Supply

current at room temperature is typically 500nA which is

1 to 2 orders of magnitude lower than competitive

devices. Application circuits using battery power will

benefit greatly from having an accurate, stable refer-

ence which essentially presents no load to the battery.

In particular, battery powered data converter circuits

that would normally require the entire circuit to be dis-

abled when not in use can remain powered up

between conversions as shown in figure 1. Data acqui-

sition circuits providing 12 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 cur-

rents 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 set-

tle to its final value, or, may not actually settle to a final

value as power-onpower-on time may be short.

Figure 1.

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.

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 Curves. The noise

measurement is made with a bandpass filter made of

a 1 pole high-pass filter with a corner frequency at

.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 Fig. 2 below. 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 2 also shows the noise in the

10KHz to 1MHz band can be reduced to about 50µVp-

p using a .001µF capacitor on the output. Noise in the

1KHz to 100KHz band can be further reduced using a

0.001µF–0.01µF

Serial

Bus

OUT

GND

X60008-50

REF IN

Enable

SCK

SDAT

A/D Converter

12 to 24-bit

0.01µF

10µF

X60008B-50