THS4631 datasheet(12/34 Pages) NSC

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E Q + RF 1

1 )

F 2

F 3

RF2

−V(Bias)

RF1

RF3

F E Q

F 1

1 )

F 3

F 2

RF2

−V(Bias)

RF1

CF2

−V(Bias)

CF1

CF3

THS4631

SLOS451B

– DECEMBER 2004 – REVISED AUGUST 2011

www.ti.com

such

that

the

corner

frequency

the

transconductance network is much less than the

transimpedance bandwidth of the circuit. Choosing

(5)

this corner frequency properly leads to more accurate

measurements of the transimpedance bandwidth. If

the interface circuit corner frequency is too close to

the bandwidth of the circuit, determining the power

level in the flatband is difficult. A decade or more of

flat bandwidth provides a good basis for determining

the proper transimpedance bandwidth.

ALTERNATIVE TRANSIMPEDANCE

CONFIGURATIONS

Other transimpedance configurations are possible.

Three possibilities are shown below.

resistive

T-network

enables

high

The first configuration is a slight modification of the

transimpedance

gain

with

reasonable

basic

transimpedance

circuit.

splitting

the

resistor values.

feedback resistor, the feedback capacitor value

Figure 40. Alternative Transimpedance

becomes more manageable and easier to control.

Configuration 2

This type of compensation scheme is useful when the

feedback capacitor required in the basic configuration

The third configuration uses a capacitive T-network to

becomes so small that the parasitic effects of the

achieve fine control of the compensation capacitance.

board and components begin to dominate the total

The capacitor CF3 can be used to tune the total

feedback capacitance. By reducing the resistance

effective feedback capacitance to a fine degree. This

across the capacitor, the capacitor value can be

circuit

behaves

the

same

the

basic

increased. This mitigates the dominance of the

transimpedance configuration, with the effective CF

parasitic effects.

given by Equation 6.

(6)

Splitting the feedback resistor enables use

of a larger, more manageable feedback

capacitor.

Figure 39. Alternative Transimpedance

Configuration 1

The second configuration uses a resistive T-network

A capacitive T-network enables fine control

to achieve high transimpedance gains using relatively

of the effective feedback capacitance using

small resistor values. This topology can be useful

relatively large capacitor values.

when the desired transimpedance gain exceeds the

Figure 41. Alternative Transimpedance

value of available resistors. The transimpedance gain

Configuration 3

is given by Equation 5.

THS4631 Datasheet(PDF) 12 Page - National Semiconductor (TI)

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