THS4531A_16 Datasheet, PDF (44/59 Pages) Texas Instruments – Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

THS4531A
SLOS823C – DECEMBER 2012 – REVISED JANUARY 2016
www.ti.com
é
ù
ê
A _V(MAX)
=
æ
ççè
R
R
F
S
ö
- 2÷÷ø ´
ê
êê_{1 +}
ê
êë
1
+
4R^F
R_S
æ
ççè
R
R
F
S
ö²
- 2÷÷ø
ú
ú
ú
ú
ú
úû
(9)
If the achievable A_V(MAX) is less than desired, increase the R_F value. After R_T is derived from Equation 8, the R_G1
element is given by Equation 10:
R _G1
=
2R^F
A_V
1+
- R_S
R_S
R_T
(10)
Then, the simplest approach is to use a single R_G2 = R_T || R_S + R_G1 on the nonsignal 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 11:
2R^F
R _G2
=
A_V
1+
R_S
R_T
(11)
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 THS4531A family
characterization is 2 kΩ. 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 9 to Equation 11 to sweep the target gain from 1 to A_V(MAX) < 10 V/V gives Table 9, 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 2 kΩ. 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 9.
Table 9. Rf = 2 kΩ, Matched Input to 50 Ω, Gain of 1 to 10-V/V Single-Ended to Differential⁽¹⁾
Av
Rt,
EXACT
(Ω)
Rt 1%
Rg1,
EXACT
(Ω)
Rg1 1%
Rg2,
EXACT
(Ω)
Rg2 1%
ACTUAL %ERR TO ACTUAL %ERR TO
Z_IN
Rs
GAIN
Av
1
51
51.1
1996.5
2000
2021.8
2000
50.1
0.3
0.998
–0.2
2
51.7
52.3
996.9
1000
1022.5
1020
50.5
1.0
1.994
–0.3
3
52.5
52.3
656.1
649
681.7
681
49.7
–0.5
3.032
1.1
4
53.2
53.6
491.5
487
517.4
523
50.2
0.4
4.035
0.9
5
54
53.6
388
392
413.9
412
49.6
–0.9
4.953
–0.9
6
54.7
54.9
322.7
324
348.9
348
49.9
–0.2
5.978
–0.4
7
55.5
54.9
272.9
274
299.1
301
49.1
–1.7
6.974
–0.4
8
56.3
56.2
238.1
237
264.6
267
49.3
–1.3
8.034
0.4
9
57.1
57.6
211.2
210
237.9
237
49.7
–0.6
9.044
0.5
10
57.9
57.6
187.4
187
214.1
215
48.9
–2.3
10.017
0.2
(1) R_F = 2 kΩ, R_S = 50 Ω.
44
Submit Documentation Feedback
Product Folder Links: THS4531A
Copyright © 2012–2016, Texas Instruments Incorporated