THS4541-Q1 Datasheet, PDF (23/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

www.ti.com
THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
Example Characterization Circuits (continued)
In this case, the input is DC-coupled, showing a 50-Ω input match to the source, gain of 5 V/V to a differential
output, again driving a nominal 500-Ω load. Using a single supply, the Vocm control input can either be floated
(defaulting to midsupply) 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 9 and Figure 27. Figure 64
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
THS4541
Output
Ro2
221
Rm1
64.9
50-
Scope
Rm1
64.9
50-
Scope
Figure 64. Example 500-Ω Load to Differential, Doubly-Terminated, DC-Coupled 50-Ω Scope Interface
8.2 Frequency-Response Shape Factors
Figure 1 illustrates the small-signal response shape versus gain using a fixed 402-Ω feedback resistor in the
circuit of Figure 61. Being a voltage-feedback based FDA, the THS4541-Q1 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 6, and repeating that sweep showing the NG gives Table 1, where only the exact R solutions are
shown.
Table 1. Resistor Values and Noise Gain for Swept Gain with Rf = 402 Ω⁽¹⁾
SIGNAL GAIN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Rt, EXACT (Ω)
55.2
60.1
65.6
72
79.7
89.1
101
117
138
170
220
313
545
2209
Rg1, EXACT (Ω)
399
191
124
89.7
67.8
54.2
43.2
35.2
29
23.6
18.7
14.6
10.8
7.26
Rg2, EXACT (Ω)
425
218
153
119
98.3
86.5
76.6
70.1
65.8
62.5
59.3
57.7
56.6
56.1
NOISE GAIN
1.94
2.85
3.63
4.37
5.09
5.65
6.25
6.74
7.11
7.44
7.78
7.97
8.11
8.16
(1) Rf = 402 Ω, Rs = 50 Ω, and Av_MAX = 14.32 V/V.
Copyright © 2015, Texas Instruments Incorporated
Product Folder Links: THS4541-Q1
Submit Documentation Feedback
23