Datasheet

Data Sheet ADA4522-1/ADA4522-2/ADA4522-4
Rev. F | Page 21 of 33
THEORY OF OPERATION
The ADA4522-1/ADA4522-2/ADA4522-4 are single, dual, and
quad, ultralow noise, high voltage, zero drift, rail-to-rail output
operational amplifiers. They feature a chopping technique that
offers an ultralow input offset voltage of 5 μV and an input
offset voltage drift of 22 nV/°C maximum for the ADA4522-1
and ADA4522-2 and 25 nV/°C maximum for the ADA4522-4.
Offset voltage errors due to common-mode voltage swings and
power supply variations are also corrected by the chopping
technique, resulting in a superb typical CMRR figure of 160 dB
and a PSRR figure of 160 dB at a 30 V supply voltage.
The ADA4522-1/ADA4522-2/ADA4522-4 have wide operating
voltages from ±2.25 V (or 4.5 V) to ±27.5 V (or 55 V). The
devices are single supply amplifiers, where their input voltage
range includes the lower supply rail. They also offer low voltage
noise density of 5.8 nV/√Hz (at f = 1 kHz, A
V
= 100) and reduced
1/f noise component. These features are ideal for the amplifica-
tion of low level signals in high precision applications. A few
examples of such applications are weigh scales, high precision
current sensing, high voltage buffers, and signal conditioning
for temperature sensors, among others.
Figure 71 shows the ADA4522-1/ADA4522-2/ADA4522-4
architecture block diagram. The architecture consists of an
input EMI filter and clamp circuitry, three gain stages (G
m1
, G
m2
,
and G
m3
), input and output chopping networks (CHOP
IN
and
CHOP
OUT
), a clock generator, offset and ripple correction loop
circuitry, frequency compensation capacitors (C1, C2, and C3),
and thermal shutdown circuitry.
An EMI filter and clamp circuit is implemented at the input
front end to protect the internal circuitry against electrostatic
discharge (ESD) stresses and high voltage transients. The ability
of the amplifier to reject EMI is explained in detail in the EMI
Rejection Ratio section.
CHOP
IN
and CHOP
OUT
are controlled by a clock generator and
operate at 4.8 MHz. The input baseband signal is initially
modulated by CHOP
IN
. Next, CHOP
OUT
demodulates the input
signal and modulates the millivolt level input offset voltage and
1/f noise of the input transconductance amplifier, G
m1
, to the
chopping frequency at 4.8 MHz. The chopping networks remove
the low frequency errors, but, in return, the networks introduce
chopping artifacts at the chopping frequency. Therefore, a offset
and ripple correction loop, operating at 800 kHz, is used. This
frequency is the switching frequency of the amplifier. This
circuitry reduces chopping artifacts, allowing the ADA4522-1/
ADA4522-2/ADA4522-4 to have a high chopping frequency
with minimal artifacts.
The thermal shutdown circuit shuts down the circuit when
the die is overheated (see the Thermal Shutdown section for
more information).
13168-068
G
m1
G
m2
G
m3
EMI
FILTER
AND
CLAMP
THERMAL
SHUTDOWN
OFFSET
AND RIPPLE
CORRECTION
LOOP
+
IN x
–
IN x
CHOP
IN
CHOP
OUT
C3
OUT
C2
C1
4.8MHz CLOCKS
800kHz CLOCKS
CLOCK
GENERATOR
NOTES
1. THE INPUTS ARE +IN x/–IN x ON THE ADA4522-2 AND ADA4522-4,
AND +IN/–IN ON THE ADA4522-1.
2. THE OUTPUT IS OUT ON THE ADA4522-1 AND OUT x ON THE
ADA4522-2 AND ADA4522-4.
Figure 71. ADA4522-1/ADA4522-2/ADA4522-4 Architecture Block Diagram
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