THS4521 Datasheet, PDF (37/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

www.ti.com
THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
Device Functional Modes (continued)
Then, the simplest approach is to use a single R_G2 = R_T || R_S + R_G1 on the non-signal input side. Often, this
approach is shown as the separate R_G1 and R_S elements. Using these separate elements provides a better
divider match on the two feedback paths, but a single R_G2 is often acceptable. A direct solution for R_G2 is given
as Equation 4:
2R^F
R _G2
=
A_V
1+
R_S
R_T
(4)
This design proceeds from a target input impedance matched to R_S, signal gain A_v from the matched input to the
differential output voltage, and a selected R_F value. The nominal R_F 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 A_V(MAX) < 14.3 V/V gives Table 7, which shows
exact values for R_T, R_G1, and R_G2, 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 kΩ, Matched Input to 50 Ω, Gain from 1 V to 10 V/V Single to Differential⁽¹⁾
Av
Rt, EXACT
(Ω)
Rt 1%
1 51.95
52.3
2 53.59
53.6
3 55.21
54.9
4 56.88
56.2
5 58.63
59
6 60.47
60.4
7 62.42
61.9
8 64.49
64.9
9 66.70
66.5
10 69.06
69.8
Rg1,
EXACT (Ω)
Rg1 1%
996.92
1000
491.51
487
322.74
324
238.14
237
189.45
191
155.01
154
130.39
130
112.97
113
98.31
97.6
87.40
86.6
Rg2,
EXACT (Ω)
Rg2 1%
1022.48 1020
517.37
523
348.90
348
264.60
267
216.51
215
182.37
182
158.05
158
141.21
140
126.85
127
116.53
118
ACTUAL
Z_IN
50.32
49.95
49.70
49.37
50.23
49.82
49.51
50.12
49.69
50.29
%ERR TO ACTUAL
Rs
GAIN
0.64%
0.997
–0.10%
2.018
–0.60%
2.989
–1.25%
4.017
0.47%
4.964
–0.37%
6.033
–0.98%
7.017
0.23%
7.998
–0.62%
9.050
0.57%
10.069
%ERR TO
Av
–0.30%
0.88%
–0.36%
0.43%
–0.71%
0.56%
0.25%
–0.02%
0.56%
0.69%
(1) R_F = 1 kΩ, R_S = 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, R_F 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.
Copyright © 2008–2015, Texas Instruments Incorporated
Submit Documentation Feedback
37
Product Folder Links: THS4521 THS4522 THS4524