LMH6629 datasheet(17/22 Pages) NSC

17 / 22 page

LOW-NOISE TRANSIMPEDANCE AMPLIFIER

Figure 12 implements a high speed, single supply, low-noise

Transimpedance amplifier commonly used with photo-

diodes. The transimpedance gain is set by R

30068011

FIGURE 12. 200MHz Transimpedance Amplifier

Configuration

Figure 13 shows the Noise Gain (NG) and transfer function

(I-V Gain). As with most Transimpedance amplifiers, it is re-

quired to compensate for the additional phase lag (Noise Gain

zero at f

Z) created by the total input capacitance ( CD (diode

capacitance) + C

CM (LMH6629 input capacitance) ) looking

into R

F; this is accomplished by placing CF across RF to create

enough phase lead (Noise Gain pole at f

P) to stabilize the

loop.

30068002

FIGURE 13. Transimpedance Amplifier Noise Gain &

Transfer Function

The optimum value of C

F is given by Equation 5 resulting in

the I-V -3dB bandwidth shown in Equation 6, or around

200MHz in this case (assuming GBWP= 4GHz with COMP

pin = HI). This C

F value is a “starting point” and CF needs to

be tuned for the particular application as it is often less than

1pF and thus is easily affected by board parasitics, etc. For

maximum speed, the LMH6629 COMP pin should be HI.

Equation 5: Optimum C

F Value

(5)

Equation 6: Resulting -3dB Bandwidth

(6)

Equation 7 provides the total input current noise density (i

ni)

equation for the basic Transimpedance configuration and is

plotted against feedback resistance (R

F) showing all con-

tributing noise sources in Figure 14. The plot indicates the

expected total equivalent input current noise density (i

ni) for a

given feedback resistance (R

F). This is depicted in the

schematic of Figure 15 where total equivalent current noise

density (i

ni) is shown at the input of a noiseless amplifier and

noiseless feedback resistor (R

F). The total equivalent output

voltage noise density (e

no) is ini*RF.

30068028

FIGURE 14. Current Noise Density vs. Feedback

Resistance

Equation 7: Noise Equation for Transimpedance

Amplifier

(7)

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