THS4508 Datasheet, PDF (14/29 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4508
SLAS459A – SEPTEMBER 2005 – REVISED FEBRUARY 2006
TEST CIRCUITS
The THS4508 is tested with the following test circuits
built on the EVM. For simplicity, the power supply
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-µF 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
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'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 45, the signal will see slightly more loss, and
these numbers will be approximate.
Frequency Response
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
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.
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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.
From ^VIN
50 W
Source
RG
R_IT
0.22 mF
49.9 W
RG
R_IT
R_F
5V
THS4508
CM
49.9 W
49.9 W ^{100 W}
Open
0.22 mF
Output Measured
Here With High
Impedance
Differential Probe
R_F
Figure 44. Frequency Response Test Circuit
Distortion and 1 db Compression
The circuit shown in Figure 45 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 1 MHZ.
From ^VIN
50 W
Source
R_G
R_IT
R_G
R_F
5V
THS4508
R_O
1:1 ^VOUT To 50 W
Test
R_O
R_OT
Equipment
0.22 mF
R_IT
49.9 W
CM
R_F
Open
0.22 mF
Figure 45. Distortion Test Circuit
The 1-dB compression point is measured with a
spectrum analyzer with 50-Ω double termination or
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