AD8541/AD8542/AD8544
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 out-
put impedance of the circuit.
R1
Q ADJUST
200
C1
1 F
R
2.61k
C2
1 F
R
2.61k
R
2.61k
1
2π LC1
R
2.61k
2.5V
REF
13
12
3
2
4
U1
11
9
10
Photodiode Application
1/4 AD8544
U3
8
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 35, illustrates a photodiode or current measurement
application. The feedback resistor is limited to 10 MΩ 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 per-
formance follow the standard high impedance layout techniques
including: shield circuit, clean circuit board, put a trace con-
nected to the noninverting input around the inverting input,
and use separate analog and digital power supplies.
C
100pF
V
OUT
2.5V
REF
1/4 AD8544
1
1/4 AD8544
7
U2
6
5
1/4 AD8544
U4
14
NC
SPARE
V+
R
10M
f=
L = R
2
C2
2.5V
REF
OR
2
3
D
7
6
V
OUT
Figure 33. FNDR 60 Hz Notch Filter with Output Buffer
Comparator Function
4
AD8541
A comparator function is a common application for a spare op
amp in a quad package. Figure 34 illustrates 1/4 of the AD8544
as a comparator in a standard overload detection application.
Unlike so many op amps, the AD854x family can double as
comparator because this op amp family has rail-to-rail differen-
tial input range, rail-to-rail output, and a great speed vs. power
ratio. R2 is used to introduce hysteresis. The AD854x when
used as comparators have 5
µs
propagation delay @ 5 V and 5
µs
overload recovery time.
R2
1M
R1
1k
2.5V
REF
2.5V
REF
Figure 35. High Input Impedance Application–Photodiode
Amplifier
V
OUT
V
IN
2.5V
DC
2.5V
REF
1/4 AD8544
Figure 34. The AD854x Comparator Application–Overload
Detector
–10–
REV. A
AD [ ANALOG DEVICES ]
