AD8541(2004) データシートの表示（PDF） - Analog Devices
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AD8541 Datasheet PDF : 16 Pages
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AD8541/AD8542/AD8544
much easier and/or less expensive to accomplish in the FNDR
circuit. For example, the Twin-T notch uses three capacitors
with two unique values, whereas the FNDR circuit uses only two
capacitors, which may be of the same value. U3 is simply a buffer
that is added to lower the output impedance of the circuit.
R1
Q ADJUST
200⍀
C1
1F
1/4 AD8544
9
8
10 U3
V OUT
2.5VREF
1/4 AD8544 6
7 U2 5
R
2.61k⍀
C2
1F
3 4 1/4 AD8544
U1 1
2
11
R
2.61k⍀
R
2.61k⍀
1
f=
2p LC1
L = R2C2
R
13 1/4 AD8544
2.61k⍀
14
12 U4
NC
2.5VREF
SPARE
2.5VREF
Figure 3. FNDR 60 Hz Notch Filter with Output Buffer
Comparator Function
A comparator function is a common application for a spare op
amp in a quad package. Figure 4 illustrates 1/4 of the AD8544
as a comparator in a standard overload detection application.
Unlike many op amps, the AD854x family can double as
comparators because this op amp family has rail-to-rail differential
input range, rail-to-rail output, and a great speed versus power
ratio. R2 is used to introduce hysteresis. The AD854x, when
used as comparators, have 5 ms propagation delay at 5 V and 5 ms
overload recovery time.
R1
1k⍀
R2
1M⍀
V OUT
VIN
1/4 AD8544
2.5VDC
2.5VREF
Figure 4. AD854x Comparator Application–Overload Detector
Photodiode Application
The AD854x family has very high impedance with input bias
current typically around 4 pA. This characteristic allows the
AD854x op amps to be used in photodiode applications and
other applications that require high input impedance. Note that
the AD854x has significant voltage offset, which can be removed
by capacitive coupling or software calibration.
Figure 5 illustrates a photodiode or current measurement
application. The feedback resistor is limited to 10 MW to avoid
excessive output offset. Also, note that a resistor is not needed
on the noninverting input to cancel bias current offset because
the bias current related output offset is not significant when
compared to the voltage offset contribution. For the best
performance follow the standard high impedance layout
techniques including:
∑ Shield the circuit.
∑ Clean the circuit board.
∑ Put a trace connected to the noninverting input around the
inverting input.
∑ Use separate analog and digital power supplies.
C
100pF
R
10M⍀
OR
D
V+
27
6
3
4 AD8541
VOUT
2.5VREF
2.5VREF
Figure 5. High Input Impedance Application–Photodiode
Amplifier
–10–
REV. D
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