THS4521 datasheet(29/53 Pages) TI2

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10.1.2 Setting Resistor Values Versus Gain

The THS4561 offers considerable flexibility in the configuration and selection of resistor values. The design

starts with the selection of the feedback resistor value. The 1.5-kΩ feedback resistor value used for the

characterization curves is a good compromise between power, noise, and phase margin considerations. With the

feedback resistor values selected (and set equal on each side) the input resistors are set to obtain the desired

gain with input impedance also set with these input resistors. Differential I/O designs provide an input impedance

that is the sum of the two input resistors. Single-ended input to differential output designs present a more

complicated input impedance. Most characteristic curves implement the single-ended to differential design as the

more challenging requirement over differential-to-differential I/O.

For single-ended, matched, input impedance designs, Table 10-1 illustrates the suggested standard resistors set

to approximately a 1.5-kΩ feedback. This table assumes a 50-Ω source and a 50-Ω input match and uses a

single resistor on the non-signal input side for gain matching. Better matching is possible using the same three

resistors on the non-signal input side as on the input side. Figure 10-3 shows the element values and naming

convention for the gain of 1-V/V configuration where the gain is defined from the matched input at RT to the

differential output.

1.5 k

1 k

1.5 k

1.52 k

52.3

THS4561 Wideband,

Fully Differential Amplifier

50-

Input Match,

Gain of 1 V/V from RT,

Single-Ended Source to

Differential Output

VOCM

1.5 k

50-

Source

Impedance

S+

FDA

S±

Figure 10-3. Single-Ended to Differential Gain of 1 V/V with Input Matching Using Standard Resistor

Values

Starting from a target feedback resistor value, the desired input matching impedance, and the target gain (AV),

the required input RT value is given by solving the quadratic of Equation 1.

(

)

(

)

2R R

R A

(1)

When this value is derived, the required input side gain resistor is given by Equation 2 and then the single value

for RG2 on the non-signal input side is given by Equation 3:

(2)

(3)

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THS4561

SBOS874D – AUGUST 2017 – REVISED FEBRUARY 2021

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