7

FN8144.1

June 27, 2006

Applications Information

FGA Technology

The X60008 series of voltage references use the floating

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 version of the floating gate

voltage. The resulting reference 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

reference 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 transistor circuitry. While

providing excellent accuracy, there are limitations in output

noise level and load regulation due to the MOS device

characteristics. These limitations are addressed with circuit

techniques discussed in other sections.

Nanopower Operation

Reference devices achieve their highest accuracy when

powered up continuously, and after initial stabilization has

taken place. This drift can be eliminated by leaving the

power-on continuously.

The X60008 is the first high precision voltage reference with

ultra low power consumption that makes it practical to leave

power-on continuously in battery operated circuits. The

X60008 consumes extremely low supply current 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 reference which essentially presents no load

to the battery.

In particular, battery powered data converter circuits that

would normally require the entire circuit to be disabled when

not in use can remain powered up between conversions as

shown in Figure 1. Data acquisition 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 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.

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 Figure 2. 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 0.1µF

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

.001µF the noise reduction network shown in Figure 3 is

recommended. This network reduces noise sig-nificantly

over the full bandwidth. As shown in Figure 2, noise is

reduced to less than 40µVp-p from 1Hz to 1MHz using this

network with a .01µF capacitor and a 2k

Ω resistor in series

with a 10µF capacitor.

0.001µF

Serial

Bus

OUT

GND

X60008-41

REF IN

Enable

SCK

SDAT

A/D Converter

12 to 24-bit

0.01µF

10µF

X60008E-41