THS4601

SLOS388B – OCTOBER 2001 – REVISED JUNE 2002

13

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APPLICATION INFORMATION

designing the transimpedance circuit (continued)

As indicated, the current source typically sets the requirements for gain, speed, and dynamic range of the

amplifier. For a given amplifier and source combination, achievable performance is dictated by the following

parameters: the amplifier’s gain-bandwidth product, the amplifier’s input capacitance, the source capacitance,

the transimpedance gain, the amplifier’s slew rate, and the amplifier’s output swing. From this information, the

optimal performance of a transimpedance circuit using a given amplifier can be determined. Optimal is defined

here as providing the required transimpedance gain with a maximally flat frequency response.

For the circuit shown in Figure 26, all but one of the design parameters is known; the feedback capacitor must

be determined. Proper selection of the feedback capacitor prevents an unstable design, controls pulse

response characteristics, provides maximally flat transimpedance bandwidth, and limits broadband integrated

capacitance and parasitic capacitance at the inverting node), and GBP is the gain-bandwidth product of the

amplifier in hertz.

C

F +

1

pR

F

GBP

)

1

pR

F

GBP

2

)

4C

S

pR

F

GBP

2

Once the optimal feedback capacitor has been selected, the transimpedance bandwidth can be calculated with

equation 2.

F

GBP

2

pR

C

F

_

+

CIDIFF

CICM

CP

RF

CF

CD

IDIODE

NOTE: The total source capacitance is the sum of several distinct capacitances.

Figure 27. Transimpedance Analysis Circuit

(1)

(2)