lmp7718mmx National Semiconductor Corporation, lmp7718mmx Datasheet - Page 19

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

Using feedback theory, F = V
divider giving the following equation:
The noise gain is 1/F. Because this is a differentiator circuit,
a zero must be inserted. The location of the zero is given by:
C
a pole to the circuit. The pole is located at:
To attain maximum bandwidth and still have good stability the
pole is to be located on the open loop gain curve which is A.
If additional compensation is required one can always in-
crease the value of C
of the circuit. Therefore A = 1/F, or AF = 1. For A the equation
is:
The expression f
For a unity gain stable part this is the frequency where A = 1.
For the LMP7717 f
in the following equation:
For the above equation s = jω. To find the actual amplitude of
the equation the square root of the square of the real and
imaginary parts are calculated. At the intersection of F and A,
we have:
F
has been added for stability. The addition of this part adds
GBW
GBW
is the gain bandwidth product of the part.
F
, but this will also reduce the bandwidth
= 88 MHz. Multiplying A and F results
A
/V
OUT
, this becomes a voltage
19
After a bit of algebraic manipulation the above equation re-
duces to:
In the above equation the only unknown is C
solve this equation the fourth power of C
An excel spread sheet with this equation can be used and all
the known values entered. Then through iteration, the value
of C
correct value for C
is used for C
Before moving to the lab, the transfer function of the tran-
simpedance amplifier must be found and the units must be in
Ohms.
The LMP7717 was evaluated for R
representing a somewhat lower gain configuration and with
the 100 kΩ feedback resistor a fairly high gain configuration.
The R
Mode Capacitance vs. V
point selected C
2.5V, C
culated value is 1.75 pF, so 1.8 pF is selected for use.
Checking the frequency of the pole finds that it is at 8.8 MHz,
which is right at the minimum gain recommended for this part.
Some over compensation was necessary for stability and the
final selected value for C
5.9 MHz. Figure 18 and Figure 19 show the rise and fall times
obtained in the lab with a 1V output swing. The laser diode
was difficult to drive due to thermal effects making the starting
and ending point of the pulse quite different, therefore the two
separate scope pictures.
F
F
when both sides are equal will be found. That is the
= 10 kΩ is covered first. Looking at the Input Common
IN
= 22 pF and f
F
.
CM
F
FIGURE 18. Fall Time
= 15 pF. Note that for split supplies V
and of course the closest standard value
GBW
F
CM
is 2.7 pF. This moves the pole to
= 88 MHz. Solving for C
chart for C
F
= 10 kΩ and 100 kΩ,
CM
F
must be dealt with.
for the operating
30010894
F
. In trying to
www.national.com
F
the cal-
CM
=