THS4521_10 Datasheet, PDF (22/48 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521
THS4522
THS4524
SBOS458D – DECEMBER 2008 – REVISED AUGUST 2010
TEST CIRCUITS
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Overview
The THS4521, THS4522, and THS4524 family is
tested with the test circuits shown in this section; all
circuits are built using the available THS4521
evaluation module (EVM). For simplicity,
power-supply decoupling is not shown; see the layout
in the Applications section for recommendations.
Depending on the test conditions, component values
change in accordance with Table 1 and Table 2, or
as otherwise noted. In some cases the signal
generators used are ac-coupled and in others they
dc-coupled 50-Ω sources. To balance the amplifier
when ac-coupled, a 0.22-mF capacitor and 49.9-Ω
resistor to ground are inserted across R_IT on the
alternate input; when dc-coupled, only the 49.9-Ω
resistor to ground is added across R_IT. A split power
supply is used to ease the interface to common test
equipment, but the amplifier can be operated in a
single-supply configuration as described in the
Applications section with no impact on performance.
Also, for most of the tests, except as noted, the
devices are tested with single-ended inputs and a
transformer on the output to convert the differential
output to single-ended because common lab test
equipment has single-ended inputs and outputs.
Similar or better performance can be expected with
differential inputs and outputs.
As a result of the voltage divider on the output formed
by the load component values, the amplifier output is
attenuated. The Atten column in Table 2 shows the
attenuation expected from the resistor divider. When
using a transformer at the output (as shown in
Figure 54), the signal sees slightly more loss because
of transformer and line loss; these numbers are
approximate.
Table 1. Gain Component Values for
Single-Ended Input⁽¹⁾
Gain
1 V/V
2 V/V
5 V/V
10 V/V
R_F
1 kΩ
1 kΩ
1 kΩ
1 kΩ
R_G
1 kΩ
487 Ω
187 Ω
86.6 Ω
R_IT
52.3 Ω
53.6 Ω
59.0 Ω
69.8 Ω
1. Gain setting includes 50-Ω source impedance.
Components are chosen to achieve gain and
50-Ω input termination.
Table 2. Load Component Values For 1:1
Differential to Single-Ended Output Transformer⁽¹⁾
R_L
100 Ω
200 Ω
499 Ω
1 kΩ
R_O
24.9 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
1. 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.
Frequency Response
The circuit shown in Figure 53 is used to measure the
frequency response of the circuit.
An HP network analyzer is used as the signal source
and the measurement device. The output impedance
of the HP network analyzer is is dc-coupled and is
50 Ω. R_IT and R_G are chosen to impedance-match to
50 Ω and maintain the proper gain. To balance the
amplifier, a 49.9-Ω resistor to ground is inserted
across R_IT on the alternate input.
The output is probed using a Tektronix
high-impedance differential probe across the 953-Ω
resistor and referred to the amplifier output by adding
back the 0.42-dB because of the voltage divider on
the output.
V_IN+
From
50-W
Source
Calibrated
Differential
Probe
Across
R_IT
Open
Installed to
Balance
Amplifier
49.9 W
R_G
R_IT
PD
0.22 mF
R_IT ^RG
1 kW
V_S+
THS452x
V_OCM
V_S-
1 kW
24.9 W
24.9 W
953 W
Measure with
Differential
Probe
Across R_OT
Open
0.22 mF
Figure 53. Frequency Response Test Circuit
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