ad797bn Analog Devices, Inc., ad797bn Datasheet - Page 11

• Manufacturer Part Number: ad797bn
• Description: Ultralow Distortion, Ultralow Noise Op Amp
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
The architecture of the AD797 was developed to overcome
inherent limitations in previous amplifier designs. Previous
precision amplifiers used three stages to ensure high open-loop
gain (
sation components. Slew rate and settling performance are
usually compromised, and dynamic performance is not
adequate beyond audio frequencies. As can be seen in
Figure 30, the first stage gain is rolled off at high frequencies by
the compensation network. Second stage noise and distortion
then appears at the input and degrade performance. The AD797
on the other hand, uses a single ultrahigh gain stage to achieve
dc as well as dynamic precision. As shown in the simplified
schematic (Figure 31), Node A, Node B, and Node C all track in
voltage forcing the operating points of all pairs of devices in the
signal path to match. By exploiting the inherent matching of
devices fabricated on the same IC chip, high open-loop gain,
CMRR, PSRR, and low V
device matching (that is., NPN to NPN and PNP to PNP), and
not absolute parameter such as beta and early voltage.
+IN
Figure 30. Model of AD797 vs. That of a Typical Three-Stage Amplifier
R2
Figure 30) at the expense of additional frequency compen-
Q1
gm
Q2
I1
R3
R1
gm
–IN
Figure 31. AD797 Simplified Schematic
GAIN = gm x R1 x 5 x 10
Q3
C
Q5
R1
C
C1
R2
N
GAIN = gm x R1 x A2 x A3
A
OS
I7
A2
Q6
are all guaranteed by pairwise
Q4
C1
a.
b.
B
C
BUFFER
A3
R1
6
C
C2
Q7
Q12
I4
BUFFER
Q8
R
I5
L
Q9
I6
V
OUT
Q10
Q11
R
L
V
V
CC
SS
V
OUT
OUT
Rev. E | Page 11 of 20
This matching benefits not just dc precision, but because it
holds up dynamically, both distortion and settling time are also
reduced. This single stage has a voltage gain of >5 × 10
<80 μV, while at the same time providing THD + noise of less
than −120 dB and true 16-bit settling in less than 800 ns. The
elimination of second stage noise effects has the additional
benefit of making the low noise of the AD797 (<0.9 nV/√Hz)
extend to beyond 1 MHz. This means new levels of perform-
ance for sampled data and imaging systems. All of this
performance as well as load drive in excess of 30 mA are made
possible by Analog Devices’ advanced Complementary Bipolar
(CB) process.
Another unique feature of this circuit is that the addition of a
single capacitor, C
distortion due to the output stage. This can best be explained by
referring to a simplified representation of the AD797 using
idealized blocks for the different circuit elements (Figure 32).
A single equation yields the open-loop transfer function of this
amplifier, solving it (at Node B) yields:
where:
gm = the transconductance of Q1 and Q2
A = the gain of the output stage, (~1)
V
V
When C
response:
The terms in A, which include the properties of the output stage
such as output impedance and distortion, cancel by simple
subtraction. Therefore, the distortion cancellation does not
affect the stability or frequency response of the amplifier. With
only 500 μA of output stage bias, the AD797 delivers a 1 kHz
sine wave into 60 Ω at 7 V rms with only 1 ppm of distortion.
O
IN
= voltage at the output
= differential input voltage
V
V
V
V
IN
IN
O
O
N
=
is equal to C
=
C
j
gm
A
ω
N
C
j
ω
N
(Figure 31), enables cancellation of
−
C
C
N
gm
this gives the ideal single pole op amp
j
ω
−
C
A
C
j
ω
AD797
6
and V
OS