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
t
he 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
la
rge 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.
00813-005
+V
IN
Q11
TEMP
Q12
R2
R1
Q5
Q4
Q1
1
×
Q2
8
×
Q9
R5
Q8
Q3
Q7
Q6
Q10
GND
C1
R6
R7
R4
R3
V
OUT
Figure 5. Schematic Diagram
APPLYING THE AD680
The AD680 is simple to use in virtually all precision reference
applications. When power is applied to +V
IN
and the GND pin
is tied to ground, V
OUT
provides a 2.5 V output. The AD680
typically requires less than 250 μA of current when operating
from a supply of 4.5 V to 36 V.
To operate the AD680, the +V
IN
pin must be bypassed to the
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
t
ransients 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 als
o 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
n
oise 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.
00813-006
5μV
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.
