AD8011 Analog Devices, AD8011 Datasheet - Page 10

• Manufacturer Part Number: AD8011
• Description: 300 MHz, 1 mA Current Feedback Amplifier
• Manufacturer: Analog Devices

Specifications of AD8011

• -3db Bandwidth: 400MHz
• Slew Rate: 3.5kV/µs
• Vos: 2mV
• Ib: 5µA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 2nV/rtHz
• Vcc-vee: 3V to 12V
• Isy Per Amplifier: 1.3mA
• Packages: DIP,SOIC

AD8011
(error current times the open-loop inverting input resistance) that
results (see Figure 7), a more exact low frequency closed-loop
transfer function can be described as
for noninverting (G is positive).
for inverting (G is negative).
where G is the ideal gain as previously described. With R
(open-loop inverting input resistance), the second expression
(positive G) clearly relates to the classical voltage feedback op amp
equation with T
and thus insignificant effect. A
voltage and transresistance gains of the amplifier, respectively.
These key transfer variables can be described as
and
Therefore
where g
and 1/g
Q3/Q4. The g
negative dc open-loop gain of typically –2500 V/V (see Figure 8).
Though atypical of conventional CF or VF amps, this negative
open-loop voltage gain results in an input referred error term
(V
greater than +3/–4. As an example, for G = 10, A
and T
derivation above.
P
–V
V
O
O
P
mf
/G = G/A
mc
= 1.2 MΩ, results in an error of –3 mV using the A
Figure 7. Z
is the thermal emitter resistance of devices D1/D2 and
is the positive feedback transconductance (not shown)
C
A
P
R
mc
V
S
O
=
× R1 product has a design value that results in a
O
omitted due to its relatively much higher value
1
L
+ R
I
+
L
N
I
R
A
A
G R
= Open-Loop Input Impedance
IE
N
F
O
V
/T
T
Z
×
Z
=
I
O
=
O
G
T
I
= OPEN LOOP INPUT IMPEDANCE = C
R
) that will typically be negative for G,
R
O
R
1
( –
I
I
F
1
1
+
=
=
T
A
+
×
O
O
1 –
O
R
1
(s)
R
T
(s)
g mf
g
–V
A
+V
and T
1
–
mc
O
F
2
S
G
O
G
S
×
g
L
L
2
×
| |
S
mc
S
=
×
×
A
+
g
1
| |
R
2
mf
O
R
T
A
×
+
1
O
2
F
are the open-loop dc
)
R
A
G
1
R
G
O
L
+
T
R
C
O
O
F
L
= –2500,
I
I
= T
V
|| R
O
L
O
V
/A
O
–10–
This analysis assumes perfect current sources and infinite transistor
V
assumptions result in actual versus model open-loop voltage gain
and associated input referred error terms being less accurate for
low gain (G) noninverting operation at the frequencies below the
open-loop pole of the AD8011. This is primarily a result of the
input signal (V
resulting in R
operation, the actual versus model dc error terms are relatively
much less.
AC TRANSFER CHARACTERISTICS
The ac small signal transfer derivations below are based on a
simplified single-pole model. Though inaccurate at frequencies
approaching the closed-loop BW (CLBW) of the AD8011 at low
noninverting external gains, they still provide a fair approxima-
tion and an intuitive understanding of its primary ac small signal
characteristics.
For inverting operation and high noninverting gains, these
transfer equations provide a good approximation to the actual
ac performance of the device.
To accurately quantify the V
and T
nonexpanded noninverting gain relationship
with
where R1 is the input resistance to A2/A2B, and τ1 (equal to
CD
and
A
s. (Q3, Q4 output conductances are assumed zero.) These
–10
–20
–30
80
70
60
50
40
30
20
10
1E+03
O
0
T s
R1
Figure 8. Open-Loop Voltage Gain and Phase
(s) need to be derived. This can be seen by the following
O
( )
=
A2) is the open-loop dominate time constant,
1E+04
I
P
|
) modulating the output conductances of Q3/Q4,
less negative than derived here. For inverting
V s V s
A
s
O
τ
2
1 1
( ) /
2
A s
|
+
×
O
1E+05
R
( )
1
P
FREQUENCY (Hz)
( )
=
R
=
1E+06
O
1
1
1
×
A s
versus V
GAIN
–
–
A
G
O
g
O
g
g
mf
[ ]
(s)
S
mc
mc
τ
1E+07
1
+
×
×
×
G
PHASE
|
T s
P
R
R
R
A
O
relationship, A
1
1
F
[ ]
2
1E+08
|
+ 1
1E+09
–90
–100
–110
–120
–130
–140
–150
–160
–170
–180
–190
–200
REV. C
O
(s)