THS4552 Datasheet, PDF (41/71 Pages) Texas Instruments – Dual-Channel, Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

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10 Application and Implementation
THS4552
SBOS831 – DECEMBER 2016
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
10.1 Application Information
Most applications for the THS4552 strive to deliver the best dynamic range in a design that delivers the desired
signal processing along with adequate phase margin for the amplifier itself. The following sections detail some of
the design issues with analysis and guidelines for improved performance.
10.1.1 Noise Analysis
The first step in the output noise analysis is to reduce the application circuit to the simplest form with equal
feedback and gain setting elements to ground. Figure 77 shows the simplest analysis circuit with the FDA and
resistor noise terms to be considered.
e_nRg2
RG
e_nRf2
RF
I_n+2
+
I_n±²
±
e_no2
e_nRg2
RG
e_ni2
RF
e_nRf2
Figure 77. FDA Noise Analysis Circuit
The noise powers are shown in Figure 77 for each term. When the R_F and R_G (or R_I) terms are matched on each
side, the total differential output noise is the root sum squared (RSS) of these separate terms. Using NG ≡ 1 +
R_F / R_G, the total output noise is given by Equation 10. Each resistor noise term is a 4kT × R power (4kT = 1.6E-
20J at 290K).
e_o
e_ni NG ² 2 i_NR _F ² 2 4kTR _FNG
(10)
The first term is simply the differential input spot noise times the noise gain, the second term is the input current
noise terms times the feedback resistor (and because there are two uncorrelated current noise terms, the power
is two times one of them), and the last term is the output noise resulting from both the R_F and R_G resistors, at
again twice the value for the output noise power of each side added together. Running a wide sweep of gains
when holding R_F close to 1 kΩ and setting the input up for a 50-Ω match gives the standard values and resulting
noise listed in Table 4.
Note that when the gain increases, the input-referred noise approaches only the gain of the FDA input voltage
noise term at 3.3 nV/√Hz.
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