THS4509 Datasheet, PDF (25/42 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4509
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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.
The 1-dB compression point is measured with a
spectrum analyzer with 50-Ω double termination or
100-Ω termination; see Table 2. The input power is
increased until the output is 1 dB lower than
expected. The number reported in the table data is
the power delivered to the spectrum analyzer input.
Add 3 dB to refer to the amplifier output.
S-Parameter, Slew Rate, Transient Response,
Settling Time, Output Impedance, Overdrive,
Output Voltage, and Turn-On/Off Time
The circuit shown in Figure 78 is used to measure
s-parameters, slew rate, transient response, settling
time, output impedance, overdrive recovery, output
voltage swing, and turn-on/turn-off times of the
amplifier. For output impedance, the signal is injected
at V_OUT with V_IN left open and the drop across the
49.9-Ω resistor is used to calculate the impedance
seen looking into the amplifier output.
Because S₂₁ is measured single-ended at the load
with 50-Ω double termination, add 12 dB to refer to
the amplifier output as a differential signal.
From ^VIN
50 Ω
Source
RG
R IT
0.22 µF
49.9 Ω
RG
R IT
RF
V_S+
THS 4509
CM
V_S−
RF
49.9 Ω
49.9 Ω
Open
0.22 µF
V_OUT+
V_OUT−
To 50 Ω
Test
Equipment
Figure 78. S-Parameter, SR, Transient Response,
Settling Time, Z_O, Overdrive Recovery, V_OUT
Swing, and Turn-On/Off Test Circuit
SLOS454H – JANUARY 2005 – REVISED NOVEMBER 2009
CM Input
The circuit shown in Figure 79 is used to measure the
frequency response and input impedance of the CM
input. Frequency response is measured single-ended
at V_OUT+ or V_OUT– with the input injected at V_IN, R_CM =
0 Ω, and R_CMT = 49.9 Ω. The input impedance is
measured with R_CM = 49.9 Ω with R_CMT = open, and
calculated by measuring the voltage drop across R_CM
to determine the input current.
0.22 m F
49.9 W
0.22 m F
49.9 W
RG
RIT
RG
RIT
RF
VS+
49.9 W
THS4509 _{49.9 W}
CM
VS–
RF
RCM
RCMT
VOUT–
To
50-W
Test
VOUT+ _Equipment
VIN From
50-W
source
Figure 79. CM Input Test Circuit
CMRR and PSRR
The circuit shown in Figure 80 is used to measure the
CMRR and PSRR of V_S+ and V_S–. The input is
switched appropriately to match the test being
performed.
V_S+
PSRR+
From ^VIN
50 Ω
Source
CMRR
PSRR−
V_S−
348 Ω
100 Ω
100 Ω
69.8 Ω
V_S+
THS4509
CM
V_S−
348 Ω
49.9 Ω
49.9 Ω ^{100 Ω}
Open
0.22 µF
Output
Measured
Here
With High
Impedance
Differential
Probe
Figure 80. CMRR and PSRR Test Circuit
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