THS4509 Datasheet, PDF (24/42 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4509
SLOS454H – JANUARY 2005 – REVISED NOVEMBER 2009
TEST CIRCUITS
The THS4509 is tested with the following test circuits
built on the evaluation module (EVM). For simplicity,
power-supply decoupling is not shown—see the
Layout Recommendations in the Applications section
for recommendations. Depending on the test
conditions, component values are changed per the
following tables, or as otherwise noted. The signal
generators used are ac-coupled, 50-Ω sources, and a
0.22-μF capacitor and 49.9-Ω resistor to ground are
inserted across R_IT on the alternate input to balance
the circuit. A split power supply is used to ease the
interface to common test equipment, but the amplifier
can be operated single-supply as described in the
Applications section with no impact on performance.
Table 1. Gain Component Values
GAIN
6 dB
10 dB
14 dB
20 dB
R_F
348 Ω
348 Ω
348 Ω
348 Ω
R_G
165 Ω
100 Ω
56.2 Ω
16.5 Ω
R_IT
61.9 Ω
69.8 Ω
88.7 Ω
287 Ω
Note the gain setting includes 50-Ω source
impedance. Components are chosen to achieve
gain and 50-Ω input termination.
Table 2. Load Component Values
R_L
100 Ω
200 Ω
499 Ω
1k Ω
R_O
25 Ω
86.6 Ω
237 Ω
487 Ω
R_OT
Open
69.8 Ω
56.2 Ω
52.3 Ω
ATTEN.
6 dB
16.8 dB
25.5 dB
31.8 dB
Note the total load includes 50-Ω termination by
the test equipment. Components are chosen to
achieve load and 50-Ω line termination through a
1:1 transformer.
Due to the voltage divider on the output formed by
the load component values, the amplifier output is
attenuated. The column Atten in Table 2 shows the
attenuation expected from the resistor divider. When
using a transformer at the output as shown in
Figure 77, the signal will see slightly more loss, and
these numbers will be approximate.
Frequency Response
The circuit shown in Figure 76 is used to measure the
frequency response of the circuit.
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From ^VIN
50 Ω
Source
RG
RIT
0.22 µF
49.9 Ω
RG
RIT
RF
V_S+
THS4509
CM
V_S−
RF
49.9 Ω
49.9 Ω ^{100 Ω}
Open
0.22 µF
Output Measured
Here With High
Impedance
Differential Probe
Figure 76. Frequency Response Test Circuit
A network analyzer is used as the signal source and
as the measurement device. The output impedance
of the network analyzer is 50 Ω. R_IT and R_G are
chosen to impedance match to 50 Ω, and to maintain
the proper gain. To balance the amplifier, a 0.22-μF
capacitor and 49.9-Ω resistor to ground are inserted
across R_IT on the alternate input.
The output is probed using a high-impedance
differential probe across the 100-Ω resistor. The gain
is referred to the amplifier output by adding back the
6-dB loss due to the voltage divider on the output.
Distortion and 1-dB Compression
The circuit shown in Figure 77 is used to measure
harmonic distortion, intermodulation distortion, and
1-db compression point of the amplifier.
From ^VIN
50 Ω
Source
R_G
R_IT
0.22 µF
49.9 Ω
R_G
R_IT
R_F
V_S+
THS 4509
CM
V_S−
R_F
R_O
1:1 ^VOUT To 50 Ω
Test
R_O
R_OT
Equipment
Open
0.22 µF
Figure 77. Distortion Test Circuit
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