THS4541-Q1 Datasheet, PDF (31/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

www.ti.com
THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
Noise Analysis (continued)
The first term is simply the differential input spot noise times the noise gain. The second term is the input current
noise terms times the feedback resistor (and because there are two terms, the power is two times one of the
terms). The last term is the output noise resulting from both the Rf and Rg resistors, again times two, for the
output noise power of each side added together. Using the exact values for a 50-Ω, matched, single-ended to
differential gain, sweep with a fixed Rf = 402 Ω (see Table 6) and the intrinsic noise e_ni = 2.2 nV and I_n = 1.9 pA
for the THS4541-Q1, gives an output spot noise from Equation 6. Then, dividing by the signal gain (Av) gives the
input-referred, spot-noise voltage (e_i) shown in Table 5.
Table 5. Swept Gain Output and Input-Referred, Spot-Noise Calculations⁽¹⁾
Av
Rt, EXACT (Ω) Rg1, EXACT (Ω) Rg2, EXACT (Ω)
NOISE GAIN
e_no (nV/√Hz)
e_i (nV/√Hz)
1
55.2
399
425
1.94
6.64
6.64
2
60.1
191
218
2.85
8.71
4.36
3
65.6
124
153
3.63
10.7
3.56
4
72
89.7
119
4.37
12.1
3.03
5
79.7
67.8
98.3
5.09
13.7
2.74
6
89.1
54.2
86.5
5.65
15.4
2.56
7
101
43.2
76.6
6.25
16.7
2.39
8
117
35.2
70.1
6.74
17.3
2.16
9
138
29.0
65.8
7.11
18.6
2.06
10
170
23.6
62.5
7.44
18.9
1.89
11
220
18.7
59.3
7.78
19.6
1.78
12
313
14.6
57.7
7.97
20.0
1.66
13
545
10.8
56.6
8.11
20.3
1.56
14
2209
7.26
56.1
8.16
21.1
1.50
(1) Rf = 402 Ω.
Notice that the input-referred e_i is less than 2.2 nV/√Hz for just the THS4541-Q1 above a gain of 7 V/V. This
result is because NG is less than Av when the source impedance is included in the NG calculation.
8.6 Factors Influencing Harmonic Distortion
As shown in the swept frequency harmonic distortion plots, the THS4541-Q1 provides extremely low distortion at
lower frequencies. In general, FDA output harmonic distortion mainly relates to the open-loop linearity in the
output stage corrected by the loop gain at the fundamental frequency. As the total load impedance decreases
(including the effect of the feedback resistor elements in parallel for loading purposes), the output-stage, open-
loop linearity degrades, increasing the harmonic distortion, as illustrated in Figure 16 and Figure 34. As the
output voltage swings increase, very fine-scale, open-loop, output-stage nonlinearities increase, also degrading
the harmonic distortion, as illustrated in Figure 14 and Figure 32. Conversely, decreasing the target output
voltage swings drops the distortion terms rapidly. For harmonic-distortion testing, 2 V_PP is used as a nominal
swing because this value represents a typical ADC, full-scale, differential input range.
Increasing the gain acts to decrease the loop gain, resulting in the increasing harmonic distortion terms, as
illustrated in Figure 18 and Figure 36. One advantage to the capacitive compensation for the attenuator design
(described in the Designing Attenuators typical application example) is that the noise gain is shaped up with
frequency to achieve a crossover at an acceptable phase margin at higher frequencies. This compensation holds
the loop gain high at frequencies lower than the noise-gain zero, improving distortion in these lower bands.
Anything that moves the output pin voltage swings close to clipping into the supplies rapidly degrades harmonic
distortion. Output clipping can occur from either absolute differential swing, or the swing can be moved closer to
the supplies with the common-mode control. This effect is illustrated in Figure 17 and Figure 35.
The THS4541-Q1 does an exceptional job of converting from single-ended inputs to differential outputs with very
low harmonic distortions. External resistors of 1% tolerance are used in characterization with good results.
Imbalancing the feedback divider ratios does not degrade distortion directly. Imbalanced feedback ratios convert
common-mode inputs to differential mode at the outputs with the gain described in the Output DC Error and Drift
Calculations and the Effect of Resistor Imbalances section.
Copyright © 2015, Texas Instruments Incorporated
Product Folder Links: THS4541-Q1
Submit Documentation Feedback
31