OP275
and dc offset errors. If the parallel combination of R
F
and R
G
is
larger than 2 k
, then an additional resistor, R
S
, should be used
in series with the noninverting input. The value of R
S
is deter-
mined by the parallel combination of R
F
and R
G
to maintain the
low distortion performance of the OP275.
Driving Capacitive Loads
The OP275 was designed to drive both resistive loads to 600
and capacitive loads of over 1000 pF and maintain stability. While
there is a degradation in bandwidth when driving capacitive loads,
the designer need not worry about device stability. The graph in
Figure 16 shows the 0 dB bandwidth of the OP275 with capaci-
tive loads from 10 pF to 1000 pF.
10
9
8
The design is a transformerless, balanced transmission system
where output common-mode rejection of noise is of paramount
importance. Like the transformer based design, either output can
be shorted to ground for unbalanced line driver applications
without changing the circuit gain of 1. Other circuit gains can be
set according to the equation in the diagram. This allows the
design to be easily set to noninverting, inverting, or differential
operation.
A 3-Pole, 40 kHz Low-Pass Filter
BANDWIDTH – MHz
7
6
5
4
3
2
1
0
0
200
400
600
C
LOAD
– pF
800
1000
The closely matched and uniform ac characteristics of the OP275
make it ideal for use in GIC (Generalized Impedance Converter)
and FDNR (Frequency-Dependent Negative Resistor) filter
applications. The circuit in Figure 18 illustrates a linear-phase,
3-pole, 40 kHz low-pass filter using an OP275 as an inductance
simulator (gyrator). The circuit uses one OP275 (A2 and A3) for
the FDNR and one OP275 (A1 and A4) as an input buffer and
bias current source for A3. Amplifier A4 is configured in a gain
of 2 to set the pass band magnitude response to 0 dB. The ben-
efits of this filter topology over classical approaches are that the
op amp used in the FDNR is not in the signal path and that the
filter’s performance is relatively insensitive to component varia-
tions. Also, the configuration is such that large signal levels can
be handled without overloading any of the filter’s internal nodes.
As shown in Figure 19, the OP275’s symmetric slew rate and low
distortion produce a clean, well behaved transient response.
R1
95.3k
2
–
A1
3
+
C1
2200pF
R2
787
Figure 16. Bandwidth vs. C
LOAD
High Speed, Low Noise Differential Line Driver
1
V
IN
1
–
2
3
R3
1.82k
C3
2200pF
R4
1.87k
A2
+
R3
2k
2
–
A2
3
+
1
R9
50
V
O1
R11
1k
R5
1.82k
R1
2k
+
V
IN
3
2
R4
2k
R7
2k
V
O2
– V
O1
= V
IN
P1
10k
Figure 18. A 3-Pole, 40 kHz Low-Pass Filter
A1
–
1
R5
2k
R6
2k
6
R10
50
100
90
R2
2k
R12
1k
V
O2
A1 = 1/2 OP275
A2, A3 = 1/2 OP275
R3
–
A3
5
+
V
OUT
10V p-p
10kHz
10
0%
7
R8
2k
GAIN =
R1
SET R2, R4, R5 = R1 AND R6, R7, R8 = R3
Figure 17. High Speed, Low Noise Differential Line Driver
SCALE: VERTICAL–2V/ DIV
HORIZONTAL–10s/ DIV
Figure 19. Low-Pass Filter Transient Response
–10–
–
6
+
5
A3
R9
1k
A1, A4 = 1/2 OP275
A2, A3 = 1/2 OP275
–
C2
2200pF
C4
2200pF
7
R7
100k
6
+
The circuit in Figure 17 is a unique line driver widely used in
industrial applications. With ±18 V supplies, the line driver can
deliver a differential signal of 30 V p-p into a 2.5 k
load. The
high slew rate and wide bandwidth of the OP275 combine to
yield a full power bandwidth of 130 kHz while the low noise
front end produces a referred-to-input noise voltage spectral
density of 10 nV/
Hz.
R6
4.12k
5
A4
7
V
OUT
R8
1k
REV. C
AD [ ANALOG DEVICES ]
