Datasheet

V
OUT
=
-R
F
1 + sC
F
R
F
x
I
DIODE
1 +
¨
¨
©
§
C
F
+ C
IN
C
F
¨
¨
©
§
2
= 8S
2
´
¶
2
GBW
R
F
2
C
F
2
=
1
Z
C
F
R
F
AF
´
¶
P
=
´
¶
GBW
´
¶
x
1 + sC
F
R
F
1 + sR
F
(C
F
+ C
IN
)
=
´
¶
GBW
´
¶
x
¨
¨
©
§
1 +
¨
¨
©
§
C
F
R
F
C
F
R
F
2
¨
¨
©
§
1 +
¨
¨
©
§
R
F
(C
F
+ C
IN
)
C
F
R
F
2
= 1
GBW
A =
GBW
Z
Z
=
´
¶
´
¶
LMV793, LMV794
SNOSAX6D –MARCH 2007–REVISED MARCH 2013 www.ti.com
(15)
The expression f
GBW
is the gain bandwidth product of the part. For a unity gain stable part this is the frequency
where A = 1. For the LMV793 f
GBW
= 88 MHz. Multiplying A and F results in the following equation:
(16)
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:
(17)
After a bit of algebraic manipulation the above equation reduces to:
(18)
In the above equation the only unknown is C
F
. In trying to solve this equation the fourth power of C
F
must be
dealt with. An excel spread sheet with this equation can be used and all the known values entered. Then through
iteration, the value of C
F
when both sides are equal will be found. That is the correct value for C
F
, and of course
the closest standard value is used for C
F
.
Before moving the lab, the transfer function of the transimpedance amplifier must be found and the units must be
in Ohms.
(19)
The LMV793 was evaluated for R
F
= 10 kΩ and 100 kΩ, representing a somewhat lower gain configuration and
with the 100 kΩ feedback resistor a fairly high gain configuration. The R
F
= 10 kΩ is covered first. Looking at the
Figure 39 chart for C
CM
for the operating point selected C
CM
= 15 pF. Note that for split supplies V
CM
= 2.5V, C
IN
= 22 pF and f
GBW
= 88 MHz. Solving for C
F
the calculated 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
F
is 2.7 pF.
This moves the pole to 5.9 MHz. Figure 58 and Figure 59 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.
Figure 58. Fall Time
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