AD680

Rev. H | Page 6 of 12

THEORY OF OPERATION

Band gap references are the high performance solution for low

supply voltage operation. A typical precision band gap consists

of a reference core and buffer amplifier. Based on a new, pat-

ented band gap reference design (Figure 5), the AD680 merges

the amplifier and the core band gap function to produce a

compact, complete precision reference.

Central to the device is a high gain amplifier with an intentionally

large proportional to absolute temperature (PTAT) input offset.

This offset is controlled by the area ratio of the amplifier input

pair, Q1 and Q2, and is developed across Resistor R1. Transistor

Q12’s base emitter voltage has a complementary to absolute

temperature (CTAT) characteristic. Resistor R2 and the parallel

combination of Resistor R3 and Resistor R4 “multiply” the PTAT

voltage across the R1 resistor. Trimming the R3 and R4 resistors

to the proper ratio produces a temperature invariant of 2.5 V at

the output. The result is an accurate, stable output voltage

accomplished with a minimum number of components.

Q11

TEMP

Q12

R2

R1

Q5

Q4

Q1

1

×

Q2

8

×

Q9

R5

Q8

Q3

Q7

Q6

Q10

GND

C1

R6

R7

R4

R3

Figure 5. Schematic Diagram

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

transients caused by input switching networks, 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 tends 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

deliver charge as 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 not to

disrupt the stability of the reference.

The 8-lead PDIP and 8-lead 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 provides 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 6 shows the 0.1 Hz to 10 Hz

noise of a typical AD680. The noise measurement is made with a

band-pass 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.

5

μV

1s

100

90

0%

10

Figure 6. 0.1 Hz to 10 Hz Noise

Noise in a 300 kHz bandwidth is approximately 800 μV p-p.

Figure 7 shows the broadband noise of a typical AD680.