LMV797MM datasheet(16/19 Pages) NSC

16 / 19 page

Application Information (Continued)

(3)

Calculating C

F from Equation (3) can sometimes return un-

reasonably small values (<1 pF), especially for high speed

applications. In these cases, it is often more practical to use

the circuit shown in Figure 10 in order to allow more reason-

able values. In this circuit, the capacitance C

F' is (1+ RB/RA)

times the effective feedback capacitance, C

F. A larger ca-

pacitor can now be used in this circuit to obtain a smaller

effective capacitance.

For example, if a C

F of 0.5 pF is needed, while onlya5pF

capacitor is available, R

B and RA can be selected such that

B/RA = 9. This would convert a CF'of5pFintoaCF of

0.5 pF. This relationship holds as long as R

A << RF.

LMV796 AS A TRANSIMPEDANCE AMPLIFIER

The LMV796 was used in the designs for a number of

amplifiers with varying transimpedance gains and source

capacitances. The gains, bandwidths and feedback capaci-

tances of the circuits created are summarized in Table 1. The

frequency responses are presented in Figure 11 and Figure

12. The feedback capacitances are slightly different from the

formula in Equation (3), since the parasitic capacitance of

the board and the feedback resistor R

F had to be accounted

for.

TABLE 1.

Transimpedance, A

−3 dB Frequency

470000

50 pF 1.5 pF

350 kHz

470000

100 pF 2.0 pF

250 kHz

470000

200 pF 3.0 pF

150 kHz

47000

50 pF 4.5 pF

1.5 MHz

47000

100 pF 6.0 pF

1 MHz

47000

200 pF 9.0 pF

700 kHz

HIGH GAIN WIDEBAND TRANSIMPEDANCE

AMPLIFIER USING THE LMV797

The LMV797 dual, low noise, wide bandwidth, CMOS input

op amp IC can be used for compact, robust and integrated

solutions for sensing and amplifying wide-band signals ob-

tained from sensitive photodiodes. One of the two op amps

available can be used to obtain transimpedance gain while

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FIGURE 9. C

F Selection for Stability

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FIGURE 10. Obtaining Small C

F from Large CF'

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FIGURE 11. Frequency Response for A

TI = 470000

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FIGURE 12. Frequency Response for A

TI = 47000

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