AD680

REV. C

–4–

APPLYING THE AD680

The AD680 is simple to use in virtually all precision reference

typically requires less than 250

µA of current when operating

from a supply of +4.5 V to +36 V.

GND pin with a 0.1

µF capacitor tied as close to the AD680 as

possible. Although the ground current for the AD680 is small

(typically 195

µA), a direct connection should be made between

the AD680 GND pin and the system ground plane.

Reference outputs are frequently required to handle fast tran-

sients caused by input switching networks, as are commonly

found in ADCs and measurement instrumentation equipment.

Many of the dynamic problems associated with this situation

can be minimized with a few simple techniques. Using a series

resistor between the reference output and the load will tend to

“decouple” the reference output from the transient source. Or a

relatively large capacitor connected from the reference output to

ground can serve as a charge storage element to absorb and de-

liver charge as is required by the dynamic load. A 50 nF capaci-

tor is recommended for the AD680 in this case; this is large

enough to store the required charge, but small enough so as not

to disrupt the stability of the reference.

The 8-pin plastic DIP and SOIC packaged versions of the

AD680 also provide a temperature output pin. The voltage on

this pin is nominally 596 mV at 25

°C. This pin will provide an

output linearly proportional to temperature with a characteristic

of 2 mV/

°C.

NOISE PERFORMANCE

The noise generated by the AD680 is typically less than 8

µV

p-p over the 0.1 Hz to 10 Hz band. Figure 3 shows the 0.1 Hz

to 10 Hz noise of a typical AD680. The noise measurement is

made with a bandpass filter made of a 1-pole high-pass filter

with a corner frequency at 0.1 Hz and a 2-pole low-pass filter

with a corner frequency at 12.6 Hz to create a filter with a

9.922 Hz bandwidth.

Figure 3. 0.1 Hz to 10 Hz Noise

Noise in a 300 kHz bandwidth is approximately 800

µV p-p.

Figure 4 shows the broadband noise of a typical AD680.

Figure 4. Broadband Noise at 300 kHz

TURN-ON TIME

Upon application of power (cold start), the time required for the

output voltage to reach its final value within a specified error

band is defined as the turn-on settling time. Two components

normally associated with this are: the time for the active circuits

to settle, and the time for the thermal gradients on the chip to

stabilize. Figure 5 shows the turn-on settling time of the AD680

to be about 20

µs to 0.025% of its final value.

Figure 5. Turn-On Settling Time

The AD680 thermal settling characteristic benefits from its

compact design. Once initial turn-on is achieved, the output lin-

early approaches its final value; the output is typically within

0.01% of its final value after 25 ms.

DYNAMIC PERFORMANCE

The output stage of the ampliflier is designed to provide the

AD680 with static and dynamic load regulation superior to less

complete references.