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THS4531A_16 Datasheet, PDF (43/59 Pages) Texas Instruments – Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier
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THS4531A
SLOS823C – DECEMBER 2012 – REVISED JANUARY 2016
50-Input Match,
Gain of 5 V/V from Rt,
Single-Ended Source to
Differential Step-Response Test
50-Ω
Source
Rg1
187 Ω
Rt
59 Ω
Wideband,
Fully-Differential Amplifier
Rf1
1 kΩ
Vcc
Vocm
–
+
FDA
–
+
PD
Rg2
Vcc
215 Ω
Rf2
1 kΩ
R1
500 Ω
Output
Measurement
Point
Figure 97. DC-Coupled, Single-Ended-to-Differential, Set for a Gain of 5 V/V
9.2.6.3 Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA
The design equations for setting the resistors around an FDA to convert from a single-ended input signal to
differential output can be approached from several directions. Here, several critical assumptions are made to
simplify the results:
• The feedback resistors are selected first and set equal on the two sides.
• The DC and AC impedances from the summing junctions back to the signal source and ground (or a bias
voltage on the nonsignal input side) are set equal to retain feedback divider balance on each side of the FDA.
Both of these assumptions are typical for delivering the best dynamic range through the FDA signal path.
After the feedback resistor values are chosen, the aim is to solve for the RT (a termination resistor to ground on
the signal input side), RG1 (the input gain resistor for the signal path), and RG2 (the matching gain resistor on the
nonsignal input side); see Figure 96 and Figure 97. The same resistor solutions can be applied to either AC- or
DC-coupled paths. Adding blocking capacitors in the input-signal chain is a simple option. Adding these blocking
capacitors after the RT element (as shown in Figure 96) has the advantage of removing any DC currents in the
feedback path from the output VOCM to ground.
Earlier approaches to the solutions for RT and RG1 (when the input must be matched to a source impedance, RS)
follow an iterative approach. This complexity arises from the active input impedance at the RG1 input. When the
FDA is used to convert a single-ended signal to differential, the common-mode input voltage at the FDA inputs
must move with the input signal to generate the inverted output signal as a current in the RG2 element. A more
recent solution is shown as Equation 8, where a quadratic in RT can be solved for an exact value. This quadratic
emerges from the simultaneous solution for a matched input impedance and target gain. The only inputs required
are:
• The selected RF value.
• The target voltage gain (Av) from the input of RT to the differential output voltage.
• The desired input impedance at the junction of RT and RG1 to match RS.
Solving this quadratic for RT starts the solution sequence, as shown in Equation 8:
R
2
T
-
R
T
2R S(2R F
2R F(2 + AV ) -
+
RS
2
A
2
V
)
R SAV(4 +
AV )
-
2R F(2 +
2R F RS2 AV
AV ) - R SAV(4 + AV)
=0
(8)
Being a quadratic, there are limits to the range of solutions. Specifically, after RF and RS are chosen, there is
physically a maximum gain beyond which Equation 8 starts to solve for negative RT values (if input matching is a
requirement). With RF selected, use Equation 9 to verify that the maximum gain is greater than the desired gain.
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