THS4541_15 Datasheet, PDF (38/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
www.ti.com
Device Functional Modes (continued)
If the achievable Av_max is less than desired, increase the Rf value. After Rt is derived from Equation 7, the Rg1
element is given by Equation 9:
2 ^Rf  Rs
Rg1 ^Av
1  ^Rs
Rt
(9)
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:
2 ^Rf
Rg2
Av
1  ^Rs
Rt
(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 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 8 to Equation 10 to sweep the target gain from 1 to Av_max < 14.3 V/V
gives Table 6, 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 6.
Table 6. Required Resistors for a Single-Ended to Differential FDA Design Stepping Gain from
1 V/V to 14 V/V⁽¹⁾
Rt, EXACT
Av
(Ω)
1
55.2
2
60.1
3
65.6
4
72.0
5
79.7
6
89.1
7
101
8
117
9
138
10
170
11
220
12
313
13
545
14
2209
Rt 1%
54.9
60.4
64.9
71.5
80.6
88.7
102
118
137
169
221
316
549
2210
Rg1,
EXACT
(Ω)
395
193
123
88.9
68.4
53.7
43.5
35.5
28.8
23.5
18.8
14.7
10.9
7.26
Rg1 1%
392
191
124
88.7
68.1
53.6
43.2
35.7
28.7
23.7
18.7
14.7
11.0
7.32
Rg2,
EXACT
(Ω)
421
220
151
118
99.2
85.7
77.1
70.6
65.4
62.0
59.6
57.9
56.7
56.2
Rg2 1%
422
221
150
118
100
86.6
76.8
69.8
64.9
61.9
59.0
57.6
56.2
56.2
ACTUAL
Z_IN
49.731
50.171
49.572
49.704
50.451
49.909
50.179
50.246
49.605
50.009
49.815
50.051
49.926
50.079
%ERR TO
Rs
–0.54%
0.34%
–0.86%
–0.59%
0.90%
–0.18%
0.36%
0.49%
–0.79%
0.02%
–0.37%
0.10%
–0.15%
0.16%
ACTUAL
GAIN
1.006
2.014
2.983
4.005
5.014
6.008
7.029
7.974
9.016
9.961
11.024
11.995
12.967
13.986
%ERR TO
Av
0.62%
0.72%
–0.57%
0.14%
0.28%
0.14%
0.42%
–0.32%
0.18%
–0.39%
0.22%
–0.04%
–0.25%
–0.10%
(1) Rf = 402 Ω, Rs = 50 Ω, and Av_MAX = 14.32 V/V.
38
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