THS4521 datasheet(37/71 Pages) TI2

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THS4521, THS4522, THS4524

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SBOS458H – DECEMBER 2008 – REVISED JUNE 2015

Device Functional Modes (continued)

Then, the simplest approach is to use a single RG2 = RT || RS + RG1 on the non-signal 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 4:

(4)

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 THS452x family

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

total loading on the outputs.

Using Equation 2 to Equation 4 to sweep the target gain from 1 to AV(MAX) < 14.3 V/V gives Table 7, 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 7.

Table 7. Rf = 1 k

Ω, Matched Input to 50 Ω, Gain from 1 V to 10 V/V Single to Differential(1)

Rt, EXACT

Rg1,

Rg2,

ACTUAL

%ERR TO

ACTUAL

%ERR TO

Rt 1%

Rg1 1%

Rg2 1%

(

Ω)

EXACT (

Ω)

EXACT (

Ω)

ZIN

GAIN

51.95

52.3

996.92

1000

1022.48

1020

50.32

0.64%

0.997

–0.30%

53.59

53.6

491.51

487

517.37

523

49.95

–0.10%

2.018

0.88%

55.21

54.9

322.74

324

348.90

348

49.70

–0.60%

2.989

–0.36%

56.88

56.2

238.14

237

264.60

267

49.37

–1.25%

4.017

0.43%

58.63

189.45

191

216.51

215

50.23

0.47%

4.964

–0.71%

60.47

60.4

155.01

154

182.37

182

49.82

–0.37%

6.033

0.56%

62.42

61.9

130.39

130

158.05

158

49.51

–0.98%

7.017

0.25%

64.49

64.9

112.97

113

141.21

140

50.12

0.23%

7.998

–0.02%

66.70

66.5

98.31

97.6

126.85

127

49.69

–0.62%

9.050

0.56%

69.06

69.8

87.40

86.6

116.53

118

50.29

0.57%

10.069

0.69%

(1)

RF = 1 kΩ, RS = 50 Ω.

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.

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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