AD8011AN(2000) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8011AN Datasheet PDF : 16 Pages
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AD8011
(error current times the open loop inverting input resistance)
that results (see Figure 29), a more exact low frequency closed
loop transfer functions can be described as:
AV
=
1
+
G
G
× RI
+
RF
=
1+
G
G + RF
TO
TO
AO TO
for noninverting (G is positive)
AV
=
G
1+ 1–G
+
RF
AO TO
for inverting (G is negative)
+VS
RS
LN
LS
VP
IE
TO (s)
AO (s)
LI
ZI
LS
VO
RL
CL
CP
RN
RF
–VS
Z I = OPEN LOOP INPUT IMPEDANCE = CI || RL
Figure 29. ZI = Open-Loop Input Impedance
where G is the ideal gain as previously described. With
R I = TO/AO (open-loop inverting input resistance), the second
expression (positive G) clearly relates to the classical voltage
feedback “op amp” equation with TO omitted do to its relatively
much higher value and thus insignificant effect. AO and TO are
the open-loop dc voltage and transresistance gains of the ampli-
fier respectively. These key transfer variables can be described as:
R1 × g mf × | A2 |
R1 × | A2 |
1 − gmc × R1
AO =
and TO =
; therefore RI =
(1 – gmc × R1)
2
2 × gmf
Where gmc is the positive feedback transconductance (not
shown) and 1/gmf is the thermal emitter resistance of devices
D1/D2 and Q3/Q4. The gmc × R1 product has a design value that
results in a negative dc open loop gain of typically –2500 V/V (see
Figure 30).
Though atypical of conventional CF or VF amps, this negative
open-loop voltage gain results in an input referred error term
(VP–VO/G = G/AO + RF/TO) that will typically be negative for G
greater than +3/–4. As an example, for G = 10, AO = –2500 and
TO = 1.2 MΩ, results in a error of –3 mV using the AV deriva-
tion above.
This analysis assumes perfect current sources and infinite tran-
sistor VAs (Q3, Q4 output conductances are assumed zero).
These assumptions result in actual vs. 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 (VP) modulating the output conductances of
Q3/Q4 resulting in RI less negative than derived here. For invert-
ing operation, the actual vs. 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 approximation
and a intuitive understanding of its primary ac small signal
characteristics.
For inverting operation and high noninverting gains these trans-
fer equations provide a good approximation to the actual ac
performance of the device.
To accurately quantify the VO vs. VP relationship, both AO(s)
and TO(s) need to be derived. This can be seen by the following
nonexpanded noninverting gain relationship:
VO (s) /VP (s) =
G
G
+ RF + 1
AO[s] TO[s]
with
AO(s) =
R1 ×
1–
gmf
gmc
× |A2|
× R1
Sτ1
1 – gmc × R1
80
–90
70
–100
60
–110
PHASE
50
–120
40
–130
GAIN
30
–140
20
–150
10
–160
AO(s)
0
–170
–10
–180
–20
–190
–30
1E+03
1E+04
1E+05 1E+06 1E+07
FREQUENCY – Hz
1E+08
–200
1E+09
Figure 30. Open-Loop Voltage Gain and Phase
–10–
REV. B
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