THS4509 Datasheet, PDF (19/30 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

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TEST CIRCUITS
The THS4509 is tested with the following test circuits
built on the EVM. For simplicity, power supply de-
coupling is not shown – see layout 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 a 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 im-
pedance. 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's 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 76, the signal will see slightly more loss, and
these numbers will be approximate.
Frequency Response
The circuit shown in Figure 75 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
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.
THS4509
SLOS454 – JANUARY 2005
The output is probed using a high-impedance differ-
ential 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.
From ^VIN
50 Ω
Source
RG
RIT
0.22 µF
49.9 Ω
RG
RIT
RF
V_S+
THS4509
CM
V_S−
49.9 Ω
49.9 Ω ^{100 Ω}
Open
0.22 µF
Output Measured
Here With High
Impedance
Differential Probe
RF
Figure 75. Frequency Response Test Circuit
Distortion and 1dB Compression
The circuit shown in Figure 76 is used to measure
harmonic distortion, intermodulation distortion, and
1-db compression point of the amplifier.
A signal generator is used as the signal source and
the output is measured with a spectrum analyzer. The
output impedance of the signal generator 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.
A low-pass filter is inserted in series with the input to
reduce harmonics generated at the signal source.
The level of the fundamental is measured, then a
high-pass filter is inserted at the output to reduce the
fundamental so that it does not generate distortion in
the input of the spectrum analyzer.
The transformer used in the output to convert the
signal from differential to single ended is an
ADT1-1WT. It limits the frequency response of the
circuit so that measurements cannot be made below
approximately 1MHz.
From ^VIN
50 Ω
Source
R_G
R_IT
R_G
R_F
V_S+
THS 4509
R_O
1:1 ^VOUT To 50 Ω
Test
R_O
R_OT
Equipment
0.22 µF
R_IT
CM
49.9 Ω
V_S−
R_F
Open
0.22 µF
Figure 76. Distortion Test Circuit
The 1-dB compression point is measured with a
spectrum analyzer with 50-Ω double termination or
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