Datasheet

TEST CIRCUITS
OutputMeasured
HereWithHigh
Impedance
DifferentialProbe
THS4508
CM
V
IN
R
F
R
F
R
G
R
G
R
IT
R
IT
From
50 W
Source
5V
49.9 W
49.9 W
100 W
0.22 Fm
49.9 W
0.22 Fm
Open
Distortion and 1 db Compression
Frequency Response
THS4508
CM
From
50 W
Source
V
IN
0.22 Fm
49.9 W
V
OUT
Open
To50 W
Test
Equipment
R
G
R
IT
R
G
R
IT
R
F
5V
R
O
R
O
R
OT
0.22 Fm
1:1
R
F
THS4508
www.ti.com .................................................................................................................................... SLAS459E – SEPTEMBER 2005 – REVISED SEPTEMBER 2008
The output is probed using a high-impedance
differential probe across the 100- Ω resistor. The gain
The THS4508 is tested with the following test circuits
is referred to the amplifier output by adding back the
built on the EVM. For simplicity, the power supply
6-dB loss due to the voltage divider on the output.
decoupling is not shown — see the layout in the
Application Information 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- µ F capacitor and a 49.9- Ω
resistor to ground are inserted across R
IT
on the
alternate input to balance the circuit.
Table 1. Gain Component Values
GAIN R
F
R
G
R
IT
Figure 44. Frequency Response Test Circuit
6 dB 348 Ω 165 Ω 61.9 Ω
10 dB 348 Ω 100 Ω 69.8 Ω
14 dB 348 Ω 56.2 Ω 88.7 Ω
20 dB 348 Ω 16.5 Ω 287 Ω
The circuit shown in Figure 45 is used to measure
harmonic distortion, intermodulation distortion, and
Note the gain setting includes 50- Ω source
1-db compression point of the amplifier.
impedance. Components are chosen to achieve
A signal generator is used as the signal source and
gain and 50- Ω input termination.
the output is measured with a spectrum analyzer. The
output impedance of the signal generator is 50 Ω . R
IT
Table 2. Load Component Values
and R
G
are chosen to impedance-match to 50 Ω , and
R
L
R
O
R
OT
Atten.
to maintain the proper gain. To balance the amplifier,
100 Ω 25 Ω Open 6 dB
a 0.22- µ F capacitor and 49.9- Ω resistor to ground are
inserted across R
IT
on the alternate input.
200 Ω 86.6 Ω 69.8 Ω 16.8 dB
499 Ω 237 Ω 56.2 Ω 25.5 dB
A low-pass filter is inserted in series with the input to
1k Ω 487 Ω 52.3 Ω 31.8 dB
reduce harmonics generated at the signal source.
The level of the fundamental is measured, then a
Note the total load includes 50- Ω termination by
high-pass filter is inserted at the output to reduce the
the test equipment. Components are chosen to
fundamental so that it does not generate distortion in
achieve load and 50- Ω line termination through a
the input of the spectrum analyzer.
1:1 transformer.
The transformer used in the output to convert the
Due to the voltage divider on the output formed by
signal from differential to single-ended is an
the load component values, the amplifier output is
ADT1-1WT. It limits the frequency response of the
attenuated. The column Atten in Table 2 shows the
circuit so that measurements cannot be made below
attenuation expected from the resistor divider. When
approximately 1 MHz.
using a transformer at the output as shown in
Figure 45 , the signal will see slightly more loss, and
these numbers will be approximate.
The circuit shown in Figure 44 is used to measure the
frequency response of the 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
Figure 45. Distortion Test Circuit
chosen to impedance match to 50 Ω , and to maintain
the proper gain. To balance the amplifier, a 0.22- µ F
capacitor and 49.9- Ω resistor to ground are inserted
across R
IT
on the alternate input.
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