THS4541IRUNR.B datasheet(39/66 Pages) TI2

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Then, the simplest approach is to use a single Rg2 = Rt || Rs + Rg1 on the nonsignal input side. Often, this

approach is shown as the separate Rg1 and Rs elements. Using these separate elements provides a better

divider match on the two feedback paths, but a single Rg2 is often acceptable. A direct solution for Rg2 is given

as Equation 10:

Rg2

(10)

This design proceeds from a target input impedance matched to Rs, signal gain Av from the matched input

to the differential output voltage, and a selected Rf value. The nominal Rf value chosen for the THS4541

characterization is 402 Ω. As discussed previously, going lower improves noise and phase margin, but reduces

the total output load impedance possibly degrading harmonic distortion. Going higher increases the output noise,

and can reduce the loop-phase margin because of the feedback pole to the input capacitance, but reduces the

total loading on the outputs. Using Equation 8 to Equation 10 to sweep the target gain from 1 to Avmax < 14.3

V/V gives Table 8-1, which shows exact values for Rt, Rg1, and Rg2, where a 50-Ω source must be matched

while setting the two feedback resistors to 402 Ω. One possible solution for 1% standard values is shown, and

the resulting actual input impedance and gain with % errors to the targets are also shown in Table 8-1.

Table 8-1. Required Resistors for a Single-Ended to Differential FDA Design Stepping Gain From

1 V/V to 14 V/V(1)

Rt, EXACT

(Ω)

Rt 1%

Rg1,

EXACT (Ω)

Rg1 1%

Rg2,

EXACT (Ω)

Rg2 1%

ACTUAL

ZIN

%ERR TO

ACTUAL

GAIN

%ERR TO

55.2

54.9

395

392

421

422

49.731

–0.54%

1.006

0.62%

60.1

60.4

193

191

220

221

50.171

0.34%

2.014

0.72%

65.6

64.9

123

124

151

150

49.572

–0.86%

2.983

–0.57%

72.0

71.5

88.9

88.7

118

49.704

–0.59%

4.005

0.14%

79.7

80.6

68.4

68.1

99.2

100

50.451

0.90%

5.014

0.28%

89.1

88.7

53.7

53.6

85.7

86.6

49.909

–0.18%

6.008

0.14%

101

102

43.5

43.2

77.1

76.8

50.179

0.36%

7.029

0.42%

117

118

35.5

35.7

70.6

69.8

50.246

0.49%

7.974

–0.32%

138

137

28.8

28.7

65.4

64.9

49.605

–0.79%

9.016

0.18%

170

169

23.5

23.7

62.0

61.9

50.009

0.02%

9.961

–0.39%

220

221

18.8

18.7

59.6

59.0

49.815

–0.37%

11.024

0.22%

313

316

14.7

57.9

57.6

50.051

0.10%

11.995

–0.04%

545

549

10.9

11.0

56.7

56.2

49.926

–0.15%

12.967

–0.25%

2209

2210

7.26

7.32

56.2

50.079

0.16%

13.986

–0.10%

(1)

Rf = 402 Ω, Rs = 50 Ω, and AvMAX = 14.32 V/V.

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.

Notice 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 Section 7.5 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.

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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