lmp7718mmx National Semiconductor Corporation, lmp7718mmx Datasheet - Page 18

• Manufacturer Part Number: lmp7718mmx
• Description: 88 Mhz, Precision, Low Noise, 1.8v Cmos Input, Decompensated Operational Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
With no increase in power consumption the decompensated
op amp offers faster speed than the compensated equivalent
part . These examples used R
enough to be easily driven by the LMP7717/LMP7718, yet
small enough to minimize the effects from the parasitic ca-
pacitance of both the PCB and the op amp.
Note: When using the LMP7717/LMP7718, proper high fre-
quency PCB layout must be followed. The GBW of these parts
is 88 MHz, making the PCB layout significantly more critical
than when using the compensated counterparts which have
a GBW of 17 MHz.
TRANSIMPEDANCE AMPLIFIER
An excellent application for either the LMP7717 or the
LMP7718 is as a transimpedance amplifier. With a GBW
product of 88 MHz these parts are ideal for high speed data
transmission by light. The circuit shown on the front page of
the
LMP7717/LMP7718 as transimpedance amplifiers. The only
change is that V
tion of the diode is reversed from the circuit shown on the front
page.
Very high speed components were used in testing to check
the limits of the LMP7717/LMP7718 in a transimpedance
configuration. The photodiode part number is PIN-HR040
from OSI Optoelectronics. The diode capacitance for this part
is only about 7 pF for the 2.5V bias used (V
ground). The rise time for this diode is 1 nsec. A laser diode
was used for the light source. Laser diodes have on and off
times under 5 nsec. The speed of the selected optical com-
ponents allowed an accurate evaluation of the LMP7717 as
a transimpedance amplifier. Nationals evaluation board for
decompensated op amps, PN 551013271-001 A, was used
and only minor modifications were necessary and no traces
had to be cut.
datasheet
FIGURE 15. LMP7715 Response Gain = +1
B
is tied to the V
is
the
circuit
F
CC
= 2 kΩ. This value is high
of the part, thus the direc-
used
to
CC
test
30010889
to virtual
the
18
Figure 16 is the complete schematic for a transimpedance
amplifier. Only the supply bypass capacitors are not shown.
C
its datasheet. C
the op amp and, for the LMP7717 it is shown in the last graph
of the Typical Performance Characteristics section of this
datasheet. In Figure 16 the inverting input pin of the LMP7717
is kept at virtual ground. Even though the diode is connected
to the 2.5V line, a power supply line is AC ground, thus C
connected to ground.
Figure 17 shows the schematic needed to derive F, the feed-
back factor, for a transimpedance amplifier. In this figure
C
the diode capacitance and the op amp input capacitance. The
photodiode is close to an ideal current source once its ca-
pacitance is included in the model. What kind of circuit is this?
Without C
resistor. This circuit is a differentiator! Remember, differen-
tiator circuits are inherently unstable and must be compen-
sated. In this case C
D
D
+ C
represents the photodiode capacitance which is given on
FIGURE 17. Transimpedance Feedback Model
CM
FIGURE 16. Transimpedance Amplifier
= C
F
there is only an input capacitor and a feedback
IN
. Therefore it is critical that the designer knows
CM
is the input common mode capacitance of
F
compensates the circuit.
30010864
30010861
D
is