THS4551 Datasheet, PDF (28/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
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Feature Description (continued)
9.3.2 Setting Resistor Values Versus Gain
The THS4551 offers considerable flexibility in the configuration and selection of resistor values. The design starts
with the selection of the feedback resistor value. The 1-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-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 69 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
1k
RT
52.3
THS4551 Wideband,
Fully Differential Amplifier
RF1
1k
VS+
VOCM
±
+
FDA
±
+
PD
RG2
1.02 k
VS- ^VS+
VOPP
RL
1k
RF2
1k
Copyright © 2016, Texas Instruments Incorporated
Figure 69. Single-Ended to Differential Gain of 1 V/V with Input Matching Using Standard Resistor Values
Starting from a target feedback resistor value, the desired input matching impedance, and the target gain (A_V),
the required input R_T value is given by solving the quadratic of Equation 1.
R_T2
§
R_T
2R_F
2R_S
2
¨© ^2RF
A_V
R_S
2
A_V2
·
¸¹
R_SA_V (4 A_V )
2R_FR_S2 A _V
2R_F 2 A_V R_SA_V (4 A_V )
0
(1)
When this value is derived, the required input side gain resistor is given by Equation 2 and then the single value
for R_G2 on the non-signal input side is given by Equation 3:
R_G1
2 ^RF
A_V
R_S
1 ^RS
R_T
(2)
2 ^RF
R_G2
A_V
1 ^RS
R_T
(3)
28
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