參數(shù)資料
型號(hào)： AD8627AKS-REEL
廠商： ANALOG DEVICES INC
元件分類： 運(yùn)動(dòng)控制電子
英文描述： Precision Low Power Single-Supply JFET Amplifier
中文描述： OP-AMP, 1350 uV OFFSET-MAX, 5 MHz BAND WIDTH, PDSO5
封裝： PLASTIC, MO-203AA, SC-70, 5 PIN
文件頁(yè)數(shù)： 15/20頁(yè)
文件大?。?/td> 568K
代理商： AD8627AKS-REEL
AD8627/AD8626/AD8625
MINIMIZING INPUT CURRENT
The AD862x is guaranteed to 1 pA max input current with a
±13 V supply voltage at room temperature. Careful attention to
how the amplifier is used will maintain or possibly better this
performance. The amplifier’s operating temperature should be
kept as low as possible. Like other JFET input amplifiers, the
AD862x’s input current doubles for every 10°C rise in junction
temperature, as illustrated in Figure 8. On-chip power dissipation
raises the device operating temperature, causing an increase in
input current. Reducing supply voltage to cut power dissipation
reduces the AD862x’s input current. Heavy output loads can
also increase chip temperature; maintaining a minimum load
resistance of 1 k is recommended.
Rev. B | Page 15 of 20
The AD862x is designed for mounting on PC boards.
Maintaining picoampere resolution in those environments
requires a lot of care. Both the board and the amplifier’s
package have finite resistance. Voltage differences between the
input pins and other pins as well as PC board metal traces may
cause parasitic currents larger than the AD862x’s input current,
unless special precautions are taken. For proper board layout
to ensure the best result, refer to the ADI website for proper
layout seminar material. Two common methods of minimizing
parasitic leakages that should be used are guarding of the input
lines and maintaining adequate insulation resistance.
Contaminants such as solder flux on the board’s surface and the
amplifier’s package can greatly reduce the insulation resistance
between the input pin and traces with supply or signal voltages.
Both the package and the board must be kept clean and dry.
PHOTODIODE PREAMPLIFIER APPLICATION
The low input current and offset voltage levels of the AD862x,
together with its low voltage noise, make this amplifier an
excellent choice for preamplifiers used in sensitive photodiode
applications. In a typical photovoltaic preamp circuit, shown in
Figure 45, the output of the amplifier is equal to
(P)Rf
R
ID(Rf)
V
p
OUT
=
=
where:
ID
= photodiode signal current (A)
R
p
= photodiode sensitivity (A/W)
R
f
= value of the feedback resistor, in
P
= light power incident to photodiode surface, in W
The amplifier’s input current,
I
B
, contributes an output voltage
error proportional to the value of the feedback resistor. The
offset voltage error,
V
OS
, causes a small current error due to the
photodiode’s finite shunt resistance,
R
D
.
The resulting output voltage error,
V
E
, is equal to
)
Rf(I
V
R
R
V
B
OS
D
f
E
+
+
=
1
A shunt resistance on the order of 100 M is typical for a small
photodiode. Resistance
R
D
is a junction resistance that typically
drops by a factor of two for every 10°C rise in temperature. In
the AD862x, both the offset voltage and drift are low, which
helps minimize these errors. With
I
B
values of 1 pA and
V
OS
of
50 mV,
V
E
for Figure 45 is very negligible. Also, the circuit in
Figure 45 results in an SNR value of 95 dB for a signal bandwidth
of 30 kHz.
0
R
100M
C4
15pF
I
B
I
B
V
OS
C
F
5pF
R
F
1.5M
OUTPUT
AD8627
PHOTODIODE
Figure 45. A Photodiode Model Showing DC Error
OUTPUT AMPLIFIER FOR DIGITAL-TO-ANALOG
CONVERTERS
Many system designers use amplifiers as buffers on the output
of amplifiers to increase the DAC’s output driving capability.
The high resolution current output DACs need high precision
amplifiers on their output as current to voltage converters (I/V).
Additionally, many DACs operate with a single supply of 5 V. In
a single-supply application, selection of a suitable op amp may
be more difficult because the output swing of the amplifier does
not usually include the negative rail, in this case AGND. This
can result in some degradation of the DAC’s specified perform-
ance unless the application does not use codes near zero. The
selected op amp needs to have very low offset voltage—for a
14-bit DAC, the DAC LSB is 300 μV with a 5 V reference—to
eliminate the need for output offset trims. Input bias current
should also be very low because the bias current multiplied by
the DAC output impedance (about 10 k in some cases) adds
to the zero code error. Rail-to-rail input and output performance
is desired. For fast settling, the slew rate of the op amp should
not impede the settling time of the DAC. Output impedance of
the DAC is constant and code independent, but in order to
minimize gain errors, the input impedance of the output
amplifier should be as high as possible. The AD862x, with very
high input impedance, I
B
of 1 pA, and fast slew rate, is an ideal
amplifier for these types of applications. A typical configuration
with a popular DAC is shown in Figure 46. In these situations,
the amplifier adds another time constant to the system, increasing
the settling time of the output. The AD862x, with 5 MHz of BW,
helps in achieving a faster effective settling time of the combined
DAC and amplifier.
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