THS4521 datasheet(38/71 Pages) TI2

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Rf1

480

500

Rg1

120

Rg2

120

Output

Measurement

Point

THS452x

Wideband,

Fully-Differential Amplifier

Rf2

480

200-

Input Impedance

Gain of 4 V/V Design

–

Vcc

–

Vocm

Vcc

FDA

(

)

2 1

+ a

öæ

÷ç

øè

THS4521, THS4522, THS4524

SBOS458H – DECEMBER 2008 – REVISED JUNE 2015

www.ti.com

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 75 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 5:

(5)

For designs that do not need impedance matching, for instance where the input is driven from the low-impedance

output of another amplifier, RG1 = RG2 is the single-to-differential design used without an RT to ground. Setting

RG1 = RG2 = RG in Equation 5 produces Equation 6, which is the input impedance of a simple-input FDA driven

from a low-impedance, single-ended source.

(6)

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. Then the RF is set to get the target gain. For example, targeting an input impedance

of 200

Ω with a gain of 4 V/V, Equation 7 calculates the RG value. Multiplying this required RG value by a gain

of 4 gives the RF value and the design of Figure 76.

(7)

Figure 76. 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.

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