THS4541 datasheet(39/64 Pages) TI1

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Rf1

480

500

Rg1

120

Rg2

120

Output

Measurement

Point

THS4541 Wideband,

Fully-Differential Amplifier

Rf2

480

200-

Input Impedance,

Gain of 4 V/V Design

Vcc

Vocm

Vcc

FDA

2 1

2Rg

Rg1

Rg2

Rg1

Rg2

·§

¸¨

¹©

THS4541-Q1

www.ti.com

SLOS930B – NOVEMBER 2015 – REVISED NOVEMBER 2019

Product Folder Links: THS4541-Q1

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These equations and design flow apply to any FDA. Using the feedback resistor value as a starting point is

particularly useful for current-feedback-based FDAs such as the LMH6554, where the value of these feedback

resistors determines the frequency response flatness. Similar tables can be built using the equations provided

here for other source impedances, Rf values, and gain ranges.

Note the extremely low Rg1 values at the higher gains. For instance, at a gain of 14 V/V, that 7.32-Ω standard

value is transformed by the action of the common-mode loop moving the input common-mode voltage to appear

like a 50-Ω input match. This active input impedance provides an improved input-referred noise at higher gains;

see the Noise Analysis section. The TINA model correctly shows this actively-set input impedance in the single-

ended to differential configuration, and is a good tool to validate the gains, input impedances, response shapes,

and noise issues.

9.4.1.4 Input Impedance for the Single-Ended to Differential FDA Configuration

The designs so far have included a source impedance, Rs, that must be matched by Rt and Rg1. The total

impedance at the junction of Rt and Rg1 for the circuit of Figure 63 is the parallel combination of Rt to ground,

and the ZA (active impedance) presented by Rg1. The expression for ZA, assuming Rg2 is set to obtain the

differential divider balance, is given by Equation 11:

(11)

For designs that do not need impedance matching, but instead come from the low impedance output of another

amplifier for instance, Rg1 = Rg2 is the single-to-differential design used without an Rt to ground. Setting Rg1 =

Rg2 = Rg in Equation 11 gives the input impedance of a simple input FDA driving from a low-impedance, single-

ended source to a differential output as shown in Equation 12:

(12)

In this case, setting a target gain as Rf / Rg

≡ α, and then setting the desired input impedance, allows the Rg

element to be resolved first, and then the required Rf to get the gain. For example, targeting an input impedance

of 200 Ω with a gain of 4 V/V, Equation 13 gives the physical Rg element. Multiplying this required Rg value by a

gain of 4 gives the Rf value and the design of Figure 72.

(13)

Figure 72. 200-Ω Input Impedance, Single-Ended to Differential DC-Coupled Design with Gain of 4 V/V

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