THS4541-Q1 Datasheet, PDF (22/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
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Example Characterization Circuits (continued)
The frequency domain characterization curves start with the selections of Figure 61. Then, various elements are
modified to show their impact over a range of design targets, specifically:
• Gain setting is changed by adjusting R_t and the 2 – R_g elements (holding a 50-Ω input match).
• Output loading, including both resistive and capacitive load testing.
• Power-supply settings. Most often, a single +5-V test uses a ±2.5-V supply, and a +3-V test uses ±1.5-V
supplies.
• The disable control pin is tied to Vs+ for any active channel test.
Because most network and spectrum analyzers are a single-ended input, the output network on the THS4541-Q1
characterization tests typically show the desired load connected through a balun to a single-ended, 50-Ω load,
while presenting a 50-Ω source from the balun output back into the balun. For instance, Figure 62 shows a
wideband MA/Com balun used for Figure 61. This network shows a 500-Ω differential load to the THS4541-Q1,
but an AC-coupled, 50-Ω source to the network analyzer. Distortion testing typically uses a lower-frequency, DC-
isolated balun (such as the TT1-6T) that is rotated 90° from the wider band interface of Figure 62.
Ro1
237
THS4541
Output
500-
Differential
Load
Ro2
237
ETC1-1-13
C10
100 nF
N1
R9
56.2
N2
C9
100 nF
50- Load
Figure 62. Example 500-Ω Load to a Single-Ended, Doubly-Terminated, AC-Coupled, 50-Ω Interface
This approach allows a higher differential load, but with a wideband 50-Ω output match at the cost of
considerable signal-path insertion loss. This loss is acceptable for characterization, and is normalized out to
show the characterization curves.
For time-domain or DC-coupled testing, the circuit of Figure 63 is used as a starting point, where the gain of a
5-V/V setting used in Figure 9 and Figure 27 are illustrated.
50- Input Match,
Gain of 5 V/V from Rt,
Single-Ended Source to
Differential Step-Response Test
50-
Source
Rg1
68.1
Rt
80.6
THS4541 Wideband,
Fully-Differential Amplifier
Rf1
402
Vcc
Vocm
±
+
FDA
±
+
PD
Rg2
Vcc
100
R1
500
Output
Measurement
Point
Rf2
402
Figure 63. DC-Coupled, Single-Ended-to-Differential, Basic Test Circuit Set for a Gain of 5 V/V
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