THS4561 Datasheet, PDF (16/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

THS4561
SBOS874 – AUGUST 2017
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Example Characterization Circuits (continued)
Starting from the test circuit of Figure 1, various elements are modified to show the effect of these elements over
a range of design targets, specifically:
• The gain setting is changed by adjusting the R_T and the two R_G elements to provide a 50-Ω input match and
setting the feedback resistors to 1.5 kΩ.
• Output loading of both resistive and capacitive load testing. Changing to lower resistive loads is accomplished
by adding parallel resistors across the output pins in Figure 1. Changing to capacitive loads adds series
output resistors to a differential capacitance before the 1-kΩ sense path of Figure 1.
• Power-supply settings. Most often, a single 10-V test uses a ±5-V supply and a 3-V test uses ±1.5-V supplies
with the V_OCM input control at ground.
• The disable control pin (PD) is tied to the positive supply (V_S+) for any active channel test.
8.2 Output Interface Circuit for DC-Coupled Differential Testing
The pulse response plots were taken using the output circuit of Figure 4. The two sides of this circuit present a
500-Ω load to ground (for a differential 1-kΩ load) with a 50-Ω source to the two scope inputs. Trace math
combines the two sides into the pulse response plots of to and to . Use balanced bipolar supplies for this test so
that the THS4561 outputs deliver a ground-centered differential swing. This setup produces no dc load currents
using the circuit of Figure 4.
RO1
475
THS4561
Output
RO2
475
RM1
56.2
50-
Scope
Input
RM1
56.2
50-
Scope
Input
Copyright © 2017, Texas Instruments Incorporated
Figure 4. Output Interface for DC-Coupled Differential Outputs
8.3 Output Common-Mode Measurements
The circuit of Figure 5 is a typical setup for common-mode measurements.
THS4561 Wideband,
Fully Differential Amplifier
RG1
1.5 k
RF1
1.5 k
Signal
Source
RS
49.9
VOCM
Input
RT
49.9
RG2
1.5 k
VS+
±
+
FDA
±
+
PD
VS± ^VS+
100
50-
Measurement
Equipment
100
RF2
1.5 k
Copyright © 2017, Texas Instruments Incorporated
Figure 5. Output Common-Mode Measurements
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Copyright © 2017, Texas Instruments Incorporated