THS4508 Datasheet, PDF (15/29 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

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100-Ω termination as shown in 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 46 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’s output.
Because S₂₁ is measured single-ended at the load
with 50-Ω double termination, add 12 dB to refer to
the amplifier’s output as a differential signal.
From ^VIN
50-W
Source
RG
R IT
0.22 mF
49.9 W
RG
R IT
RF
5V
THS4508
CM
RF
49.9 W
49.9 W
Open
0.22 mF
V_OUT+
V_OUT-
To 50-W
Test
Equipment
Figure 46. S-Parameter, SR, Transient Response,
Settling Time, Z_O, Overdrive Recovery, V_OUT
Swing, and Turn-on/off Test Circuit
CM Input
The circuit shown in Figure 47 is used to measure the
frequency response and input impedance of the CM
input. Frequency response is measured single-ended
THS4508
SLAS459A – SEPTEMBER 2005 – REVISED FEBRUARY 2006
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 mF
49.9 W
0.22 mF
49.9 W
RG
RIT
RG
RIT
RF
5V
THS4508
CM
RF
49.9 W
49.9 W
RCM
RCMT
VOUT–
To
50-W
Test
VOUT+ _Equipment
VIN From
50-W
source
Figure 47. CM Input Test Circuit
CMRR and PSRR
The circuit shown in Figure 48 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 W
Source
CMRR
PSRR-
V_S-
348 W
5V
100 W
100 W
THS4508
CM
69.8 W
49.9 W
49.9 W ^{100 W}
Open
0.22 mF
Output
Measured
Here
With High
Impedance
Differential
Probe
348 W
Figure 48. CMRR and PSRR Test Circuit
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