LMP7718MASLASHNOPB datasheet(25/42 Pages) TI2

25 / 42 page

VOUT =

-RF

1 + sCFRF

x IDIODE

1 + ¨

§ C

F + CIN

= 8S

GBW RF

2 C

CFRF

LMP7717, LMP7718

www.ti.com

SNOSAY7H – MARCH 2007 – REVISED MARCH 2013

For the above equation s = j

ω. 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 CF. In trying to solve this equation the fourth power of CF 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 CF when both sides are equal will be found. That is the correct value for CF and of course

the closest standard value is used for CF.

Before moving to the lab, the transfer function of the transimpedance amplifier must be found and the units must

be in Ohms.

(19)

The LMP7717 was evaluated for RF = 10 kΩ and 100 kΩ, representing a somewhat lower gain configuration and

with the 100 k

Ω feedback resistor a fairly high gain configuration. The RF = 10 kΩ is covered first. Looking at the

Input Common Mode Capacitance vs. VCM chart for CCM for the operating point selected CCM = 15 pF. Note that

for split supplies VCM = 2.5V, CIN = 22 pF and fGBW = 88 MHz. Solving for CF 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 CF is 2.7 pF. This moves the pole to 5.9 MHz. Figure 63 and Figure 64 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 63. Fall Time

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LMP7718MA/NOPB Datasheet(PDF) 25 Page - Texas Instruments