THS4541IRUNR.B datasheet(23/66 Pages) TI2

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In this case, the input is dc-coupled, showing a 50-Ω input match to the source, having a gain of 5 V/V to a

differential output, and again is driving a nominal 500-Ω load. Using a single supply, the Vocm control input

can either be floated (defaulting to mid-supply) or be driven within the allowed range for the Vocm loop (see

the headroom limits on Vocm in the Electrical Characteristics tables). To use this circuit for step-response

measurements, load each of the two outputs with a 250-Ω network, translating to a 50-Ω source impedance

driving into two 50-Ω scope inputs. Then, difference the scope inputs to generate the step responses of Figure

6-9 and Figure 6-27. Figure 7-4 shows the output interface circuit. This grounded interface pulls a dc load current

from the output Vocm voltage for single-supply operation. Running this test with balanced bipolar power supplies

eliminates this dc load current and gives similar waveform results.

Ro1

221

Ro2

221

Rm1

64.9

THS4541

Output

Rm1

64.9

50-

Scope

50-

Scope

Figure 7-4. Example 500-Ω Load to Differential, Doubly-Terminated, DC-Coupled 50-Ω Scope Interface

7.2 Frequency-Response Shape Factors

Figure 6-1 illustrates the small-signal response shape versus gain using a fixed 402-Ω feedback resistor in the

circuit of Figure 7-1. Being a voltage-feedback based FDA, the THS4541 shows a response shape that varies

with gain setting, largely determined by the loop-gain crossover frequency and phase margin at the crossover.

This loop-gain crossover frequency is where the open-loop response and the noise gain intersect (where the

loop gain drops to 1). The noise gain is the inverse of the voltage divider from the outputs back to the differential

inputs; use a balanced divider ratio on each feedback path. In general, the noise gain (NG) does not equal

the signal gain for designs providing an input match from a source impedance. NG is given by 1 + Rf / (total

impedance from the inverting summing junction to ground). Using the resistor values computed in the gain

sweep of Table 8-1, and repeating that sweep showing the NG gives Table 7-1, where only the exact R solutions

are shown.

Table 7-1. Resistor Values and Noise Gain for Swept Gain with Rf = 402 Ω(1)

SIGNAL GAIN

Rt, EXACT (Ω)

Rg1, EXACT (Ω)

Rg2, EXACT (Ω)

NOISE GAIN

55.2

399

425

1.94

60.1

191

218

2.85

65.6

124

153

3.63

89.7

119

4.37

79.7

67.8

98.3

5.09

89.1

54.2

86.5

5.65

101

43.2

76.6

6.25

117

35.2

70.1

6.74

138

65.8

7.11

170

23.6

62.5

7.44

220

18.7

59.3

7.78

313

14.6

57.7

7.97

545

10.8

56.6

8.11

2209

7.26

56.1

8.16

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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