8

FN8087.1

May 5, 2005

Applications Information

FGA Technology

The ISL60007 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 ISL60007 is the first high precision voltage reference

with ultra low power consumption that makes it possible to

leave power on continuously in battery operated circuits. The

ISL60007 consumes extremely low supply current due to the

proprietary FGA technology. Supply current at room

temperature is typically 400nA 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 27. 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.

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

Performance Curves. 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

as shown in Figure 28. 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 28 also shows the

noise in the 10kHz to 1MHz band can be reduced to about

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 29 is recommended. This network reduces

noise significantly over the full bandwidth. As shown in

to 1MHz using this network with a 0.01µF capacitor and a

2k

Ω resistor in series with a 10µF capacitor.

0.001µF-0.01µF

SERIAL

BUS

OUT

GND

ISL60007

REF IN

ENABLE

SCK

SDAT

A/D CONVERTER

12 TO 24-BIT

0.01µF

10µF

FIGURE 27.

ISL60007