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參數資料
| 型號： | AD8137YR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運動控制電子 |
| 英文描述： | Low Cost, Low Power 12-Bit Differential ADC Driver |
| 中文描述： | OP-AMP, 2600 uV OFFSET-MAX, PDSO8 |
| 封裝： | MS-012AA, SOIC-8 |
| 文件頁數： | 21/24頁 |
| 文件大小： | 594K |
| 代理商： | AD8137YR |
AD8137
Estimating DC Errors
Primary differential output offset errors in the AD8137 are due
to three major components: the input offset voltage, the offset
between the V
AN
and V
AP
input currents interacting with the
feedback network resistances, and the offset produced by the dc
voltage difference between the input and output common-mode
voltages in conjunction with matching errors in the feedback
network.
Rev. A | Page 21 of 24
The first output error component is calculated as
+
R
=
G
G
F
IO
R
R
V
Vo_e
1
, or equivalently as V
IO
/β
(21)
where
V
IO
is the input offset voltage.
The second error is calculated as
(
)
F
IO
G
F
F
R
G
+
G
G
F
IO
R
I
R
R
R
R
R
R
I
Vo_e
=
+
=
2
(22)
where
I
IO
is defined as the offset between the two input bias
currents.
The third error voltage is calculated as
)
(
3
OCM
V
ICM
V
enr
Vo_e
×
=
(23)
where Δ
enr
is the fractional mismatch between the two feed-
back resistors.
The total differential offset error is the sum of these three error
sources.
Additional Impact of Mismatches in the Feedback Networks
The internal common-mode feedback network will still force
the output voltages to remain balanced, even when the R
F
/R
G
feedback networks are mismatched. The mismatch will, how-
ever, cause a gain error proportional to the feedback network
mismatch.
Ratio-matching errors in the external resistors will degrade the
ability to reject common-mode signals at the V
AN
and V
IN
input
terminals, much the same as with a four-resistor difference
amplifier made from a conventional op amp. Ratio-matching
errors will also produce a differential output component that is
equal to the V
OCM
input voltage times the difference between the
feedback factors (βs). In most applications using 1% resistors,
this component amounts to a differential dc offset at the output
that is small enough to be ignored.
Driving a Capacitive Load
A purely capacitive load will react with the bondwire and pin
inductance of the AD8137, resulting in high frequency ringing
in the transient response and loss of phase margin. One way to
minimize this effect is to place a small resistor in series with
each output to buffer the load capacitance. The resistor and load
capacitance will form a first-order, low-pass filter, so the resistor
value should be as small as possible. In some cases, the ADCs
require small series resistors to be added on their inputs.
Figure 39 and Figure 42 illustrate transient response versus ca-
pacitive load, and were generated using series resistors in each
output and a differential capacitive load.
Layout Considerations
Standard high speed PCB layout practices should be adhered to
when designing with the AD8137. A solid ground plane is
recommended and good wideband power supply decoupling
networks should be placed as close as possible to the supply pins.
To minimize stray capacitance at the summing nodes, the
copper in all layers under all traces and pads that connect to the
summing nodes should be removed. Small amounts of stray
summing-node capacitance will cause peaking in the frequency
response, and large amounts can cause instability. If some stray
summing-node capacitance is unavoidable, its effects can be
compensated for by placing small capacitors across the feedback
resistors.
Terminating a Single-Ended Input
Controlled impedance interconnections are used in most high
speed signal applications, and they require at least one line ter-
mination. In analog applications, a matched resistive termina-
tion is generally placed at the load end of the line. This section
deals with how to properly terminate a single-ended input to
the AD8137.
The input resistance presented by the AD8137 input circuitry is
seen in parallel with the termination resistor, and its loading
effect must be taken into account. The Thevenin equivalent
circuit of the driver, its source resistance, and the termination
resistance must all be included in the calculation as well. An
exact solution to the problem requires solution of several simul-
taneous algebraic equations and is beyond the scope of this data
sheet. An iterative solution is also possible and is simpler,
especially considering the fact that standard resistor values are
generally used.
Figure 66 shows the AD8137 in a unity-gain configuration, and
with the following discussion, provides a good example of how
to provide a proper termination in a 50 environment.
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