THS4520 Datasheet, PDF (14/23 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER WITH RAIL-TO-RAIL OUTPUTS

THS4520
SLOS503A – SEPTEMBER 2006 – REVISED OCTOBER 2006
TEST CIRCUITS
The THS4520 is tested with the following test circuits
built on the EVM. For simplicity, power supply
decoupling is not shown – see layout in the
applications section for recommendations.
V_IN
From
R_G
50 W
Source
R_IT
R_F
V_S+
THS4520
R_O
1:1
V_OUT
To 50 W
R_O
R_OT
Test
Equipment
0.22 mF
R_IT
CM
49.9 W
V_S-
R_F
Open
Figure 29. General Test Circuit for Device Testing
and Characterization
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
0 dB
6 dB
10 dB
14 dB
20 dB
R_F
499 Ω
499 Ω
499 Ω
499 Ω
499 Ω
R_G
487 Ω
243 Ω
147 Ω
88.7 Ω
34.8 Ω
R_IT
53.6 Ω
57.6 Ω
63.4 Ω
71.5 Ω
115 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
1 kΩ
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
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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 in test. The column Atten in Table 2
shows the attenuation expected from the resistor
divider. When using a transformer at the output the
signal will have slightly more loss, and the numbers
will be approximate.
Frequency Response
The general circit shown in Figure 29 is modified as
shown in Figure 30, and is used to measure the
frequency response of the device.
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.
From ^VIN
50 W
Source
RG
RIT
0.22 mF
49.9 W
RG
RIT
RF
V_S+
THS4520
CM
V_S−
49.9 W
49.9 W ^{100 W}
Open
0.22 mF
Output Measured
Here With High
Impedance
Differential Probe
RF
Figure 30. Frequency Response Test Circuit
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