THS4541IRUNR.B datasheet(40/66 Pages) TI2

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8.4.1.4 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 7-3 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:

Rg1

Rg2

Rg1

Rg2

·§

¸¨

¹©

(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:

2Rg

(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 8-1.

2 1

(13)

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

Figure 8-1. 200-Ω Input Impedance, Single-Ended to Differential DC-Coupled Design with Gain of 4 V/V

After being designed, this circuit can also be ac coupled by adding blocking caps in series with the two 120-Ω Rg

resistors. This active input impedance has the advantage of increasing the apparent load to the prior stage using

lower resistors values, leading to lower output noise for a given gain target.

THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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