THS4561 Datasheet, PDF (25/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

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THS4561
SBOS874 – AUGUST 2017
Application Information (continued)
This impedance combines with the load to shift the apparent open-loop gain and phase to the output pins when
the load changes. The rail-to-rail output stage shows a very high impedance at low frequencies that reduces with
frequency to a lower midrange value and then peaks again at higher frequencies. The maximum value at low
frequencies is set by the common-mode sensing resistors to be a TBD-kΩ dc value (see the Functional Block
Diagram section). This high impedance at a low frequency is significantly reduced in closed-loop operation by the
loop gain, as shown in the closed-loop output impedance of . Figure 7 compares the no load A_OL gain to the A_OL
gain driving a 100-Ω load that shows the effect of the output impedance. The heavier loads pull the A_OL gain
down faster to lower crossovers with more phase shift at the lower frequencies.
The much faster phase rolloff for the 100-Ω differential load explains the greater peaked response illustrated in
and when the load decreases. This same effect happens for the RC loads common with converter interface
designs. Use the TINA-TI™ model to verify loop phase margin in any design.
10.1.2 Setting Resistor Values Versus Gain
The THS4561 offers considerable flexibility in the configuration and selection of resistor values. The design starts
with the selection of the feedback resistor value. The 1.5-kΩ feedback resistor value used for the characterization
curves is a good compromise between power, noise, and phase margin considerations. With the feedback
resistor values selected (and set equal on each side) the input resistors are set to obtain the desired gain with
input impedance also set with these input resistors. Differential I/O designs provide an input impedance that is
the sum of the two input resistors. Single-ended input to differential output designs present a more complicated
input impedance. Most characteristic curves implement the single-ended to differential design as the more
challenging requirement over differential-to-differential I/O.
For single-ended, matched, input impedance designs, Table 1 illustrates the suggested standard resistors set to
approximately a 1.5-kΩ feedback. This table assumes a 50-Ω source and a 50-Ω input match and uses a single
resistor on the non-signal input side for gain matching. Better matching is possible using the same three resistors
on the non-signal input side as on the input side. Figure 10 shows the element values and naming convention for
the gain of 1-V/V configuration where the gain is defined from the matched input at R_T to the differential output.
50-
Source
Impedance
50- Input Match,
Gain of 1 V/V from RT,
Single-Ended Source to
Differential Output
RS1
50
RG1
1.5 k
RT
52.3
THS4561 Wideband,
Fully Differential Amplifier
RF1
1.5 k
VS+
VOCM
±
+
FDA
±
+
PD
VOPP
RL
1k
RG2
1.52 k
VS± ^VS+
RF2
1.5 k
Copyright © 2017, Texas Instruments Incorporated
Figure 10. Single-Ended to Differential Gain of 1 V/V with Input Matching Using Standard Resistor Values
Copyright © 2017, Texas Instruments Incorporated
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